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This is an updated version of a presentation for my Intro to Soil Science class at Western Illinois University

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  1. 1. Understanding soil organic matter So when does crop residue become SOM?
  2. 2. Grain yields have increased dramatically since WWII Crop residue production has also increased but not as dramatically as grain yields
  3. 3. * 200 bu/a in 2014 IL corn yields have doubled during my lifetime!
  4. 4. How much residue does a 200 bushel corn crop return to the soil?
  5. 5. General rule of thumb for corn grain, stover and roots each comprise ~1/3rd of the total biomass 33%
  6. 6. 200 bushels*~50 lbs/bu * 2 = 20,000 lbs residue/a/yr! So how much of the ~10 tons/acre of dry residue left by a 200 bu/a corn crop turns into soil organic matter? Why not 56? Why multiply by 2?
  7. 7. Estimating C inputs retained as soil organic matter from corn Plant and Soil Volume 215, Issue 1, pp 85-91 M.A. Bolinder, D.A. Angers, M. Giroux, M.R. Laverdière Abstract In agroecosystems, the annual C inputs to soil are a major factor controlling soil organic matter (SOM) dynamics. However, the ability to predict soil C balance for agroecosystems is limited because of difficulties in estimating C inputs and in particular from the below-ground part. The objective of this paper was to estimate the proportion of corn residue retained as SOM. For that purpose, the results of a 13C long-term (15 yr) field study conducted on continuous silage corn and two silage corn rotations along with data from the existing literature were analyzed. The total amount of corn-derived C (0–30 cm) was about 2.5 to 3.0 times higher for the continuous corn treatment (445 g m-2), compared to the two rotational treatments (175 and 133 g m-2 for the corn-barley-barley-wheat and corn-underseeded barley hay-hay rotations, respectively). Assuming that the C inputs to the soil from silage-corn was mainly roots and would have been similar across treatments on an annual basis, the total amount of corn-derived C for the two rotational treatments was The results from this study indicate that ~ 17% of corn root-derived C was retained long-term as SOM. This is almost twice the ~ 10 % retention reported in the literature for shoot-derived C and is in agreement with many studies showing that more root C is retained than shoot C.
  8. 8. The standard conversion from SOM to C is %SOM / 1.72 = %C. For soils or soil horizons containing large amounts of relatively undecomposed plant materials, a factor of 2 may be more accurate (i.e., %SOM / 2 =%C).
  9. 9. crop residue carbon CO2 Living organisms Microbial compounds often > 75% After 1 year Most of the C in crop residues quickly returns to the atmosphere Plant compounds SOM
  10. 10. The current OM level in a soil is the result of the long-term balance between organic inputs and outputs So… shouldn’t yield enhancing practices build SOM?
  11. 11. “The microherd” Phil Brookes Practices that enhance crop yield also impact the soil stomach!! When there is more grass, I eat more!!
  12. 12. ”But with the removal of water through furrows, ditches, and tiles, and the aeration of the soil by cultivation, what the pioneers did in effect was to fan the former simmering fires… into a blaze of bacterial oxidation and more complete combustion. The combustion of the accumulated organic matter began to take place at a rate far greater than its annual accumulation. Along with the increased rate of destruction of the supply accumulated from the past, the removal of crops lessened the chance for annual additions. The age-old process was reversed and the supply of organic matter in the soil began to decrease instead of accumulating.” William Albrecht – 1938 Yearbook of Agriculture Tillage + Lime + Drainage + N fertilizer => higher crop yields & higher decomposition rates
  13. 13. Mollisols (prairie soils) under long-term agricultural management have ~ 50% less organic C in their top 8” than native prairie soils. Most of the loss of organic C is likely to have occurred within 25 years of the original plowing (as opposed to the last 50-100 years). Organic C levels in IL soils have been relatively stable over the past 50 years.
  14. 14. Soil Changes After Sixty Years of Land Use in Iowa Jessica Veenstra, Iowa State University, 1126 Agronomy Hall, Iowa State University, Ames, IA 50010 Soils form slowly, thus on human time scales, soil is essentially a non-renewable resource. Therefore in order to maintain and manage our limited soil resources sustainably, we must try to document, monitor and understand human induced changes in soil properties. By comparing current soil properties to an archived database of soil properties, this study assesses some of the changes that have occurred over the last 60 years, and attempts to link those changes to natural and human induced processes. This study was conducted across Iowa where the primary land use has been row crop agriculture and pasture. We looked at changes in A horizon depth, color, texture, structure, organic C content and pH. Hill top and backslope landscape positions have been significantly degraded w/ less organic C but catchment areas have deeper topsoil w/ more organic C.
  15. 15. Are crop residues different than the residues from native vegetation ? perennial roots > annual roots!
