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3. Soil Organic Matter


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Synthesis of recent research on the maintenance, formation and functions of organic matter. Joel Gruver, Western Illinois U Ext

Published in: Food
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3. Soil Organic Matter

  1. 1. SOM Joel Gruver WIU Agriculture a fulcrum with many forms and functions
  2. 2. What is a fulcrum?
  3. 3. Technology/management SOM SOM is helping LEVERAGE input & technology investments into yield & profit
  4. 4. “Soil organic matter is an indicator of past productivity… ERIC LUND President of Veris Technologies
  5. 5. …and a driver of future yields”
  6. 6. Jimmy, this farm’s yields have tripled!
  7. 7. Grandpa, has your SOM tripled?
  8. 8. Historical US Corn Grain Yields Do you think historical SOM levels look anything like this?
  9. 9. Bouma et al. (2016) documented large differences in soil C and function for Dutch fields mapped as the same soil type but w/ long-term differences in management. Soil genotype vs. phenotype??? crops crops grassland 1.7% OM 3.3% OM 5.0% OM
  10. 10. 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 eroded & now have thinner topsoil w/ less SOM Catchment areas now have deeper topsoil w/ more SOM
  11. 11. Veenstra and Burras resampled 82 soil profiles on IA farms with historical descriptions and classified the soils according to the US, Canadian and FAO-WRB taxonomic systems. 11 to 33% of the pedons originally classified as Black soils (e.g., Prairie soils) no longer classified as Black soils.
  12. 12. The current amount of OM in a soil = the long-term balance between organic matter inputs and outputs grown in place, redistributed on-farm, imported from off-farm harvested, decomposed, lost to erosion So why hasn’t modern agriculture had a more favorable impact on SOM?
  13. 13. grain, stover and roots each comprise ~1/3rd of the total biomass A 200 bu/a corn crop adds ~10 t/a of roots and stover, equivalent to ~ 1 % of the weight of an acre-plow layer So what happens to this biomass? High yielding grain crops return lots of residues
  14. 14. crop residues CO2 Living organisms Microbial compounds often > 75% within 1 year Most of the C in crop residues quickly returns to the atmosphere Plant compounds SOM This transformation is a major focus of current SOM research
  15. 15. Phil Brookes Practices that enhance crop yield also impact a soil’s metabolism When there is more grass, I eat more!!
  16. 16. Have you read this classic commentary? Journal of Soil and Water Conservation - 1995 What are the symptoms of the HIGH CARB DIET that most US soils receive?
  17. 17. There has been a surge in SOM research since DeLuca’s commentary – what have we learned??? The next slides review some of the most interesting recent developments in SOM research
  18. 18. Biologically active SOM We have known that SOM is a complex mixture of living, dead and very dead OM for a LONG TIME Living organisms Recent residues Stabilized SOM Adapted from Magdoff and Weil (2003) Historically often called HUMUS Does it matter if these residues are of plant vs. microbial origin? How does management impact the synthesis & stabilization of microbial residues?
  19. 19. The authors present a conceptual framework - the microbial carbon pump (MCP) to demonstrate how microorganisms are an active player in soil C storage. The MCP couples microbial production of organic compounds to their further stabilization, which the authors define as the entombing effect (EE). Nature Microbiology - 2017 Microbes take apart organic matter but they also synthesize it
  20. 20. ex vivo modification = extracellular enzymes attack and transform plant residues, resulting in accumulation of plant-derived compounds that are not readily assimilated by microorganisms in vivo turnover = cell uptake of organic substrates –> biosynthesis resulting in deposition of compounds produced by microbes Through these 2 pathways, compounds are produced that are more resistant to further degradation and/or more readily stabilized by interactions with the soil mineral matrix. A growing body of evidence indicates that the in vivo pathway is MORE important for C stabilization than ex vivo modification 2 major pathways by which microorganisms influence C stabilization
  21. 21. Net stabilization of C when EE > PE Priming effect = stimulation of decomposition by fresh additions
  22. 22. The long-term storage of organic forms of N in soils was classically attributed to chemical complexity of plant residues that retarded microbial degradation. Recent advances have revised this framework, with the understanding that persistent soil organic N consists largely of chemically labile, microbially processed organic compounds. Chemical bonding to minerals and physical protection in aggregates are more important to long- term preservation of these organic compounds than molecular complexity…
  23. 23. Soil fungi can form mineral-stabilized SOM not only by modifying SOM but also by synthesizing mineral surface reactive metabolites
  24. 24. Adapted from Carter (2002) mineral protected OM Intra- aggregate OM Free OM What regulates the abundance of these SOM fractions? Can the protective mechanisms be saturated? Compartments of varying connectivity between SOM and the mineral matrix physically protected OM
  25. 25. Geoderma - 2016 The proportion of residues physically protected within aggregates decreases and priming effects increase when aboveground C inputs increase - leading to lower rates of long-term C stabilization when aboveground residue additions are high. Potential explanation of why high crop yields often do NOT result in much change in SOM???
  26. 26. High above ground residue inputs = low efficiency of SOM stabilization (Shahbaz et al., 2016)
  27. 27. Functionally diverse perennial and cover cropped rotations increased both C input and SOC concentrations, potentially by exploiting niches in time that would otherwise be unproductive, that is, increasing the “perenniality” of crop rotations.
