Nutrient Cycling in Pastures


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Nutrient Cycling in Pastures

  1. 1. NUTRIENT CYCLING IN PASTURES LIVESTOCK SYSTEMS GUIDE By Barbara Bellows NCAT Agriculture Specialist December 2001 Table of Contents Introduction and Summary ........................................................... 2 Publication Overview ................................................................... 5 Chapter 1. Nutrient Cycle Components, Interactions, and ............. Transformations ........................................................................ 6 Water Cycle ........................................................................... 6 Carbon Cycle ....................................................................... 10 Nitrogen Cycle ..................................................................... 13 Phosphorus Cycle ................................................................ 18 Secondary Nutrients ............................................................ 21Abstract: Good pasture management Chapter 2. Nutrient Availability in Pastures ................................ 23practices foster effective use and Soil Parent Material .............................................................. 23recycling of nutrients. Nutrient cycles Soil Chemistry ...................................................................... 23important in pasture systems are the Prior Management Practices ................................................ 24water, carbon, nitrogen, and phosphorus Soil Compaction ................................................................... 24cycles. This publication provides basic Organic Matter ..................................................................... 25descriptions of these cycles, and presents Soil pH ................................................................................. 27guidelines for managing pastures to Timing of Nutrient Additions ................................................. 27enhance nutrient cycling efficiency — Chapter 3. Nutrient Distribution and Movement in Pastures ...... 30with the goal of optimizing forage and Pasture Nutrient Inputs and Outputs .................................... 30livestock growth, soil health, and water Manure Nutrient Availability .................................................. 32quality. Includes 19 Tables and 14 Pasture Fertilization ............................................................. 33Figures. Grazing Intensity .................................................................. 34 Diversity and Density of Pasture Plants ............................... 36 Chapter 4. The Soil Food Web and Pasture Soil Quality ........... 40 Diversity of the Soil Food Web ............................................. 40 Organic Matter Decomposition............................................. 40 Primary Decomposers ......................................................... 41 Secondary Decomposers ..................................................... 43 Soil Organisms and Soil Health ........................................... 44 Chapter 5. Pasture Management and Water Quality ................ 47 Risk Factors for Nutrient Losses .......................................... 47 Pathogens ............................................................................ 48 Nitrate Contamination .......................................................... 49 Phosphorus Contamination .................................................. 49 Subsurface Drainage ........................................................... 51 Riparian Buffers ................................................................... 52 Riparian Grazing .................................................................. 53 References ................................................................................ 55 Resource List ............................................................................ 61 Agencies and Organizations ................................................ 61 Publications in Print ............................................................. 61 Web Resources ......................................................................... 63 ATTRA is the national sustainable agriculture information service operated by the National Center for Appropriate Technology under a grant from the Rural Business-Cooperative Service, U.S. Department of Agriculture. These organizations do not recommend or endorse products, companies, or individuals. ATTRA is headquartered in Fayetteville, Arkansas (P.O. Box 3657, Fayetteville, AR 72702), with offices in Butte, Montana and Davis, California.
  2. 2. Introduction and Summary As a pasture manager, what factors do you look at as indicators of high production and maximumprofitability? You probably look at the population of animals stocked within the pasture. You probablylook at the vigor of plant regrowth. You probably also look at the diversity of plant species growing in thepasture and whether the plants are being grazed uniformly. But do you know how much water seepsinto your soil or how much runs off the land into gullies or streams? Do you monitor how efficiently yourplants are taking in carbon and forming new leaves, stems, and roots through photosynthesis? Do youknow how effectively nitrogen and phosphorus are being used, cycled, and conserved on your farm? Aremost of these nutrients being used for plant and animal growth? Or are they being leached into thegroundwater or transported through runoff or erosion into lakes, rivers, and streams? Do you know howto change your pasture management practices to decrease these losses and increase the availability ofnutrients to your forages and animals? .igure 1. Nutrient Cycles in Pastures. Animal Production T Manure Production T T T Water Infiltration TT T T Plant Vigor Minimal Soil T Water Availability T T T T TSoil Organisms Erosion T T T T Nutrient Availability T T T T T T T T Soil Organic Matter T T T Legumes T Nutrient Mineralization T T Plant Cover Soil Porosity/ T Minimal CompactionHealthy plant growth provides plant cover over the entire pasture. Cover from growing plants and plant residues protectsthe soil against erosion while returning organic matter to the soil. Organic matter provides food for soil organisms thatmineralize nutrients from these materials and produce gels and other substances that enhance water infiltration and thecapacity of soil to hold water and nutrients. Effective use and cycling of nutrients is practices that facilitate water movement into thecritical for pasture productivity. As indicated in soil and build the soil’s water holding capacityFigure 1 above, nutrient cycles are complex and will conserve water for plant growth and ground-interrelated. This document is designed to help water recharge, while minimizing waters poten-you understand the unique components of wa- tial to cause nutrient losses. Water-conservingter, carbon, nitrogen, and phosphorus cycles and pasture management practices include:how these cycles interact with one another. This • Minimizing soil compaction by not overgraz-information will help you to monitor your pas- ing pastures or using paddocks that have wettures for breakdowns in nutrient cycling pro- or saturated soilscesses, and identify and implement pasture man- • Maintaining a complete cover of forages andagement practices to optimize the efficiency of residues over all paddocks by not overgraz-nutrient cycling. ing pastures and by implementing practices WATER that encourage animal movement across each Water is necessary for plant growth, for dis- paddocksolving and transporting plant nutrients, and for • Ensuring that forage plants include a diver-the survival of soil organisms. Water can also be a sity of grass and legume species with a vari-destructive force, causing soil compaction, nutri- ety of root systems capable of obtaining wa-ent leaching, runoff, and erosion. Management ter and nutrients throughout the soil profilePAGE 2 //NUTRIENT CYCLING IN PASTURES
