Understanding Soil Organic Matter


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I shared this presentation with my Intro to Soil Science class at Western Illinois University in mid April 2013.

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Understanding Soil Organic Matter

  1. 1. Understanding soil organic matter
  2. 2. BiologicallyactiveSOMSOM is a complex mixtureLiving organismsRecent residuesStabilizedSOMAdapted from Magdoff and Weil (2003)HUMUS??
  3. 3. http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.htmlThis pie chart representsorganic matter in soil beforeagriculture. After land hasbeen farmed for severaldecades, much of the activefraction is lost and stabilizedorganic matter makes up morethan half of the soil organicmatter.From the U of MN bulletin on SOM
  4. 4. http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.htmlFrom the U of MN bulletin on SOMAfterlong-termagricultureChange in both the size of thepie and the slices of the pie
  5. 5. The current OM level in a soil is aresult of the long-term balancebetween organic inputs and outputsSo… shouldn’t yield enhancingpractices build SOM?
  6. 6. General rule of thumb for corngrain, stover and roots each comprise~1/3rd of the OM produced by cornHow much OM is returned to the soilby a 200 bushel corn crop?200 bushels*56 lbs/bu * 2 = 22,400 lbs/a/yr!2/3rds of corn OMVery little of the OMsupplied by corn residueslasts more than a few yearson most farms
  7. 7. “The microherd”Phil BrookesYield enhancing practices alsoimpact the soil stomach!!When there is moregrass, I eat more!!
  8. 8. ”But with the removal of water through furrows, ditches,and tiles, and the aeration of the soil by cultivation, whatthe pioneers did in effect was to fan the former simmeringfires… into a blaze of bacterial oxidation and morecomplete combustion. The combustion of the accumulatedorganic matter began to take place at a rate far greaterthan its annual accumulation. Along with the increased rateof destruction of the supply accumulated from the past, theremoval of crops lessened the chance for annual additions.The age-old process was reversed and the supply oforganic matter in the soil began to decrease instead ofaccumulating.”William Albrecht – 1938 Yearbook of AgricultureTillage + Lime + Drainage + N fertilizer =>higher yields & higher decomposition rates
  9. 9. < 1 yeardecadescenturiesWhat istheaverageage ofSOM ??Janzen (2006)Ancient SOMis mostabundant!depleted inmost agsoils
  10. 10. http://www.grida.no/climate/vital/graphics/large/12.jpgGlobalC cycleAll# = GTGt = 109 t = 1 billion metric tonsSoil > Atmosphere + Vegetation
  11. 11. There is very clear evidence that atmospheric levels of CO2are increasing and that the majority of the CO2 added to theatmosphere in the last 3 decades has come from fossil fuelsWhy do CO2levels go upand downannually?Prior to ~1980,majority of CO2↑came fromlosses in SOM
  12. 12. 2400
  13. 13. Why is SOMimportant ??
  14. 14. What Does Organic Matter Do (for you)?Nutrient cyclingIncreases 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 nutrientsWater dynamicsImproves water infiltration.Decreases evaporation.Increases water holding capacity, especially in sandy soils.StructureReduces crusting, especially in fine-textured soils.Encourages root development.Improves aggregation, preventing erosion and reducing compaction.From the U of MN bulletin:
  15. 15. Most(but not all)soil organismseat SOM
  16. 16. Some bacteria are CHEMOAUTOTROPHSChemoautotrophic bacteria obtain energythrough the oxidation of electron donorsother than C.For example, the bacteria that oxidize ammoniainto nitrate, a important process called nitrification,do NOT eat SOMMany bacteria and all fungi(as well as all other soil organisms)are HETEROTROPHS(which means that they eat organic matter).
  17. 17. SOM is the fuelthat energizesmost biologicalprocesses in soil
  18. 18. SOM reduces bulk densityMagdoff and Weil (2004)
  19. 19. (Watts and Dexter, 1997)Structuraldamage Soils with high OMare more resistant tostructural damage !Soils with more OM have less strength when dryand more strength when moist!