  28. 28. Permanganate oxidizable C better reflected practices that promote organic matter accumulation or stabilization and therefore can be a useful indicator of long-term soil C sequestration. Conversely, mineralizable C (aka soil respiration during a short incubation) better reflected practices that promote organic matter mineralization and therefore can be a useful indicator of short-term soil nutrient availability. Measuring what matters = understanding connection between form and function
  29. 29. SOLVITA CO2 burst test (CO2 production for 24 hrs after wetting dry soil)
  30. 30. In fall 2017, students in my Soil Properties class brought in paired soils (Crop field vs Fence Row) from their farms. In all cases, the fence row soils had higher soil respiration in 24 hrs after wetting.
  31. 31. The SituResp method is a reliable method for performing a $low cost$ assessment of soil microbial activity that could be used for soil health monitoring. < 1/10th the cost of SOLVITA
  32. 32. (Thoumazeau et al, 2017) Cuvettes containing pH indicator Cresol Red in agar gel can provide a cheap yet sensitive measurement of respiration
  33. 33. A standardized test of soil digestive ability???
  34. 34. Research based on alkali extraction assumes that large recalcitrant humus molecules are created by a process of humification; however, these humic substances appear to be artifacts of the extraction process and have not been observed in soil using modern analytical techniques. Nature - 2015 High profile rejection of traditional humus concepts
  35. 35. At the next Soil Science Society of America meeting (January 2019 in San Diego), there will be a day- long special session focused on whether soil humic research has any meaning/value. There will be a morning debate between proponents and opponents of humic acid extraction and characterization and afternoon poster session and oral session for volunteered presentations. The two sides will each write a review paper using a shared theme for publication in the Journal of Environmental Quality, and likely more review papers will follow. personal communication w/ Dan Olk
  36. 36. Highlights • We analyzed data from 60 published studies and global databases with > 50,000 measurements. • A 1% increase in C increased plant available water by only 1.2% of soil volume on average. European Journal of Soil Science - 2017 1” of additional plant available water in the top 3 feet = 1/36” = 2.8% Everyone knows that SOM increases plant available water but is this true?
  37. 37. Model developed using data from > 2000 soils useful tool for teaching soil water concepts
  38. 38. + 10% + 150%
  39. 39. Water movement (infiltration & percolation) and rooting depth/volume (from which crops acquire water) are far more responsive to↑C than water holding capacity
  40. 40. 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 Impact on synthesis and stabilization of microbial residues???
  41. 41. Fields or parts of fields with the lowest OM content (relative to their potential) will benefit the most from practices that build SOM.
  42. 42. Is there a practical way to predict a soils potential?
  43. 43. Bingham, AH and MF Cotrufo. (2016). Organic nitrogen storage in mineral soil: Implications for policy and management. Science of the Total Environment. 551–552: 116–126. Bouma, J; Van Ittersum, M; Stoorvogel, J; Batjes, N; Droogers, P and M Pulleman. (2017). Soil Capability: Exploring the functional potentials of soils. In: Field, D; Morgan, C and A McBratney. (eds). Global Soil Security. Springer International Publishing. (Part of the series Progress in Soil Science) p.27-44. Deluca, T. (1995). Conventional row crop agriculture: putting America’s soils on a whitebread diet: Journal of Soil and Water Conservation. 50(3): 262-263 Dick, WA and Gregorich, EG. (2004). Developing and maintaining soil organic matter levels. Managing Soil Quality: Challenges in Modern Agriculture. CAB International. 103- 120. 10.1079/9780851996714.0103. Hurisso, T; Culman, S; Horwath, W; Wade, J; Cass, D; Beniston, J; Bowles, T; Grandy, S; Franzluebbers, A; Schipanski, M; Lucas, S and C Ugarte. (2016). Comparison of Permanganate-Oxidizable Carbon and Mineralizable Carbon for Assessment of Organic Matter Stabilization and Mineralization. Soil Science Society of America Journal. 80. 10.2136/sssaj2016.04.0106. King, A and J Blesh. (2017). Crop rotations for increased soil carbon: perenniality as a guiding principle. Ecological Applications. 28. 10.1002/eap.1648. Lehmann, J and M Kleber. (2015). The contentious nature of soil organic matter. Nature. 528. 10.1038/nature16069. Liang, C; Schimel, J and J Jastrow. (2017). The importance of anabolism in microbial control over soil carbon storage. Nature Microbiology. 2. 17105. 10.1038/nmicrobiol.2017.105. Magdoff, F and R Weil. (2004). Soil Organic Matter in Sustainable Agriculture. 10.1201/9780203496374. Minasny, B and A Mcbratney. (2017). Limited effect of organic matter on soil available water capacity: Limited effect of organic matter on soil water retention. European Journal of Soil Science. 69. 10.1111/ejss.12475. Shahbaz, M; Kuzyakov, Y and F Heitkamp. (2016). Decrease of soil organic matter stabilization with increasing inputs: Mechanisms and controls. Geoderma. 304. 76-82. 10.1016/j.geoderma.2016.05.019 Thoumazeau, A; Gay, F; Pascal, A; Suvannange, N; Phongjinda, A; Panklang, P; Chevallier, T; Bessou, C and A Brauman. (2017). SituResp®: A time- and cost-effective method to assess basal soil respiration in the field. Applied Soil Ecology. 121. 223-230. 10.1016/j.apsoil.2017.10.006. Veenstra, J.J., C.L. Burras. 2012. Effects of agriculture on the classification of Black Soils in the Midwestern United States. Canadian Journal of Soil Science. 92(3): 403-411. Wang, T; Tian, Z; Bengtson, P; Tunlid, A and P Persson. Mineral‐surface‐reactive metabolites secreted during fungal decomposition contribute to the formation of soil organic matter. Environmental Microbiology. 19. 10.1111/1462-2920.13990. Literature Cited