  3. 3. CARBON called rhizobia, are able to transform atmospheric Carbon is transformed from carbon dioxide nitrogen into a form available for plant use. Ni-into plant cell material through photosynthesis. trogen in dead organic materials becomes avail-It is the basic structural material for all cell life, able to plants through mineralization. Nitrogenand following the death and decomposition of can be lost from the pasture system through thecells it provides humus and other organic com- physical processes of leaching, runoff, and ero-ponents that enhance soil quality. Plant nutri- sion; the chemical process of volatilization; theents such as nitrogen and phosphorus are chemi- biological process of denitrification; and throughcally bound to carbon in organic materials. For burning of plant residues. Since it is needed inthese nutrients to become available for plant use, high concentration for forage production and cansoil organisms need to break down the chemi- be lost through a number of pathways, nitrogencal bonds in a process called mineralization. If is often the limiting factor in forage and crop pro-the amount of carbon compared to other nutri- duction. Productive pasture management prac-ents is very high, more bonds will need to be tices enhance the fixation and conservation of ni-broken and nutrient release will be slow. If the trogen while minimizing the potential for nitro-amount of carbon compared to other nutrients gen losses. Practices that favor effective nitro-is low, fewer bonds will need to be broken and gen use and cycling in pastures include:nutrient release will proceed relatively rapidly. • Maintaining stable or increasing percentagesRapid nutrient release is preferred when plants of legumes by not overgrazing pastures andare growing and are able to use the nutrients by minimizing nitrogen applications, espe-released. Slower nutrient release is preferred cially in the springwhen plants are not actively growing (as in the • Protecting microbial communities involvedfall or winter) or if the amount of nutrients in in organic matter mineralization by minimiz-the soil is already in excess of what plants can ing practices that promote soil compactionuse. Pasture management practices that favor and soil disturbance, such as grazing of weteffective carbon use and cycling include: soils and tillage• Maintaining a diversity of forage plants with • Incorporating manure and nitrogen fertiliz- a variety of leaf shapes and orientations (to ers into the soil, and never applying these ma- enhance photosynthesis) and a variety of terials to saturated, snow-covered, or frozen root growth habits (to enhance nutrient up- soils take). A diversity of forages will provide a • Avoiding pasture burning. If burning is re- balanced diet for grazing animals and a va- quired, it should be done very infrequently riety of food sources for soil organisms and by using a slow fire under controlled con-• Promoting healthy regrowth of forages by ditions including a combination of grasses with both • Applying fertilizers and manure according low and elevated growing points, and by to a comprehensive nutrient management moving grazing animals frequently enough plan to minimize the removal of growing points PHOSPHORUS• Maintaining a complete cover of forages and Phosphorus is used for energy transforma- residues over all paddocks to hold soil nu- tions within cells and is essential for plant trients against runoff and leaching losses and growth. It is often the second-most-limiting min- ensure a continuous turnover of organic resi- eral nutrient to plant production, not only be- dues cause it is critical for plant growth, but also be- NITROGEN cause chemical bonds on soil particles hold the majority of phosphorus in forms not available Nitrogen is a central component of cell pro- for plant uptake. Phosphorus is also the majorteins and is used for seed production. It exists nutrient needed to stimulate the growth of algaein several chemical forms and various microor- in lakes and streams. Consequently, the inad-ganisms are involved in its transformations. Le- vertent fertilization of these waterways with run-gumes, in association with specialized bacteria off water from fields and streams can cause //NUTRIENT CYCLING IN PASTURES PAGE 3
  4. 4. degradation of water quality for drinking, recre-ational, or wildlife habitat uses. Regulations on .igure 2. Components ofthe use of phosphorus-containing materials are Soil Organic Matter.becoming more widespread as society becomes Readily Bacteria & decomposable Fauna 10%increasingly aware of the impacts agricultural Actinomycetes 30% 7-21%practices can have on water quality. Pasture T T Yeast, algae, T Tmanagement practices must balance the need to T protozoa,ensure sufficient availability of phosphorus for nematodes 10%plant growth with the need to minimize move- Fungi 50%ment of phosphorus from fields to streams. Pas- T Tture management practices that protect this bal- Mineral Stable Humusance include: particles 70-90%• Minimizing the potential for compaction Soil Organic Soil Microbial while providing organic inputs to enhance Soil Matter Biomass activities of soil organisms and phosphorus mineralization Soil contains 1-6% organic matter. Organic matter• Incorporating manure and phosphorus fer- contains 3-9% active microorganisms. These organisms tilizers into the soil and never applying these include plant life, bacteria and actinomycetes, fungi, yeasts, materials to saturated, snow-covered, or fro- algae, protozoa, and nematodes. zen soils Humus, along with fungal threads, bacterial• Relying on soil tests, phosphorus index gels, and earthworm feces, forms glues that hold guidelines, and other nutrient management soil particles together in aggregates. These con- practices when applying fertilizers and ma- stitute soil structure, enhance soil porosity, and nure to pastures allow water, air, and nutrients to flow through SOIL LIFE the soil. These residues of soil organisms also en- hance the soils nutrient and water holding ca- Soil is a matrix of pore spaces filled with wa- pacity. Lichens, algae, fungi, and bacteria formter and air, minerals, and organic matter. Al- biological crusts over the soil surface. Thesethough comprising only 1 to 6% of the soil, liv- crusts are important, especially in arid range-ing and decomposed organisms are certainly of lands, for enhancing water infiltration and pro-the essence. They provide plant nutrients, cre- viding nitrogen fixation (2). Maintaining a sub-ate soil structure, hold water, and mediate nutri- stantial population of legumes in the pasture alsoent transformations. Soil organic matter is com- ensures biological nitrogen fixation by bacteriaposed of three components: stable humus, associated with legume roots.readily decomposable materials, and living or- Effective nutrient cycling in the soil is highlyganisms — also described as the very dead, the dependent on an active and diverse communitydead, and the living components of soil (1). of soil organisms. Management practices that Living organisms in soil include larger fauna maintain the pasture soil as a habitat favorablesuch as moles and prairie dogs, macroorganisms for soil organisms include:such as insects and earthworms, and microor- • Maintaining a diversity of forages, whichganisms including fungi, bacteria, yeasts, algae, promotes a diverse population of soil organ-protozoa, and nematodes. These living organ- isms by providing them with a varied dietisms break down the readily decomposable plant • Adding organic matter, such as forage resi-and animal material into nutrients, which are dues and manure, to the soil to provide foodthen available for plant uptake. Organic matter for soil organisms and facilitate the forma-residues from this decomposition process are tion of aggregatessubsequently broken down by other organisms • Preventing soil compaction and soil satura-until all that remains are complex compounds tion, and avoiding the addition of amend-resistant to decomposition. These complex end ments that might kill certain populations ofproducts of decomposition are known as humus. soil organismsPAGE 4 //NUTRIENT CYCLING IN PASTURES
  5. 5. Publication OverviewThis publication is divided into five chapters:1. Nutrient cycle components, interactions, and transformations2. Nutrient availability in pastures3. Nutrient distribution and movement in pastures4. The soil food web and pasture soil quality5. Pasture management and water quality The first chapter provides an overview of nutrient cycles critical to plant production and water-quality protection: the water, carbon, nitrogen, phosphorus, and secondary-nutrient cycles. Thecomponents of each cycle are explained, with emphasis on how these components are affected bypasture management practices. The description of each cycle concludes with a summary of pasturemanagement practices to enhance efficient cycling of that nutrient. The second chapter focuses on the effects of soil chemistry, mineralogy, and land-managementpractices on nutrient cycle transformations and nutrient availability. Management impacts discussedinclude soil compaction, organic matter additions and losses, effects on soil pH, and consequences ofthe method and timing of nutrient additions. The chapter concludes with a summary of pasturemanagement practices for enhancing nutrient availability in pastures. The third chapter discusses nutrient balances in grazed pastures and the availability of manure,residue, and fertilizer nutrients to forage growth. Factors affecting nutrient availability include nutri-ent content and consistency of manure; manure distribution as affected by paddock location andlayout; and forage diversity. These factors, in turn, affect grazing intensity and pasture regrowth. Agraph at the end of the chapter illustrates the interactions among these factors. The fourth chapter describes the diversity of organisms involved in decomposing plant residuesand manure in pastures, and discusses the impact of soil biological activity on nutrient cycles andforage production. The impacts of pasture management on the activity of soil organisms are ex-plained. A soil health card developed for pastures provides a tool for qualitatively assessing thesoil’s ability to support healthy populations of soil organisms. The publication concludes with a discussion of pasture management practices and their effects onwater quality, soil erosion, water runoff, and water infiltration. Several topical water concerns arediscussed: phosphorus runoff and eutrophication, nutrient and pathogen transport through subsur-face drains, buffer management, and riparian grazing practices. A guide for assessing potential wa-ter-quality impacts from pasture-management practices concludes this final chapter. //NUTRIENT CYCLING IN PASTURES PAGE 5