  20. 20. SOM increases plant available H20Adapted from Brady and Weil (2002)
  21. 21. Do you remember this photo??Bettersponge
  22. 22. SOM is a very important adsorbent in soilAdapted from Brady and Weil (2002)
  23. 23. What is humus ???Humus is organic matter that has beentransformed such that its original sourceis no longer apparent… The diverseproducts of “humification” have manycommon characteristics: Extreme chemical complexity Resistance to further decomposition High specific surface and negative charge Dark color
  24. 24. Humus gives soil a darker colorIs this beneficial?
  25. 25. Have youever heard ofany humateproducts?Hydra-hume
  26. 26. There is growing evidence that mined humate products canpromote plant growth **BUT** arenot the same as SOM
  27. 27. Recent research has demonstrated that molecularstructure alone does not control SOM stability: infact, environmental and biological controlspredominate…Nature, October 2011
  28. 28. The traditional concept ofgiant stable humusmolecules in soil has beenrejected by most scientists
  29. 29. accumulate insoil?why doesmatorganic terSo…
  30. 30. Understanding biochemical recalcitrance(Giller, 2000)aka digestibility
  31. 31. Understanding mineral protectionMagdoff and Weil (2004)
  32. 32. Weak relationship between clay content and SOCfor 1261 agricultural soils in England and WalesWebb et al.(2003)Clay is clearly notthe only factorcontrolling carboncontent
  33. 33. Understanding physical protectionAdapted from Carter (2002)Mineral associated OMIntra-aggregatePOMFree POMSensitivitytomanagement↑↑
  34. 34. Soil microaggregatesSoil macroaggregateOMOMIs there organic matter inside macroaggregates?
  35. 35. Soil macro-aggregatesform aroundfresh organicresiduesTillagedisruptsaggregatesandacceleratesdecompositionTillageOM inputs
  36. 36. What is POM??Mineral protectedSand sized Silt and clay sizedhttp://www.grdc.com.au/growers/res_summ/pdfs/cso00029.pdfParticulate OM = POM
  37. 37. Geographic distribution of SOM
  38. 38. What Determines Soil Organic Matter Levels?The amount of organic matter in soil is the result of two processes: the addition oforganic matter (roots, surface residue, manure, etc.), and the loss of organicmatter through decomposition. Five factors affect both additions and losses.Soil texture - Fine-textured soils can hold much more organic matter than sandysoils for two reasons. First, clay particles form electrochemical bonds that holdorganic compounds. Second, decomposition occurs faster in well-aerated sandysoils. A sandy loam rarely holds more than 2% organic matter.Historical vegetation - In prairies, much of the organic matter that dies and isadded 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 surfaceof the soil. Thus, farmland that was once prairie will have higher amounts oforganic matter deep in the soil than lands that were previously forest.Climate - High temperatures speed up the degradation of organic matter. Inareas of high precipitation (or irrigation) there is more plant growth and thereforemore roots and residues entering the soil.Landscape position - Low, poorly-drained areas have higher organic matterlevels, because less oxygen is available in the soil for decomposition. Low spotsalso accumulate organic matter that erodes off hill tops and steep slopes.So what is the 5th factor?MANAGEMENT
  39. 39. Temperature affects SOM production and destructionBrady and Weil (2002)70 FOrganic matter destructionby aerobic organismsOrganic mattersynthesis by plantsOrganicmatterproductionOrganicmatterconsumptionOMproductionandconsumptionIllinois
  40. 40. InterstreamdivideSOILDRAINAGECLASSESPoorlydrainedSomewhatpoorlydrainedModeratelywell drainedPoorlydrainedWelldrainedInterfluveValley floorBackslopeShoulderLANDSCAPEPOSITIONSLandscape position affects SOM dynamicsWhere does the most OM accumulate?
  41. 41. Blackland soils of North CarolinaLily (1981)> 1 million acres of Histosols
  42. 42. Howmuchisenough??