  16. 16. This pie chart represents organic matter in soil with native vegetation. Notice the large active fraction. When native vegetation is converted to agriculture, the active OM fraction normally quickly declines. The stable OM fraction changes much less. From the U of MN bulletin on SOM
  17. 17. From the U of MN bulletin on SOM After long-term agriculture Why does old OM become dominant? Smaller pie and different slices
  18. 18. < 1 year decades centuries What is the average age of organic C in soil ?? Janzen (2006) Old C is most abundant! depleted in most ag soils C dynamics
  19. 19. There is very clear evidence that atmospheric levels of CO2 are increasing and that the majority of the CO2 added to the atmosphere in the last 3 decades has come from fossil fuels Why do CO2 levels go up and down annually? Prior to ~1980, majority of CO2↑ came from loss of SOM
  20. 20. Agricultural Production Affects Annual CO2 Cycle Each year in the Northern Hemisphere, levels of atmospheric carbon dioxide drop in the summer as plants grow, and then climb again as they decompose. Over the past five decades, the size of this seasonal swing has increased by ~50%, for reasons that aren’t fully understood. Scientists recently evaluated global production statistics for 4 leading crops – corn, wheat, rice and soybeans and found that production of these crops in the Northern Hemisphere has more than doubled since 1960. This translates to ~ 1 billion metric tons of C captured and released each year! According to Dr. Josh Gray of Boston U: “Croplands are ecosystems on steroids. They occupy about 6 percent of the vegetated land area in the Northern Hemisphere but are responsible for up to a quarter of the total increase in seasonal exchange of atmospheric carbon dioxide, and possibly more…that’s a very large, significant contribution, and 2/3 of that contribution is attributed to corn.“
  21. 21. Global C cycle All# = GT Gt = 109 t = 1 billion metric tons Soil C > Atmosphere C + Vegetation C
  22. 22. 2400
  23. 23. Why is SOM important ??
  24. 24. What Does Soil Organic Matter Do (for you)? Nutrient cycling Increases the nutrient holding capacity of soil (CEC). Serves as a slow release form of nutrients for plants. Chelates nutrients increasing their availability to plants. Feeds soil organisms from bacteria to worms that excrete available nutrients Water dynamics Improves water infiltration. Decreases evaporation. Increases water holding capacity, especially in sandy soils. Structure Reduces crusting, especially in fine-textured soils. Encourages root development. Improves aggregation, preventing erosion and reducing compaction. From the U of MN bulletin: Fertilizer is not a substitute for SOM
  25. 25. SOM
  26. 26. Most (but not all) soil organisms eat SOM
  27. 27. Some bacteria are CHEMOAUTOTROPHS Chemoautotrophic bacteria obtain energy through the oxidation of electron donors other than C. For example, the bacteria that oxidize ammonium into nitrate, a important process called nitrification, do NOT eat SOM Many bacteria and all fungi (as well as all other soil organisms) are HETEROTROPHS (which means that they eat organic matter).
  28. 28. SOM is the fuel that energizes most biological processes in soil
  29. 29. (Watts and Dexter, 1997) Structural damage Soils with high OM are more resistant to structural damage ! Soils with more OM have less strength when dry and more strength when moist!
  30. 30. SOM increases plant available H20 Adapted from Brady and Weil (2002)
  31. 31. SOM is a very important adsorbent in soil Adapted from Brady and Weil (2002)
  32. 32. Humus gives soil a darker color Is this beneficial?
  33. 33. Biologically active SOM SOM is a complex mixture of living, dead and very dead OM Living organisms Recent residues Stabilized SOM Adapted from Magdoff and Weil (2003) Historically often called HUMUS
  34. 34. What is humus ??? Humus is organic matter that has been transformed such that its original source is no longer apparent… The diverse products of “humification” have many common characteristics:  Resistance to further decomposition  Complexation with fine mineral materials  High specific surface and negative charge  Dark color
  35. 35. HUMMUS HUMUS is NOT
  36. 36. The traditional concept of highly complex humus macro-molecules distinctly different from bio-molecules has been rejected by most scientists
  37. 37. Recent research has demonstrated that chemical structure alone does not control SOM stability: in fact, environmental and biological controls are more important… Nature, October 2011
  38. 38. Have you ever heard of any humate products? Hydra-hume
  39. 39. Descriptions of humate products often refer to humic and fulvic acid content Fulvic acid Humic acid Fulvic acid = soluble in strong base and still soluble when pH => 7 Humic acid = soluble in strong base but precipitates when pH => 7 HA & FA are solubility fractions NOT specific compounds
  40. 40. TIDIC acid production system ☺
  41. 41. Leonardite source material for commercial humate products Products differ with respect to the amount and type of processing
  42. 42. There is growing evidence that humate products can enhance crop growth **BUT** are not the same as natural SOM
  43. 43. There are lots of humate products on the market. Ask for research results and clear explanations.