  6. 6. Chapter 1 Nutrient Cycle Components, Interactions, and Transformations The water, carbon, nitrogen, phosphorus, and sulfur cycles are the most important nutrient cyclesoperating in pasture systems. Each cycle has its complex set of interactions and transformations aswell as interactions with the other cycles. The water cycle is essential for photosynthesis and thetransport of nutrients to plant roots and through plant stems. It also facilitates nutrient loss throughleaching, runoff, and erosion. The carbon cycle forms the basis for cell formation and soil quality. Itbegins with photosynthesis and includes respiration, mineralization, immobilization, and humusformation. Atmospheric nitrogen is fixed into plant-available nitrate by one type of bacteria, con-verted from ammonia to nitrate by another set of bacteria, and released back to the atmosphere by yetanother group. A variety of soil organisms are involved in decomposition processes that release ormineralize nitrogen, phosphorus, sulfur, and other nutrients from plant residues and manure. Bal-ances in the amount of these nutrients within organic materials, along with temperature and mois-ture conditions, determine which organisms are involved in the decomposition process and the rateat which it proceeds. Pasture management practices influence the interactions and transformations occurring withinnutrient cycles. The efficiency of these cycles, in turn, influences the productivity of forage growthand the productivity of animals feeding on the forage. This chapter examines each of these cycles indetail and provides management guidelines for enhancing their efficiency. WATER CYCLE Water is critical for pasture productivity. It small, these soils can easily become compacted.dissolves soil nutrients and moves them to plant Sandy soils are porous and allow water to enterroots. Inside plants, water and the dissolved nu- easily, but do not hold water and nutrientstrients support cell growth and photosynthesis. against leaching. Organic matter in soil absorbsIn the soil, water supports the growth and re- water and nutrients, reduces soil compaction,production of insects and microorganisms that and increases soil porosity. A relatively smalldecompose organic matter. Water also can de- increase in the amount of organic matter in soilgrade pastures through runoff, erosion, and can cause a large increase in the ability of soils toleaching, which cause nutrient loss and water use water effectively to support plant produc-pollution. Pro- tion.ductive pastures A forage cover over INFILTRATION ANDare able to absorb the entire paddock pro- WATER HOLDING CAPACITYand use water ef- motes water infiltration Water soaks into soils that have a plant orfectively for plant and minimizes soil residue cover over the soil surface. This covergrowth. Good compaction. cushions the fall of raindrops and allows thempasture manage- to slowly soak into the soil. Roots create poresment practices promote water absorption by that increase the rate at which water can entermaintaining forage cover over the entire soil sur- the soil. Long-lived perennial bunch-grass formsface and by minimizing soil compaction by ani- deep roots that facilitate water infiltration by con-mals or equipment. ducting water into the soil (3). Other plant char- Geology, soil type, and landscape orientation acteristics that enhance water infiltration are sig-affect water absorption by soils and water move- nificant litter production and large basal cover-ment through soils. Sloping land encourages age (4). In northern climates where snow pro-water runoff and erosion; depressions and vides a substantial portion of the annual waterfootslopes are often wet since water from upslope budget, maintaining taller grasses and shrubscollects in these areas. Clay soils absorb water that can trap and hold snow will enhance waterand nutrients, but since clay particles are very infiltration.PAGE 6 //NUTRIENT CYCLING IN PASTURES
  7. 7. .igure 3. Water Cycle. Rainfall T Water vapor T T Evaporation Transpiration The Water Cycle. Rain falling on soil can either be absorbed into the soil or be lost as it flows over the soil surface. Absorbed rain is used for the growth of plants and T Dissolved nutrients soil organisms, to transport nutrients to plant roots, or to T Photosynthesis T Crop harvest T TRunoff & recharge ground water. It can also leach nutrients through the soil profile, out of the reach of plant roots. Water flowing TPlant uptake T Infiltration T erosion off the soil surface can transport dissolved nutrients as T T Dissolved T to runoff, or nutrients and other contaminants associated with T Soil Leached T nutrients sediments as erosion. Uptake nutrients Soil by wells water T T Subsurface flow T T Groundwater flows Soils with a high water holding capacity ab- layer or high water table. Soils prone to satura-sorb large amounts of water, minimizing the po- tion are usually located at the base of slopes, neartential for runoff and erosion and storing water waterways, or next to seeps.for use during droughts. Soils are able to absorb Impact on crop production. Soil saturationand hold water when they have a thick soil pro- affects plant production by exacerbating soil com-file; contain a relatively high percentage of or- paction, limiting air movement to roots, andganic matter; and do not have a rocky or com- ponding water and soil-borne disease organismspacted soil layer, such as a hardpan or plowpan, around plant roots and stems. When soil poresclose to the soil surface. An active population of are filled with water, roots and beneficial soil or-soil organisms enhances the formation of aggre- ganisms lose access to air, which is necessary forgates and of burrowing channels that provide their healthy growth. Soil compaction decreasespathways for water to flow into and through the the ability of air, water, nutrients, and roots tosoil. Management practices that enhance water move through soils even after soils have dried.infiltration and water holding capacity include: Plants suffering from lack of air and nutrients• a complete coverage of forages and residues are susceptible to disease attack since they are over the soil surface under stress, and wet conditions help disease or-• an accumulation of organic matter in and on ganisms move from contaminated soil particles the soil and plant residues to formerly healthy plant roots• an active community of soil organisms in- and stems. volved in organic matter decomposition and Runoff and erosion potential. Soil satura- aggregate formation tion enhances the potential for runoff and ero-• water runoff and soil erosion prevention sion by preventing entry of additional water into• protection against soil compaction the soil profile. Instead, excess water will run SOIL SATURATION off the soil surface, often carrying soil and nutri-Soils become saturated when the amount of wa- ents with it. Water can also flow horizontallyter entering exceeds the rate of absorption or under the surface of the soil until it reaches thedrainage. A rocky or compacted lower soil layer banks of streams or lakes. This subsurface wa-will not allow water to drain or pass through, ter flow carries nutrients away from roots, wherewhile a high water table prevents water from they could be used for plant growth, and intodraining through the profile. Water soaking into streams or lakes where they promote the growththese soils is trapped or perched above the hard of algae and eutrophication. //NUTRIENT CYCLING IN PASTURES PAGE 7
