fundamentals of soil science soil organic matter

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Fundamentals of Soil Science Soil Organic Matter Slide 2 Lecture 6 SOMs Influence on Soil Properties and Plants Slide 3 Learning Objectives Lecture 6 Identify factors that lead to a loss or gain of organic matter in soils Explain the conundrum of soil organic matter management List five guidelines for managing soil organic matter Discuss changes in active and passive pools of organic matter as a result of management Name the greenhouse gases of importance to soil processes and the relative warming potential of each Slide 4 Lecture 6 - Topics Factors controlling the level of soil organic matter Major soil C pools Maintenance of soil organic matter Summary and review Slide 5 Carbon Inputs Outputs = Storage PlantsLitterSoil organic matter Gains in carbon come from plant residues and applied organic materials Losses in carbon are due to respiration (CO 2 losses), plant removals, and erosion. Slide 6 Balance of Carbon Factors Affecting the Balance Between Gains and Losses or Organic Matter in Soils Factors promoting gainsFactors promoting losses Green manures or cover crops Conservation tillage Return of plant residues Low temperatures and shading Controlled grazing High soil moisture Surface mulches Application of compost and manures Appropriate nitrogen levels High plant productivity High plant root:shoot ratio Erosion Intensive tillage Whole plant removal High temperatures and exposure to sun Overgrazing Low soil moisture Fire Application of only inorganic materials Excessive mineral nitrogen Low plant productivity Low plant root:shoot ratio Slide 7 Managing SOM Management of soil organic matter leads to reduction in greenhouse gas emission or to enhanced soil quality and plant production Slide 8 Conundrum SOM must simultaneously decompose and accumulate. SOM must decompose to become a source of nutrients for plants and organic compounds that promote biological diversity, disease suppression, aggregate stability and metal chelation. SOM must accumulate for these same functions as well as for sequestering of C, enhancement of soil water-holding, adsorption of exchangeable cations, immobilization of pesticides and detoxification of metals. Slide 9 General Guidelines for Managing SOM Continuous supply of plant residues Slide 10 General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own ideal level of SOM Slide 11 General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own ideal level of SOM Adequate N is requisite Microbial activity, CO 2 evolved Microbial activity, CO 2 evolved Nitrate depression period Soluble N level in soil C/N ratio of residues Residues added Time C/N ratio 60 40 20 0 80 60 40 20 0 (a) (b) Slide 12 General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own ideal level of SOM Adequate N is requisite Tillage should be reduced or eliminated Slide 13 General Guidelines for Managing SOM Continuous supply of plant residues Each system has its own ideal level of SOM Adequate N is requisite Tillage should be reduced or eliminated Encourage perennial vegetation and natural ecosystems Slide 14 Pools of SOM Small % of residue is retained Offset by slow decomposition Often in equilibrium in mature ecosystems Disturbance can cause drastic change Plant residues Structural C high lignin, low N 2-4 years C/N=100-200 Metabolic C low lignin, high N 0.1-0.5 year C/N=10-25 Slow SOM 15-100 years C/N = 10-25 Active SOM 1-2 years C/N = 15-30 Passive SOM 500-5000 years C/N = 7-10 CO 2 Slide 15 SOM Active Pool Active Pool - 10-20% of SOM labile materials with half-lives of only a few days to a few years. Provides most of the accessible food for soil organisms and most of the readily mineralizable nitrogen. Beneficial effects on structural stability that lead to enhanced infiltration of water, erosion resistance, ease of tillage. Slide 16 SOM Slow Pool Slow Pool Between Active and Passive pools Particulate matter high in lignin and other slowly decomposable and chemically resistant components. (Half-lives in decades) Source of mineralizable N, P, and S Important source of mineralized nitrogen and provides food source for k-strategist microbes. Slide 17 SOM Passive Pool Passive Pool 60-90 % of SOM materials remaining in soil for hundreds or thousands of years. Material physically protected in clay-humus complexes Responsible for cation exchange and water- holding capacities contributed to soil by organic matter Composed of humic substances Slide 18 Pools of SOM (cont.) Slide 19 Changes in Active and Passive Pools with Soil Management Monitoring the Active C Pool can serve as an early warning of soil quality changes The Active Pool reflects the greatest change in organic matter, either loss through cultivation or gain through addition of organic material. Slide 20 Global Climate Change Levels of certain gases in Earths atmosphere cause concern Carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons (CFCs) Greenhouse gases (GHG) trap much of the outgoing long-wavelength radiation GHG produced by biological processes, such as those occurring in soil, account for of the rising greenhouse effect. Root respiration, decomposition of exudates and SOM produce CO 2 Methanogenesis produces CH 4 Nitrification and denitrification produce N 2 O Slide 21 Global Warming Potential N 2 O and CH 4 are present in lower concentrations than CO 2 Their potential to trap infrared radiation is greater GWP of N 2 O is 298 x and CH 4 is 25 x CO 2 over 100 years Small increases in the production of these trace gases impacts net emissions of an ecosystem or production system Slide 22 GHG Emission from Soil Slide 23 Trace Gas Emission in CO 2 Equivalents Sugar cane Napier grass Slide 24 Renewable Energy: Biofuels Slide 25 Summary SOM is beneficial to soil biological, physical, and chemical properties To realize this potential you must build SOM up, but also have mineralization, in balance Management can have enormous impact particularly on the active soil C pools Trace GHG that originate from soils, such as CH 4 and N 2 O have disproportionate effects on climate change compared to CO 2

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