  44. 44. Reputable companies should be able to provide analytical results obtained using this new standardized method
  45. 45. Humate products are not a substitute for good soil organic matter management
  46. 46. accumulate in soil? why does matorganic ter So…
  47. 47. Understanding biochemical recalcitrance (Giller, 2000) aka digestibility
  48. 48. C:N ~ 25 Faster Slower Decomposition
  49. 49. LIGNIN is the main molecule that makes plants stiff – it decomposes much more slowly than cellulose but research shows that it eventually decomposes and does not accumulate significantly in soil
  50. 50. Field-Grown Bt and non-Bt Corn: Yield, Chemical Composition, and Decomposability Sandra F. Yanni, Joann K. Whalen and Bao-Luo Ma Abstract Bt (Bacillus thuringiensis) corn (Zea mays L.) accounted for 74.5% of the corn acreage in eastern Canada in 2009. Reports that Bt corn has greater yield and lignin concentrations than unmodified corn have raised questions about its effect on the soil ecosystem. Our objectives were to evaluate the biomass of field-grown Bt and non-Bt corn, the chemical composition of different corn components that remain as residues in the field after harvest, and the effect of the Bt modification on residue decomposition. Nine Bt corn hybrids and their near isolines were field- grown in 2008 and 2009. Grain and stover yields were measured and leaves, stems, and roots were collected and analyzed for lignin, C, and N concentrations. Stem sections from a Bt/non-Bt corn pair were buried in the field and sampled periodically during 1 yr. No difference in yield or lignin concentrations due to the Bt gene was noted; however, N concentration in Bt stems was significantly greater than in non-Bt stems in 1 yr of the 2-yr study. Leaves had less lignin and a lower C/N ratio than stems and roots in both years. In buried field litterbags, the decline We conclude that the Bt gene does NOT affect the chemical composition of corn residues in fields without herbivory, and that Bt corn residue may actually be MORE susceptible to decomposition than non-Bt corn residue. What about with herbivory??? SOME SURPRISING RESULTS
  51. 51. Charcoal is highly resistant to decomposition and full of porosity
  52. 52. Terra Preta soils contain lots of ancient charcoal Typical acid infertile rain forest soil
  53. 53. Can we make new Terra Preta soils by amending infertile soils with BIOCHAR? Scientists are trying to figure out…
  54. 54. Understanding Mineral Protection Magdoff and Weil (2004) Soil C content is often closely related to fine mineral content
  55. 55. Weak relationship between clay content and SOC for 1261 agricultural soils in England and Wales Webb et al.(2003) Clay is clearly not the only factor controlling C content in these soils
  56. 56. Understanding physical protection Adapted from Carter (2002) Mineral protected OM Intra- aggregate OM Free OM Over time SOM becomes more and more intimately connected to soil mineral particles
  57. 57. Soil microaggregates Soil macroaggregate OM OM Is there organic matter inside aggregates?
  58. 58. Soil macro- aggregates form around fresh organic residues Tillage disrupts aggregates and accelerates decomposition TillageOM inputs
  59. 59. What is POM?? Mineral protected Sand sized Silt and clay sized Particulate OM = POM
  60. 60. Geographic distribution of SOM
  61. 61. What Determines Soil Organic Matter Levels? The amount of organic matter in soil is the result of two processes: the addition of organic matter (roots, surface residue, manure, etc.), and the loss of organic matter through decomposition. 5 main factors affect both additions and losses. Soil texture - Fine-textured soils can hold much more organic matter than sandy soils for two reasons. First, clay particles form electrochemical bonds that hold organic compounds. Second, decomposition occurs faster in well-aerated sandy soils. Sandy loams rarely have more than 2% organic matter. Historical vegetation - In prairies, much of the organic matter that dies and is added to the soil each year comes from grass roots that extend deep into the soil. In forests, the organic matter comes from leaves that are dropped on the surface of the soil. Thus, farmland that was once prairie will have higher amounts of organic matter deep in the soil than land that was previously forest. Climate - High temperatures speed up the degradation of organic matter. In areas of high precipitation (or irrigation) there is more plant growth and therefore more roots and residues entering the soil. Landscape position - Low, poorly-drained areas have higher organic matter levels, because less oxygen is available in the soil for decomposition. Low spots also accumulate organic matter that erodes off hill tops and steep slopes. So what is the 5th factor? MANAGEMENT
  62. 62. Interstream divide SOIL DRAINAGE CLASSES Poorly drained Somewhat poorly drained Moderately well drained Poorly drained Well drained Interfluve Valley floor Backslope Shoulder LANDSCAPE POSITIONS Landscape position affects SOM dynamics Where does the most OM accumulate? “flat black” soils
  63. 63. Temperature affects OM production and decomposition Brady and Weil (2002) 70 F Organic matter synthesis by plants Illinois in 50 yrs?