  8. 8. Artificial drainage practices are often used tures is remediated by root growth, aggregateon soils with a hardpan or a high water table to formation, and activities of burrowing soil organ-decrease the duration of soil saturation follow- isms. In colder climates, frost heaving is an im-ing rainfalls or snowmelts. This practice can in- portant recovery process for compacted soils (8).crease water infiltration and decrease the poten- RUNOFF AND EROSIONtial for water runoff (5). Unfortunately, most sub-surface drains were installed before water pollu- Runoff water dissolves nutrients and removestion from agriculture became a concern and thus them from the pasture as it flows over the soilempty directly into drainage ways. Nutrients, surface. Soil erosion transports nutrients and anypathogens, and other contaminants on the soil contaminants, such as pesticides and pathogens,surface can move through large cracks or chan- attached to soil particles. Because nutrient-richnels in the soil to drainage pipes where they are clay and organic matter particles are small andcarried to surface water bodies (6). lightweight, they are more readily picked up and moved by water than the nutrient-poor, but SOIL COMPACTION heavier, sand particles. Besides depleting pas- Soil compaction occurs when animals or tures of nutrients that could be used for forageequipment move across soils that are wet or satu- production, runoff water and erosion carry nu-rated, with moist soils being more easily com- trients and sediments that contaminate lakes,pacted than saturated soils (7). Compaction can streams, and rivers.also occur when animals or equipment continu- Landscape condi- Runoff water and ero-ally move across a laneway or stand around wa- tions and manage- sion carry nutrientstering tanks and headlands or under shade. Ani- ment practices that and sediments thatmals trampling over the ground press down on favor runoff and ero- contaminate lakes,soils, squeezing soil pore spaces together. Tram- sion include sloping streams, and rivers.pling also increases the potential for compaction areas, minimal soilby disturbing and killing vegetation. protection by forage or residues, intense rainfall, Soils not covered by forages or residues are and saturated soils. While pasture managerseasily compacted by the impact of raindrops. should strive to maintain a complete forage coverWhen raindrops fall on bare soil, their force causes over the soil surface, this is not feasible in prac-fine soil particles to splash or disperse. These tice because of plant growth habits and land-splash particles land on the soil surface, clog sur- scape characteristics. Plant residues from die- face soil pores, back and animal wastage during grazing provide Animals trampling over and form a crust a critical source of soil cover and organic matter. the ground press down over the soil. As mentioned above, forage type affects water on soils, squeezing soil Clayey soils are infiltration and runoff. Forages with deep roots pores together, which more easily com- enhance water infiltration while plants with a wide limits root growth and pacted than sandy vegetative coverage area or prostrate growth pro- the movement of air, soils because clay vide good protection against raindrop impact. water, and dissolved particles are very Sod grasses that are short-lived and shallow- nutrients. small and sticky. rooted inhibit water infiltration and encourage Compaction runoff. Grazing practices that produce clumps limits root growth of forages separated by bare ground enhance run-and the movement of air, water, and dissolved off potential by producing pathways for waternutrients through the soil. Compressing and flow.clogging soil surface pores also decreases water EVAPORATION AND TRANSPIRATIONinfiltration and increases the potential for runoff. Water in the soil profile can be lost throughThe formation of hardpans, plowpans, traffic evaporation, which is favored by high tempera-pans, or other compacted layers decreases down- tures and bare soils. Pasture soils with a thickward movement of water through the soil, caus- cover of grass or other vegetation lose little wa-ing rapid soil saturation and the inability of soils ter to evaporation since the soil is shaded andto absorb additional water. Compaction in pas- soil temperatures are decreased. While evapo-PAGE 8 //NUTRIENT CYCLING IN PASTURES
  9. 9. ration affects only the top few inches of pasture This is because forage species differ in their abil-soils, transpiration can drain water from the en- ity to extract water from the soil and conserve ittire soil profile. Transpiration is the loss of wa- against transpiration (9). Some invasive plantter from plants through stomata in their leaves. species, however, can deplete water storesEspecially on sunny and breezy days, significant through their high water use (4). Water not usedamounts of water can be absorbed from the soil for immediate plant uptake is held within the soilby plant roots, taken up through the plant, and profile or is transported to groundwater reserves,lost to the atmosphere through transpiration. A which supply wells with water and decrease thediversity of forage plants will decrease transpi- impacts of drought.ration losses and increase water-use efficiency. Table 1. Water Cycle Monitoring. If you answer no to all the questions, you have soils with high water-use efficiency. If you answer yes to some of the questions, water cycle efficiency of your soil will likely respond to improved pasture management practices. See Table 2, next page. YES NO Water infiltration / Water runoff 1. Do patches of bare ground separate forage coverage?________________________________________________________________________________ 2. Are shallow-rooted sod grasses the predominant forage cover?________________________________________________________________________________ 3. Can you see small waterways during heavy rainfalls or sudden snowmelts?________________________________________________________________________________ 4. Are rivulets and gullies present on the land?________________________________________________________________________________ Soil saturation 5. Following a rainfall, is soil muddy or are you able to squeeze water out of a handful of soil?________________________________________________________________________________ 6. Following a rainfall or snowmelt, does it take several days before the soil is no longer wet and muddy?________________________________________________________________________________ 7. Do forages turn yellow or die during wet weather?________________________________________________________________________________ Soil compaction 8. Do you graze animals on wet pastures?________________________________________________________________________________ 9. Are some soils in the pasture bare, hard, and crusty?________________________________________________________________________________ 10. Do you have difficulty driving a post into (non-rocky) soils?________________________________________________________________________________ Water retention /Water evaporation and transpiration 11. Do you have a monoculture of forages or are invasive species prominent components of the pasture?________________________________________________________________________________ 12. Do soils dry out quickly following a rainstorm?________________________________________________________________________________ 13. During a drought, do plants dry up quickly? //NUTRIENT CYCLING IN PASTURES PAGE 9
