2Lecture 6 SOM’s Influence on Soil Properties and Plants
3Learning Objectives Lecture 6 – Identify factors that lead to a loss or gain of organic matter in soilsExplain the conundrum of soil organic matter managementList five guidelines for managing soil organic matterDiscuss changes in active and passive pools of organic matter as a result of managementName the greenhouse gases of importance to soil processes and the relative warming potential of each
4Lecture 6 - TopicsFactors controlling the level of soil organic matterMajor soil C poolsMaintenance of soil organic matterSummary and review
5Carbon Inputs – Outputs = Storage Plants Litter Soil organic matterGains in carbon come from plant residues and applied organic materialsLosses in carbon are due to respiration (CO2 losses), plant removals, and erosion.
6Factors promoting gains Factors promoting losses Balance of CarbonFactors Affecting the Balance Between Gains and Losses or Organic Matter in SoilsFactors promoting gainsFactors promoting lossesGreen manures or cover cropsConservation tillageReturn of plant residuesLow temperatures and shadingControlled grazingHigh soil moistureSurface mulchesApplication of compost and manuresAppropriate nitrogen levelsHigh plant productivityHigh plant root:shoot ratioErosionIntensive tillageWhole plant removalHigh temperatures and exposure to sunOvergrazingLow soil moistureFireApplication of only inorganic materialsExcessive mineral nitrogenLow plant productivityLow plant root:shoot ratio
7Managing SOMManagement of soil organic matter leads to reduction in greenhouse gas emission or to enhanced soil quality and plant production
8Conundrum – 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.
9General Guidelines for Managing SOM Continuous supply of plant residues
10General Guidelines for Managing SOM Continuous supply of plant residuesEach system has its own “ideal” level of SOM
11General Guidelines for Managing SOM Microbial activity,CO2 evolvedNitrate depression periodSoluble N level in soilC/N ratio of residuesResidues addedTimeC/N ratio60402080(a)(b)Continuous supply of plant residuesEach system has its own “ideal” level of SOMAdequate N is requisite
12General Guidelines for Managing SOM Continuous supply of plant residuesEach system has its own “ideal” level of SOMAdequate N is requisiteTillage should be reduced or eliminated
13General Guidelines for Managing SOM Continuous supply of plant residuesEach system has its own “ideal” level of SOMAdequate N is requisiteTillage should be reduced or eliminatedEncourage perennial vegetation and natural ecosystems
14Pools of SOM Small % of residue is retained Plant residuesStructural Chigh lignin, low N2-4 yearsC/N=Metabolic Clow lignin, high NyearC/N=10-25Small % of residue is retainedOffset by slow decompositionOften in equilibrium in mature ecosystemsDisturbance can cause drastic changeCO2CO2Active SOM1-2 yearsC/N = 15-30CO2Slow SOMyearsC/N = 10-25CO2Passive SOMyearsC/N = 7-10CO2
15SOM Active PoolActive Pool % 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.
16SOM 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 SImportant source of mineralized nitrogen and provides food source for k-strategist microbes.
17SOM Passive PoolPassive Pool – % of SOM – materials remaining in soil for hundreds or thousands of years.Material physically protected in clay-humus complexesResponsible for cation exchange and water-holding capacities contributed to soil by organic matterComposed of humic substances
19Changes in Active and Passive Pools with Soil Management Monitoring the Active C Pool can serve as an early warning of soil quality changesThe Active Pool reflects the greatest change in organic matter, either loss through cultivation or gain through addition of organic material.
20Global Climate ChangeLevels of certain gases in Earth’s atmosphere cause concernCarbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons (CFCs)Greenhouse gases (GHG) trap much of the outgoing long-wavelength radiationGHG produced by biological processes, such as those occurring in soil, account for ½ of the rising greenhouse effect.Root respiration, decomposition of exudates and SOM produce CO2Methanogenesis produces CH4Nitrification and denitrification produce N2O
21Global Warming Potential N2O and CH4 are present in lower concentrations than CO2Their potential to trap infrared radiation is greaterGWP of N2O is 298 x and CH4 is 25 x CO2 over 100 yearsSmall increases in the production of these trace gases impacts net emissions of an ecosystem or production system
25SummarySOM is beneficial to soil biological, physical, and chemical propertiesTo realize this potential you must build SOM up, but also have mineralization, in balanceManagement can have enormous impact particularly on the active soil C poolsTrace GHG that originate from soils, such as CH4 and N2O have disproportionate effects on climate change compared to CO2