  43. 43. There are many ways to “measure” SOMAdapted from Strek and Weber (1985)Total organic matterby “loss on ignition”Total Cby several wet anddry oxidationmethodsHumic matterby alkali extractionC ~ 0.6*OM% OM
  44. 44. Many biological products claim to containhumic and fulvic acidsFulvicHumicFulvic acid = soluble in strong base and still soluble when pH => 7Humic acid = soluble in strong base but precipitates when pH => 7
  45. 45. TIDIC acid production system ☺
  46. 46. Humic and fulvic acids aresolubility fractions ratherthan specific compoundswith specific biologicaleffects
  47. 47. Permanganate oxidizable Ca routine test for “active” soil C ??
  48. 48. Our analysis demonstrates theusefulness of POXC in quickly andinexpensively assessingchanges in the labile soil C pool.
  49. 49. Soil from along termexperiment inBeltsville, MD
  50. 50. Afteraddingwater
  51. 51. 1.4 % C1.0% CRelatively small differences in SOC
  52. 52. 48 bu/a 140 bu/aLarge differences in soil function
  53. 53. 16 % clay 39 % 49%More OM is needed to stabilize fine textured soilsAdapted from Russell (1973)16 % clay39 % 49%
  54. 54. Aggregation changes more rapidly than total CJastrow (1996)Years since prairie restoration
  55. 55. Janzen (2006)Hydroelectric dam metaphorSufficient OMoptimizes thebeneficial effects ofOMOM forms anddynamics are moreimportant than thetotal quantity
  56. 56. Managing SOM
  57. 57. well mixed vs.stratifiedConventional tillage Conservation tillageAdapted from House and Parmelee (1985)
  58. 58. Effect of tillage on microbial activityHavlin et al. (1999)TillageWhich tillage system hasmore total microbialactivity ?Conventional tillageWhich system releasesmore CO2 when crops needCO2 ?
  59. 59. It is widely believed that tillage was main cause of soil C loss when naturalecosystems were converted to agriculture, and that substantial Csequestration can be accomplished by changing from conventional tillage tono-till. This is based on lots of experiments (and on farm observations) wheresoil C increased under no-till. However, sampling methods may have biasedthe 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…
  60. 60. Very few tillage studies have been sampled deeper than 1’Many studies were only sampled 6” deep!
  61. 61. Elevated OM levels at the soil surface are beneficialeven if no greater OM accumulates at depth
  62. 62. Artificial drainage has greatly increased the number ofdays when soils in the Upper Midwest are suitable forfield operationsbut has alsocontributedto someenvironmentalproblemsPollution ofwater resources Loss of SOM
  63. 63. Original soil surface of a Histosol (muck soil) in FL
  64. 64. Soil Changes After Sixty Years of Land Use in IowaJessica Veenstra, Iowa State University, 1126 Agronomy Hall, Iowa StateUniversity, Ames, IA 50010Soils form slowly, thus on human time scales, soil is essentially a non-renewable resource. Therefore in order to maintain and manage our limitedsoil resources sustainably, we must try to document, monitor and understandhuman induced changes in soil properties. By comparing current soilproperties to an archived database of soil properties, this study assessessome of the changes that have occurred over the last 60 years, and attemptsto link those changes to natural and human induced processes. This studywas conducted across Iowa where the primary land use has been row cropagriculture and pasture. We looked at changes in A horizon depth, color,texture, structure, organic carbon content and pH.Hill top and backslope landscape positionshave been significantly degradedbutcatchment areas have deeper topsoil w/ more C.
  65. 65. Adapted from Bailey and Lazarovits (2003)A systems approachto SOM managementWell adapted cropNutrientManagementWaterManagementSOM
  66. 66. Cropresiduemanagement
  67. 67. Erosion Control Practices
  68. 68. On-farm recycling of OM
  69. 69. Off-farm sources of OM
  70. 70. Crop RotationHigh residue cropsCover cropsForages
  71. 71. Innovative cover cropping
  72. 72. Actual CPracticallyattainable CPotential C(Dick and Gregorich, 2004)Input factorsManyfactorscontrol soilC contentResidue yield
  73. 73. Saturation deficitSaturation of capacityActual CPracticallyattainable CPotential C(Dick and Gregorich, 2004)Disturbance factorsInput factorscapacity factorsmanagement= opportunityResidue yield
  74. 74. Comparison of soil from fields and hedgerows with thesame soil type can help identify sites with the mostpotential for building SOM