  10. 10. Table 2. Pasture Management Practices for Efficient Water Cycling in Pastures. Ensure forage and residue coverage across the entire pasture • Use practices that encourage animal movement throughout the pasture and discourage con- gregation in feeding and lounging areas • Use practices that encourage regrowth of forage plants and discourage overgrazing • Use a variety of forages with a diversity of root systems and growth characteristics Pasture management during wet weather • Use well-drained pastures or a “sacrificial pasture” that is far from waterways or water bodies • Avoid driving machinery on pastures that are wet or saturated • Avoid spreading manure or applying fertilizers Artificial drainage practices • Avoid grazing animals on artificially drained fields when drains are flowing • Avoid spreading manure or applying fertilizers when drains are flowing • Ensure that drains empty into a filter area or wetland rather than directly into a stream or drainage way CARBON CYCLE Effective carbon cycling in pastures depends on a diversity of plants and healthy populations ofsoil organisms. Plants form carbon and water into carbohydrates through photosynthesis. Plants aremost able to conduct photosynthesis when they can efficiently capture solar energy while also havingadequate access to water, nutrients, and air. Animals obtain carbohydrates formed by plants whenthey graze on pastures or eat hay or grains harvested from fields. Some of the carbon and energy inplant carbohydrates is incorporated into animal cells. Some of the carbon is lost to the atmosphere ascarbon dioxide, and some energy is lost as heat, during digestion and as the animal grows and breathes. Carbohydrates and other nutrients not used by animals are re- turned to the soil in the form of urine and manure. These organic Humus maintains soil materials provide soil organisms with nutrients and energy. As soil tilth and enhances water organisms use and decompose organic materials, they release nutri- and nutrient absorption. ents from these materials into the soil. Plants then use the released, inorganic forms of nutrients for their growth and reproduction. Soil organisms also use nutrients from organic materials to produce sub-stances that bind soil particles into aggregates. Residues of organic matter that resist further decom-position by soil organisms form soil humus. This stable organic material is critical for maintaining soiltilth and enhancing the ability of soils to absorb and hold water and nutrients. CARBOHYDRATE FORMATION ergy by a diversity of leaf shapes and leaf angles. For productive growth, plants need to effec- Taller plants with more erect leaves capture lighttively capture solar energy, absorb carbon diox- even at the extreme angles of sunrise and sunset.ide, and take up water from the soil to produce Horizontal leaves capture the sun at midday orcarbohydrates through photosynthesis. In pas- when it is more overhead.tures, a combination of broadleaf plants and Two methods for transforming carbon intograsses allows for efficient capture of solar en- carbohydrates are represented in diversified pas-PAGE 10 //NUTRIENT CYCLING IN PASTURES
  11. 11. .igure 4. Carbon Cycle. T CO2 in atmosphere T T Consumption The Carbon Cycle begins with plants taking up carbon dioxide from the atmosphere in the process of photosynthesis. T Respiration T Photosynthesis Some plants are eaten by grazing animals, which return T T organic carbon to the soil as manure, and carbon dioxide to the atmosphere. Easily broken-down forms of carbon in Crop and Mineralization animal manure and plant cells are released as carbon dioxide when residues Loss via T decomposing soil organisms respire. Forms of carbon that Carbon in soil T erosion are difficult to break down become stabilized in the soil as humus. organic matter T T Humus and T aggregate T T Decomposition formulation T T in microbestures. Broadleaf plants and cool-season grasses they break down complex carbohydrates and pro-have a photosynthetic pathway that is efficient teins into simpler chemical forms. For example,in the production of carbohydrates but is sensi- soil organisms break down proteins into carbontive to dry conditions. Warm-season grasses dioxide, water, ammonium, phosphate, and sul-have a pathway that is more effective in produc- fate. Plants require nutrients to be in this sim-ing carbohydrates during hot summer condi- pler, decomposed form before they can use themtions. A combination of plants representing these for their growth.two pathways ensures effective forage growth To effectively decompose organic matter, soilthroughout the growing season. A diversity of organisms require access to air, water, and nu-root structures also promotes photosynthesis by trients. Soil compaction and saturation limit thegiving plants access to water and nutrients growth of beneficial organisms and promote thethroughout the soil profile. growth of anaerobic organisms, which are inef- ficient in the decomposition of organic matter. ORGANIC MATTER DECOMPOSITION These organisms also transform some nutrients Pasture soils gain organic matter from growth into forms that are less available or unavailableand die-back of pasture plants, from forage wast- to plants. Nutrient availability and nutrient bal-age during grazing, and from manure deposition. ances in the soil solution also affect the growthIn addition to the recycling of aboveground plant and diversity of soil organisms. To decomposeparts, every year 20 to 50% of plant root mass organic matter that contains a high amount ofdies and is returned to the soil system. Some carbon and insufficient amounts of other nutri-pasture management practices also involve the ents, soil organisms must mix soil- solution nu-regular addition of manure trients with this material tofrom grazing animals housed achieve a balanced diet. A healthy and diverse popu-during the winter or from poul- Balances between the lation of soil organisms istry, hog, or other associated amount of carbon and nitro- necessary for organic matterlivestock facilities. gen (C:N ratio) and the decomposition, nutrient min- A healthy and diverse amount of carbon and sulfur eralization, and the formationpopulation of soil organisms is (C:S ratio) determine of soil aggregates.necessary for organic matter whether soil organisms willdecomposition, nutrient miner- release or immobilize nutri-alization, and the formation of soil aggregates. ents when they decompose organic matter. Im-Species representing almost every type of soil mobilization refers to soil microorganisms takingorganism have roles in the breakdown of manure, nutrients from the soil solution to use in the de-plant residues, and dead organisms. As they use composition process of nutrient-poor materials.these substances for food and energy sources, Since these nutrients are within the bodies of soil //NUTRIENT CYCLING IN PASTURES PAGE 11
  12. 12. organisms, they are temporarily unavailable to aggregated, soil pore size increases and soils be-plants. In soils with low nutrient content, this come resistant to compaction. The organic com-can significantly inhibit plant growth. However, pounds that hold aggregates together also in-immobilization can be beneficial in soils with ex- crease the ability of soils to absorb and hold wa-cess nutrients. This process conserves nutrients ter and bodies of soil organisms, where they are less As soil organisms decompose manure andlikely to be lost through leaching and runoff (10). plant residues, they release carbon dioxide and Populations of soil organisms are enhanced produce waste materials, which are further de-by soil that is not com- composed by other soil organisms. Be-pacted and has adequate In soils with low nutrient con- cause carbon is lost to respiration atair and moisture, and tent, nutrient immobilization each stage of this decomposition pro-by additions of fresh inhibits plant growth. In soils cess, the remaining material increasesresidues they can with excess nutrients, immo- in relative nitrogen content. The re-readily decompose. bilization conserves nutrients maining material also increases inSoil-applied pesticides in the bodies of soil organ- chemical complexity and requires in-can kill many beneficial isms, where they are less creasingly specialized species of de-soil organisms, as will likely to be lost through leach- composers. Efficient decompositionsome chemical fertiliz- ing and runoff. of organic matter thus requires a di-ers. Anhydrous ammo- versity of soil organisms. Humus isnia and fertilizers with the final, stable product of decompo-a high chloride content, such as potash, are par- sition, formed when organic matter can be bro-ticularly detrimental to soil organisms. Moder- ken down by soil organisms only slowly or withate organic or synthetic fertilizer additions, how- difficulty. Humus-coated soil particles form ag-ever, enhance populations of soil organisms in gregates that are soft, crumbly, and somewhatsoils with low fertility. greasy-feeling when rubbed together. SOIL HUMUS AND SOIL AGGREGATES PREVENTING ORGANIC MATTER LOSSES Besides decomposing organic materials, bac- Perennial plant cover in pastures not onlyteria and fungi in the soil form gels and threads provides organic matter inputs, it also protectsthat bind soil particles together. These bound against losses of organic matter through erosion.particles are called soil aggregates. Worms, Soil coverage by forages and residues protectsbeetles, ants, and other soil organisms move par- the soil from raindrop impact while dense roottially decomposed organic matter through the soil systems of forages hold the soil against erosionor mix it with soil in their gut, coating soil par- while enhancing water infiltration. Fine root hairsticles with organic gels. As soil particles become also promote soil aggregation. In addition, a Table 3. Typical C:N, C:S, and N:S Ratios. Typical C:N, C:S, and N:S ratios of plant residues, excreta of ruminant animals, and biomass of soil microorganisms decomposing in grassland soils (based on values for % in dry matter) %N C:N %S C:S N:S ______________________________________________________________ Dead grass 1.8 26.6:1 0.15 320:1 12:1 ______________________________________________________________ Dead clover 2.7 17.7:1 0.18 270:1 15:1 ______________________________________________________________ Grass roots 1.4 35:1 0.15 330:1 9:1 ______________________________________________________________ Clover roots 3.8 13.2:1 0.35 140:1 10:1 ______________________________________________________________ Cattle feces 2.4 20:1 0.30 160:1 8:1 ______________________________________________________________ Cattle urine 11.0 3.9:1 0.65 66:1 17:1 ______________________________________________________________ Bacteria 15.0 3.3:1 1.1 45:1 14:1 ______________________________________________________________ Fungi 3.4 12.9:1 0.4 110:1 8.5:1 From Whitehead, 2000 (reference #11)PAGE 12 //NUTRIENT CYCLING IN PASTURES