  64. 64. How much is enough ??
  65. 65. Have we learned anything in the last 77 years ?
  66. 66. Janzen (2006) Hydroelectric dam metaphor OM forms and dynamics are more important than total quantity but many soils currently have OM levels that limit soil function SOM is a very important source of nutrients in modern production systems but we are less dependent on SOM to supply nutrients than we were in 1938.
  67. 67. There are many ways to “measure” SOM Adapted from Strek and Weber (1985) Total organic matter by “loss on ignition” Total C by several wet and dry oxidation methods Humic matter by alkali extraction OM/1.72 = C % OM
  68. 68. Permanganate oxidizable C a routine test for “active” soil C ??
  69. 69. “Our analysis demonstrates the usefulness of POXC in quickly and inexpensively assessing changes in the labile soil C pool.”
  70. 70. Soil from a long term experiment in Beltsville, MD
  71. 71. After adding water
  72. 72. 1.4 % C1.0% C Relatively small differences in C
  73. 73. 48 bu/a 140 bu/a Large differences in soil function
  74. 74. Aggregation changes much more rapidly than total C Jastrow (1996) Years since PRAIRIE RESTORATION Aggregation Total C
  75. 75. 16 % clay 39 % 49% More OM is needed to stabilize fine textured soils Adapted from Russell (1973) 16 % clay 39 % 49%
  76. 76. Comparison of soil from fields with the same soil type but different OM levels can help identify sites with the most potential for building SOM and improving soil function
  77. 77. Managing SOM
  78. 78. well mixed vs. stratified Conventional tillage Conservation tillage Adapted from House and Parmelee (1985)
  79. 79. It is widely believed that tillage was the main cause of soil C loss when natural ecosystems were converted to agriculture, and that substantial C sequestration can be accomplished by changing from conventional tillage to no-till. This is based on lots of experiments (and on-farm observations) where soil C increased under no-till. However, sampling methods may have biased the results. In essentially all cases where no-till was found to sequester C, soils were only sampled to a depth of 1 foot or less… What is meant by the term CARBON SEQUESTRATION? CARBON SEQUESTRATION in soil CO2 -> SOM
  80. 80. Very few tillage studies have been sampled deeper than 1’ Many studies were only sampled 6” deep! In the Upper Midwest, tillage reduces OM levels near the soil surface but has much less impact on SOM in the whole profile. In warmer climates, tillage more negatively effects OM levels in the whole profile.
  81. 81. Effect of tillage on microbial activity Havlin et al. (1999) Tillage Which tillage system has more total microbial activity ? Conventional tillage Which system releases more CO2 when crops need CO2 ?
  82. 82. Elevated OM levels at the soil surface are beneficial even if no greater OM accumulates at depth
  83. 83. Artificial drainage has greatly increased the number of days when soils in the Upper Midwest are suitable for field operations but has also contributed to some environmental problems Pollution of water resources? Loss of SOM??
  84. 84. Original soil surface of a Histosol (muck soil) in FL
  85. 85. Adapted from Bailey and Lazarovits (2003) A systems approach to SOM management Well adapted crop Nutrient Management Water Management SOM
  86. 86. Crop residue management Increase residue production (especially roots) and minimize soil disturbance
  87. 87. Crop Rotation High residue crops Cover crops Forages
  88. 88. Erosion Control Practices
  89. 89. Erosion is a major cause of reduced SOM levels at the soil surface Erosion status
  90. 90. On-farm recycling of OM Most IL fields never receive manure
  91. 91. Off-farm sources of OM
  92. 92. ~ 60% of the biosolids generated in the US are land applied. Farmers get nutrients and OM for free!
  93. 93. Innovative cover cropping Its possible to ~double the months of active plant growth in IL 4 8
  94. 94. A good way to grow more roots!
  95. 95. Actual C Practically attainable C Potential C (Dick and Gregorich, 2004) Input factors Many factors control SOM content Residue yield
  96. 96. Saturation deficit Saturation of capacity Actual C Practically attainable C Potential C (Dick and Gregorich, 2004) Disturbance factors Input factors capacity factors management = opportunity Residue yield
  97. 97. Fencerows are often a good place to check a soil’s capacity for C accumulation
  98. 98. Some effects of higher OM quantity and/or quality occur relatively quickly Other effects take longer
  99. 99. Fields or parts of fields with the lowest OM content (relative to their potential) will benefit the most from practices that build SOM.