3 Learning Objectives Lecture 5 – Describe the importance of Carbon to Nitrogen ratio in litter decayList the primary mechanisms contributing to soil C stabilizationDistinguish between factors that control decomposition of litter vs. decomposition of soil organic matter
4 Lecture 5 TopicsFactors that control litter decay, nutrient mineralization, and humus formationFactors that control soil organic matter stabilizationMajor soil carbon pools
5 Review - What is Soil Organic Matter? Living biomass (plant tissues, animal tissues and microorganisms)Dead roots and dead plant residues or litterMixture or organic substances no longer identifiable as tissuesSome carbon from decomposition process is converted to soil humus
6 polysaccharides, polyuronides, acids, etc. Humus Creation60-80 gOrganic C in residues100 grams3-8 g3-8 g10-30 gBiomass(soil organisms)polysaccharides, polyuronides, acids, etc.Complex compoundsSoil humus (15-35 g)Incorporation year later
7 By-products of decomposition in aerobic soil Basic reaction accounts for most of the organic matter decomposition in the soil, as well as oxygen consumption and CO2 release.Aerobic: CH2O + O2 CO2 + H2O + energy (478 kJ mol-1 C)In Aerobic soil activities of soil organisms create:Carbon dioxide, water, energy and decomposer biomassRelease of essential nutrient elements such as nitrogen, phosphorus and sulfur and inorganic ions such as ammonium, nitrate an sulfateCompounds resistant to microbial actionMineralization – process that releases elements from organic compounds to produce inorganic forms
8 By-products of decomposition in anaerobic soil Methanogenic bacteria and archaea reactionAnaerobic: 4C2H5COOH + 2H2O 4CH3COOH + CO2 + CH4In anaerobic soil decomposition activities are very slowWet, anaerobic soils accumulate large amounts of organic matter in partially decomposed condition.Alcohols and methane gas contain energyFoul odor and plant inhibitors
9 Controlling the Rate of Decomposition Environmental conditions in the soilMoistureAirTemperatureResidues as food source for soil organisms.Physical locationSurfaceIncorporated in soil by root deposition, faunal action, tillageParticle sizeCarbon/Nitrogen RatioOlder plants higher proportion of slow decomposing lignin and cellulose
10 Carbon/Nitrogen Ratio Soil organisms need carbon for building essential organic compounds and to obtain energyThey need nitrogen to synthesize nitrogen-containing cellular components such as amino acids, enzymes and DNA.Microbes need 1 g of N for every 24 g of C in their foodHigher than 25:1 – not enough nitrogen so 1) microbes take from plant supply, 2) decay delayed because microbes can’t survive
11 Significance of C/N Ratio 604020Adding readily decomposable organic material increases the consumption of microbial community which results in high CO2 yield. The microbes demand nitrogen which deprives plants of nitrogen. This in nitrate depression period.Planting should be delayed until after nitrate depression period or additional sources of nitrogen applied.Microbial activity,CO2 evolvedC/N ratioSoluble N level in soilC/N ratio of residuesNitrate depression periodResidues addedTime(a)80604020Microbial activity,CO2 evolvedSoluble N level in soilC/N ratioC/N ratio of residuesResidues addedTime(b)
12 Mechanisms for SOM Stabilization Protection within soil aggregatesOrgano-mineral interaction (bound organic matter to mineral surfaces)Recalcitrance (intrinsic chemical resistance to decay)(Sollins et al. 1996, Geoderma)
13 Protection Root Microaggregates Plant and fungal debris Silt sized microaggregateClay microstructureParticulate OMwith hyphaeHyphaeMacroaggregate>250µmPore spaceInteraggregate binding agents(from Jastrow and Miller 1998)Soil Processes and the Carbon Cycle, CRC Press.
14 Organo-mineral interaction Organic MatterExchangeableHydrophilicfunctional groupsHydrophobicstructuresDirect bond with surface metal cationElectrostatic Interactionwith soluble ionsHydroxylated mineral surface(Kleber et al. 2007, Biogeochemistry)
15 Organo-mineral interaction (cont.) Hydrophobic CompoundsOCH3OHCH3OCOCH3OCH3OHHC=CHCO2HC/N meanslots of C,little NPhenolic groups = ligninOHCWaxy, long chain fatty acids = cutin and suberinPolar side chains for solubility, but will bind to minerals, other organic matter, each other preferentiallyVery important role in ORGANO-MINERAL interactions
16 Recalcitrance Mean Residence Time (y) CellulosePolyphenolsComplex proteinsLipidsLigninCuticular waxesBlack carbonHemicelluloseStarchesSimple sugarsPhospholipidsPeptides and AAsAutofluorescence microsopy of pine wood0.0010.010.1110100100010000Mean Residence Time (y)Free compounds
17 Recalcitrance (cont.) Stabilized in Soil Mean Residence Time (y) LMW acidsCellulosePolyphenolsComplex proteinsLipidsLigninCuticular waxesBlack carbonHemicelluloseStarchesSimple sugarsPhospholipidsPeptides and AAsFree compoundsSTABILIZED inthe soil matrix0.0010.010.1110100100010000Mean Residence Time (y)
18 Pools 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
20 SOM 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.
21 SOM 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
22 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 SImportant source of mineralized nitrogen and provides food source for k-strategist microbes.