  13. 13. dense forage cover shades and cools the soil. High temperatures promote mineralization and loss oforganic matter, while cooler temperatures promote the continued storage of this material within the plantresidues and the bodies of soil organisms. Table 4. Pasture Management Practices for Efficient Carbon Cycling. Ensure forage and residue coverage and manure deposition across the entire pasture • Use practices that encourage animal movement throughout the pasture and discourage con- gregation in feeding and lounging areas Promote healthy forage growth and recovery following grazing • Use a variety of forages with a diversity of leaf types and orientations • Use a combination of cool- and warm-season forages with a diversity of shoot and root growth characteristics • Conserve sufficient forage leaf area for efficient plant regrowth by monitoring pastures and moving grazing animals to another pasture in a timely manner • Maintain soil tilth for healthy root growth and nutrient uptake Encourage organic matter decomposition by soil organisms • Use management practices that minimize soil compaction and soil erosion • Minimize tillage and other cultivation practices • Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms • Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or inhibit the growth of soil organisms Encourage soil humus and aggregate formation • Include forages with fine, branching root systems to promote aggregate formation • Maintain organic matter inputs into the soil to encourage the growth of soil organisms • Maintain coverage of forages and plant residues over the entire paddock to provide organic matter and discourage its rapid degradation NITROGEN CYCLE Nitrogen is a primary plant nutrient and a return of manure to the land, and through themajor component of the atmosphere. In a pas- mineralization of organic matter in the soil. Ni-ture ecosystem, almost all nitrogen is organically trogen fixation occurs mainly in the roots of le-bound. Of this, only about 3% exists as part of a gumes that form a symbiotic association with aliving plant, animal, or microbe, while the re- type of bacteria called rhizobia. Some free-livingmainder is a component of decomposed organic bacteria, particularly cyanobacteria (“blue-greenmatter or humus. A very small percentage of algae”), are also able to transform atmosphericthe total nitrogen (less than 0.01%) exists as plant- nitrogen into a form available for plant growth.available nitrogen in the form of ammonium or Fertilizer factories use a combination of highnitrate (12). pressure and high heat to combine atmospheric Nitrogen becomes available for the growth nitrogen and hydrogen into nitrogen fertilizers.of crop plants and soil organisms through nitro- Animals deposit organically-bound nitrogen ingen fixation, nitrogen fertilizer applications, the feces and urine. Well-managed pastures accu- //NUTRIENT CYCLING IN PASTURES PAGE 13
  14. 14. .igure 5. Nitrogen Cycle. Nitrogen gas (N2) (78% of atmosphere) T atmospheric fixation or fertilizer production The Nitrogen Cycle. Nitrogen enters the cycle when T T atmospheric nitrogen is fixed by bacteria. Nitrogen in the T NH4+ nitrogen crop harvest ammonical form is transformed into nitrite and nitrate by fixation T + NO3- T volatilization bacteria. Plants can use either ammonia or nitrate for legumes T growth. Nitrogen in plant cells can be consumed by animals NH4++ OH- T and returned to the soil as feces or urine. When plants die, T NH3 + H2O T T crop and soil organisms decompose nitrogen in plant cells and release T free-living bacteria animal residues T it as ammonia. Nitrate nitrogen can be lost through the T T in soil plant uptake physical process of leaching or through the microbially- T T T nitrogen organic matter erosion mediated process of denitrification. Nitrogen in the T ammonical form can be lost to the atmosphere in the T ammonium T denitrification chemical process of volatilization. TN NH4+ immobilization NO3- + N2O T 2 T nitrate NO3- leaching to groundwater Tmulate stores of organic matter in the soil and in NITROGEN FIXATIONplant residues. Decomposition and mineraliza- Plants in the legume family, including alfalfa,tion of nutrients in these materials can provide clover, lupines, lespedeza, and soybeans, form asignificant amounts of nitrogen to plants and relationship with a specialized group of bacteriaother organisms in the pasture system. called rhizobia. These bacteria have the ability to Plants use nitrogen for the formation of pro- fix or transform atmospheric nitrogen into a formteins and genetic material. Grazing animals thatconsume these plants use some of the nitrogen of nitrogen plants can use for their growth.for their own growth and reproduction; the re- Rhizobia form little balls or nodules on the rootsmainder is returned to the earth as urine or ma- of legumes. If these balls are white or pinkishnure. Soil organisms decompose manure, plant on the inside, they are actively fixing nitrogen.residues, dead animals, and microorganisms, Nodules that are grey or black inside are dead ortransforming nitrogen-containing compounds in no longer active. Legume seeds should be dustedtheir bodies into forms that are available for use with inoculum (a liquid or powder containingby plants. the appropriate type of rhizobia) prior to plant- Nitrogen is often lacking in pasture systems ing to ensure that the plant develops many nod-since forage requirements for this nutrient are ules and has maximal ability to fix nitrogen.high and because it is easily lost to the environ- Other microorganisms that live in the soil are alsoment. Nitrogen is lost from pasture systems able to fix and provide nitrogen to plants.through microbiological, chemical, and physicalprocesses. Dry followed by wet weather pro-vides optimal conditions for bacteria to transform Table 5. Nitrogen .ixation by Legumes.nitrogen from plant-available forms into atmo- #N/acre/ yearspheric nitrogen through denitrification. Chemi- Alfalfa -------------------------------------- 150-350cal processes also transform plant-available ni- White clover ------------------------------ 112-190trogen into atmospheric nitrogen through vola- Hairy vetch ------------------------------- 110-168tilization. In pastures, this often occurs after ma- Red clover --------------------------------- 60-200nure or nitrogen fertilizers are applied to the soilsurface, especially during warm weather. Physi- Soybeans ---------------------------------- 35-150cal processes are involved in the downward Annual lespedeza ----------------------- 50-193movement of nitrogen through the soil profile Birdsfoot trefoil --------------------------- 30-130during leaching. From Joost, 1996 (Reference # 13)PAGE 14 //NUTRIENT CYCLING IN PASTURES
  15. 15. Legumes require higher amounts of phospho- sults in the formation of ammonia. Protozoa,rus, sulfur, boron, and molybdenum than non- amoebas, and nematodes are prolific nitrogenlegumes to form nodules and fix nitrogen. If these mineralizers, cycling 14 times their biomass eachnutrients are not available in sufficient amounts, year. While bacteria only cycle 0.6 times theirnitrogen fixation will be suppressed. When ni- biomass, because of their large numbers in soiltrogen levels in the soil are high due to applica- they produce a greater overall contribution to thetions of manure or nitrogen fertilizers, nitrogen pool of mineralized nitrogen (17). Plants can usefixation by legumes decreases because nitrogen ammonical nitrogen for their growth, but underfixation requires more energy than does root up- aerobic conditions two types of bacteria usuallytake of soluble soil nitrogen. Nitrogen fixed by work together to rapidly transform ammonia firstlegumes and rhizobia is available primarily to the into nitrite and then into nitrate before it is usedlegumes while they are growing. When pasture by plants.legume nodules, root hairs, and aboveground Mineralization is a very important source ofplant material dies and decomposes, nitrogen in nitrogen in most grasslands. As discussed above,this material can become available to pasture for efficient decomposition (and release of nitro-grasses (14). gen), residues must contain a carbon-to-nitrogen However, while legumes are still growing, ratio that is in balance with the nutrient needs ofmycorrhizal fungi can form a bridge between the the decomposer organisms. If the nitrogen con-root hairs of legumes and nearby grasses. This tent of residues is insufficient, soil organisms willbridge facilitates the transport of fixed nitrogen extract nitrogen from the soil solution to satisfyfrom legumes to linked grasses. Depending on their nutrient needs. the nitrogen con- NITROGEN LOSSES TO THE ATMOSPHERE tent of the soil and Under wet or anaerobic conditions, bacteria Legumes can transfer the mix of legumes transform nitrate nitrogen into atmospheric ni- up to 40% of their fixed and grasses in a trogen. This process, called denitrification, re- nitrogen to grasses pasture, legumes duces the availability of nitrogen for plant use. during the growing can transfer be- Denitrification occurs when dry soil containing season. tween 20 and 40% nitrate becomes wet or flooded and at the edges of their fixed nitro- of streams or wetlands where dry soils are adja-gen to grasses during the growing season (15). cent to wet soils.A pasture composed of at least 20 to 45% legumes Volatilization is the transformation of ammo-(dry matter basis) can meet and sustain the ni- nia into atmospheric nitrogen. This chemicaltrogen needs of the other forage plants in the pas- process occurs when temperatures are high andture (16). ammonia is exposed to the air. Incorporation of Grazing management affects nitrogen fixation manure or ammonical fertilizer into the soil de-through the removal of herbage, deposition of creases the potential for volatilization. In gen-urine and manure, and induced changes in mois- eral, 5 to 25% of the nitrogen in urine is volatil-ture and temperature conditions in the soil. Re- ized from pastures (11). A thick forage cover andmoval of legume leaf area decreases nitrogen fixa- rapid manure decomposition can reduce volatil-tion by decreasing photosynthesis and plant com- ization from manure.petitiveness with grasses. Urine deposition de- NITROGEN LEACHINGcreases nitrogen fixation by adjacent plants sinceit creates an area of high soluble-nitrogen avail- Soil particles and humus are unable to holdability. Increased moisture in compacted soils nitrate nitrogen very tightly. Water from rainfallor increased temperature in bare soils will also or snowmelt readily leaches soil nitrate down-decrease nitrogen fixation since rhizobia are sen- ward through the profile, putting it out of reachsitive to wet and hot conditions. of plant roots or moving it into the groundwater. Leaching losses are greatest when the water table NITROGEN MINERALIZATION is high, the soil sandy or porous, or when rainfall Decomposition of manure, plant residues, or or snowmelt is severe. In pastures, probably thesoil organic matter by organisms in the soil re- most important source of nitrate leaching is from //NUTRIENT CYCLING IN PASTURES PAGE 15
  16. 16. urine patches (18). Cattle urine typically leaches to Nitrate levels in excess of 10 ppm in drinkinga depth of 16 inches, while sheep urine leaches only water can cause health problems for human infants,six inches into the ground (19). Leaching may also infant chickens and pigs, and both infant and adultbe associated with the death of legume nodules sheep, cattle, and horses (21). Pasture forages canduring dry conditions (20). also accumulate nitrate levels high enough to cause Methods for reducing nitrate leaching include health problems. Conditions conducive for nitratemaintaining an actively growing plant cover over accumulation by plants include acid soils; low mo-the soil surface, coordinating nitrogen applications lybdenum, sulfur, and phosphorus content; soilwith the period of early plant growth, not applying temperatures lower than 550 F; and good soil aera-excess nitrogen to soils, and encouraging animal tion (22).movement and distribution of manure across pad- Nitrate poisoning is called methemoglobinemia,docks. Actively growing plant roots take up ni- commonly known as “blue baby syndrome” whentrate from the soil and prevent it from leaching. If seen in human infants. In this syndrome, nitratethe amount of nitrogen applied to the soil is in ex- binds to hemoglobin in the blood, reducing thecess of what plants need or is applied when plants blood’s ability to carry oxygen through the body.are not actively growing, nitrate not held by plants Symptoms in human infants and young animalscan leach through the soil. Spring additions of ni- include difficulty breathing. Pregnant animals thattrogen to well-managed pastures can cause exces- recover may abort within a few days. Personnelsive plant growth and increase the potential for from the Department of Health can test wells toleaching. This is because significant amounts of determine whether nitrate levels are dangerouslynitrogen are also being mineralized from soil or- high.ganic matter as warmer temperatures increase theactivity of soil organisms. Table 6. Estimated Nitrogen Balance (pounds/acre) for Two Grassland Management Systems. Moderately managed Extensively grazed grass-clover grass Inputs Nitrogen fixation 134 9 Atmospheric deposition 34 19 Fertilizer 0 0 Supplemental feed 0 0 Recycled nutrients Uptake by herbage 270 67 Herbage consumption by animals 180 34 Dead herbage to soil 90 34 Dead roots to soil 56 34 Manure to soil 134 28 Outputs Animal weight gain 28 4 Leaching/runoff/erosion 56 6 Volatilization 17 3 Denitrification 22 2 Gain to soil 56 13 From Whitehead, 2000 (Reference # 11)PAGE 16 //NUTRIENT CYCLING IN PASTURES
  17. 17. Table 7. Pasture Management Practices for Efficient Nitrogen Cycling.Ensure effective nitrogen fixation by legumes• Ensure that phosphorus, sulfur, boron, and molybdenum in the soil are sufficient for effective nitrogen fixation• Apply inoculum to legume seeds when sowing new pastures to ensure nodulation of legume roots• Ensure that legumes represent at least 30% of the forage cover• Maintain stable or increasing ratios of legumes to grasses and other non-legumes in pastures over time• Establish forages so that legumes and grasses grow close to one another to allow for the transfer of nitrogen from legumes to grassesEncourage nitrogen mineralization by soil organisms• Use management practices that minimize soil compaction and soil erosion• Minimize tillage and other cultivation practices• Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms• Use grazing management practices that encourage productive forage growth and that return and maintain residues within paddocks• Avoid application of sawdust, straw, or other high-carbon materials unless these mate- rials are mixed with manure or composted prior to application• Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or inhibit the growth of soil organismsAvoid nitrogen losses• Minimize nitrogen volatilization by avoiding surface application of manure, especially when the temperature is high or there is minimal forage cover over the soil• Minimize nitrogen leaching by not applying nitrogen fertilizer or manure when soil is wet or just prior to rainstorms and by encouraging animal movement and distribution of urine spots across paddocks• Minimize nitrogen leaching by not applying nitrogen fertilizer or manure to sandy soils except during the growing season• Rely on mineralization of organic residues to supply most or all of your forage nitrogen needs in the spring. Minimize the potential for nitrogen leaching by limiting spring applications of nitrogen• Minimize nitrogen losses caused by erosion by using management practices that main- tain a complete cover of forages and residues over the pasture surfaceEnsure effective use of nitrogen inputs• Use management practices that encourage the even distribution of manure and urine across paddocks• Rely on soil tests and other nutrient management practices when applying fertilizers and manure to pastures //NUTRIENT CYCLING IN PASTURES PAGE 17
  18. 18. NITROGEN LOSS THROUGH of forages and plant residues should be main- tained over the soil surface to minimize raindrop RUNOFF AND EROSION impact on the soil, enhance water infiltration, Runoff and erosion caused by rainwater or help trap sediments and manure particles, andsnowmelt can transport nitrogen on the soil sur- reduce the potential for runoff and erosion. Aface. Erosion removes soil particles and organic healthy and diverse population of soil organisms,matter containing nitrogen; runoff transports dis- including earthworms and dung beetles that rap-solved ammonia and nitrate. Incorporation of idly incorporate manure nitrogen into the soilmanure and fertilizers into the soil reduces the and into their cells, can further reduce the risk ofexposure of these nitrogen sources to rainfall or nitrogen runoff from manure. Since increasedsnowmelt, thus reducing the potential for ero- water infiltration decreases the potential for run-sion. In pasture systems, however, incorpora- off but increases the potential for leaching, riskstion is usually impractical and can increase the of nitrate losses from runoff need to be balancedpotential for erosion. Instead, a complete cover against leaching risks. PHOSPHORUS CYCLE Like nitrogen, phosphorus is a primary plant nutrient. Unlike nitrogen, phosphorus is not part ofthe atmosphere. Instead, it is found in rocks, minerals, and organic matter in the soil. The mineralforms of phosphorus include apatitite (which may be in a carbonate, hydroxide, fluoride, or chlorideform) and iron or aluminum phosphates. These minerals are usually associated with basalt and shalerocks. Chemical reactions and microbial activity affect the availability of phosphorus for plant up-take. Under acid conditions, phosphorus is held tightly by aluminum and iron in soil minerals.Under alkaline conditions, phosphorus is held tightly by soil calcium. Plants use phosphorus for energy transfer and reproduction. Legumes require phosphorus foreffective nitrogen fixation. Animals consume phosphorus when they eat forages. Phosphorus notused for animal growth is returned to the soil in manure. Following decomposition by soil organ-isms, phosphorus again becomes available for plant uptake. .igure 6. Phosphorus Cycle. Fertilizer T Consumption Crop harvest T The Phosphorus Cycle is affected by microbial and chemical transformations. Soil organisms mineralize or T T Loss via T release phosphorus from organic matter. Phosphorus is T T Immobilization Crop and erosion chemically bound to iron and aluminum in acid soils, and T animal T T residues Loss via to calcium in alkaline soils. Soil-bound phosphorus can be Plant runoff uptake lost through erosion, while runoff waters can transport T Phosphorus T T T T T soluble phosphorus found at the soil surface. in microbes Phosphorus in soil humus Mineralization T Mineral T T phosphorus Phosphorus Plant T held by clay available minerals T phosphorus TPAGE 18 //NUTRIENT CYCLING IN PASTURES
  19. 19. MYCORRHIZAE izer or manure may be readily lost from fields and transported to rivers and streams through Mycorrhizal fungi attach to plant roots and runoff and erosion. The potential for phospho-form thin threads that grow through the soil and rus loss through runoff or erosion is greatestwrap around soil particles. These thin threads when rainfall or snowmelt occurs within a fewincrease the ability of plants to obtain phospho- days following surface applications of manurerus and water from soils. Mycorrhizae are espe- or phosphorus fertilizers.cially important in acid and sandy soils where Continual manure additions increase the po-phosphorus is either chemically bound or has tential for phosphorus loss from the soil and thelimited availability. Besides transferring phos- contamination of lakes and streams. This is es-phorus and water from the soil solution to plant pecially true if off-farm manure sources are usedroots, mycorrhizae also facilitate the transfer of to meet crop or forage nutrient needs for nitro-nitrogen from legumes to grasses. Well aerated gen. The ratio of nitrogen to phosphate in swineand porous soils, and soil organic matter, favor or poultry manure is approximately 1 to 1, whilemycorrhizal growth. the ratio of nitrogen to phosphate taken up by SOIL CHEMISTRY AND forage grasses is between 2.5 to 1 and 3.8 to 1. PHOSPHORUS AVAILABILITY Thus, manure applied for nitrogen requirements will provide 2.5 to 3.8 times the amount of phos- Phosphorus is tightly bound chemically in phorus needed by plants (23). While much ofhighly weathered acid soils that contain high con- this phosphorus will be bound by chemicalcentrations of iron and aluminum. Active cal- bonds in the soil and in the microbial biomass,cium in neutral to alkaline soils also forms tight continual additions will exceed the ability of thebonds with phosphorus. Liming acid soils and soil to store excess phosphorus, and the amountapplying organic matter to either acid or alka- of soluble phosphorus (the form available for lossline soils can increase phosphorus availability. by runoff) will increase. To decrease the poten-In most grasslands, the highest concentration of tial for phosphorus runoff from barnyard manurephosphorus is in the surface soils associated with or poultry litter, alum or aluminum oxide can bedecomposing manure and plant residues. added to bind phosphorus in the manure (24). PHOSPHORUS LOSS THROUGH Supplemental feeds are another source of phosphorus inputs to grazing systems, especially RUNOFF AND EROSION for dairy herds. Feeds high in phosphorus in- Unlike nitrogen, phosphorus is held by soil crease the amount of phosphorus deposited onparticles. It is not subject to leaching unless soil pastures as manure. To prevent build up of ex-levels are excessive. However, phosphorus can cess phosphorus in the soil, minimize feeding ofmove through cracks and channels in the soil to unneeded supplements, conduct regular soil testsartificial drainage systems, which can transport on each paddock, and increase nutrient remov-it to outlets near lakes and streams. Depending als from excessively fertile paddocks throughon the soil type and the amount of phosphorus haying.already in the soil, phosphorus added as fertil- Phosphorus runoff from farming operations can promote unwanted growth of algae in lakes and slow-moving streams. Regulations and nutrient-management guidelines are being de- veloped to decrease the potential for phospho- rus movement from farms and thus reduce risks of lake eutrophication. Land and animal man- agement guidelines, called “phosphorus indi- ces,” are being developed across the U.S. to pro- vide farmers with guidelines for reducing “non- point” phosphorus pollution from farms (25). These guidelines identify risk factors for phos- phorus transport from fields to water bodies based on the concentration of phosphorus in the //NUTRIENT CYCLING IN PASTURES PAGE 19
  20. 20. Phosphorus index guidelines consider: streams include location near streams of barn- • the amount of phosphorus in the soil yards or other holding areas without runoff con- • manure and fertilizer application rates, tainment or filtering systems, extensive grazing methods, and timing of animals near streams without riparian buff- • runoff and erosion potential ers, and unlimited animal access to streams. • distance from a water bodysoil, timing and method of fertilizer and manureapplications, potential for runoff and erosion, anddistance of the field from a water body (26). Al-though the total amount of phosphorus lost fromfields is greatest during heavy rainstorms, snow-melts, and other high-runoff events, relativelysmall amounts of phosphorus running off fromfields into streams at low water level in summerpose a higher risk for eutrophication. This is be-cause phosphorus is more concentrated in thesesmaller flows of water (27). Conditions for con-centrated flows of phosphorus into low-flow Table 8. Pasture Management Practices for Efficient Phosphorus Cycling. Encourage phosphorus mineralization by soil organisms • Use management practices that minimize soil compaction and soil erosion • Minimize use of tillage and other cultivation practices • Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms • Avoid application of sawdust, straw, or other high-carbon materials unless these materi- als are mixed with manure or composted prior to application • Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or inhibit the growth of soil organisms Avoid phosphorus losses • Minimize phosphorus losses caused by erosion by using management practices that maintain a complete cover of forages and residues over the pasture surface • Minimize phosphorus losses caused by runoff by not surface-applying fertilizer or ma- nure to soil that is saturated, snow-covered, or frozen • Avoid extensive grazing of animals in or near streams especially when land is wet or saturated or when streams are at low flow Ensure effective use of phosphorus inputs • Use management practices that encourage the even distribution of manure and urine across paddocks • Rely on soil tests, phosphorus index guidelines, and other nutrient management prac- tices when applying fertilizers and manure to pasturesPAGE 20 //NUTRIENT CYCLING IN PASTURES