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Fundamentals of Soil Science Soil Organic Matter.

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Presentation on theme: "Fundamentals of Soil Science Soil Organic Matter."— Presentation transcript:

1 Fundamentals of Soil Science Soil Organic Matter

2 Lecture 5 Creating SOM

3 Learning Objectives Lecture 5 – Lecture 5 – −Describe the importance of Carbon to Nitrogen ratio in litter decay −List the primary mechanisms contributing to soil C stabilization −Distinguish between factors that control decomposition of litter vs. decomposition of soil organic matter

4 Lecture 5 Topics Factors that control litter decay, nutrient mineralization, and humus formation Factors that control litter decay, nutrient mineralization, and humus formation Factors that control soil organic matter stabilization Factors that control soil organic matter stabilization Major soil carbon pools Major soil carbon pools

5 Review - What is Soil Organic Matter? What is soil organic matter? What is soil organic matter? – Living biomass (plant tissues, animal tissues and microorganisms) – Dead roots and dead plant residues or litter – Mixture or organic substances no longer identifiable as tissues Some carbon from decomposition process is converted to soil humus Some carbon from decomposition process is converted to soil humus

6 Humus Creation Incorporation1 year later Organic C in residues 100 grams Biomass (soil organisms) polysaccharides, polyuronides, acids, etc.Complex compounds g 3-8 g g Soil humus (15-35 g)

7 By-products of decomposition in aerobic soil In Aerobic soil activities of soil organisms create: In Aerobic soil activities of soil organisms create: – Carbon dioxide, water, energy and decomposer biomass – Release of essential nutrient elements such as nitrogen, phosphorus and sulfur and inorganic ions such as ammonium, nitrate an sulfate – Compounds resistant to microbial action – Mineralization – process that releases elements from organic compounds to produce inorganic forms Basic reaction accounts for most of the organic matter decomposition in the soil, as well as oxygen consumption and CO2 release. Aerobic: CH 2 O + O 2 CO 2 + H 2 O + energy (478 kJ mol -1 C)

8 By-products of decomposition in anaerobic soil In anaerobic soil decomposition activities are very slow In anaerobic soil decomposition activities are very slow Wet, anaerobic soils accumulate large amounts of organic matter in partially decomposed condition. Wet, anaerobic soils accumulate large amounts of organic matter in partially decomposed condition. – Alcohols and methane gas contain energy Foul odor and plant inhibitors Foul odor and plant inhibitors Methanogenic bacteria and archaea reaction Anaerobic: 4C 2 H 5 COOH + 2H 2 O 4CH 3 COOH + CO 2 + CH 4

9 Controlling the Rate of Decomposition Environmental conditions in the soil Environmental conditions in the soil – Moisture – Air – Temperature Residues as food source for soil organisms. Residues as food source for soil organisms. – Physical location Surface Surface Incorporated in soil by root deposition, faunal action, tillage Incorporated in soil by root deposition, faunal action, tillage −Particle size −Carbon/Nitrogen Ratio Older plants higher proportion of slow decomposing lignin and cellulose Older 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 energy Soil organisms need carbon for building essential organic compounds and to obtain energy They need nitrogen to synthesize nitrogen- containing cellular components such as amino acids, enzymes and DNA. They 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 food Microbes need 1 g of N for every 24 g of C in their food Higher than 25:1 – not enough nitrogen so 1) microbes take from plant supply, 2) decay delayed because microbes can’t survive Higher 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 Adding 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. Adding 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. Planting should be delayed until after nitrate depression period or additional sources of nitrogen applied. 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 (a) (b)

12 Mechanisms for SOM Stabilization Protection within soil aggregates Protection within soil aggregates Organo-mineral interaction (bound organic matter to mineral surfaces) Organo-mineral interaction (bound organic matter to mineral surfaces) Recalcitrance (intrinsic chemical resistance to decay) Recalcitrance (intrinsic chemical resistance to decay) (Sollins et al. 1996, Geoderma)

13 Protection Macroaggregate >250µm (from Jastrow and Miller 1998) Soil Processes and the Carbon Cycle, CRC Press. Particulate OM with hyphae Hyphae Plant and fungal debris Silt sized microaggregate Pore space Interaggregate binding agents Microaggregates Clay microstructure

14 (Kleber et al. 2007, Biogeochemistry) Organo-mineral interaction Mineral Organic MatterExchangeable Hydrophilic functional groups Hydrophobic structures Direct bond with surface metal cation Electrostatic Interaction with soluble ions Hydroxylated mineral surface

15 Organo-mineral interaction (cont.) Hydrophobic Compounds Hydrophobic Compounds OCH 3 OH CH 3 O COCH 3 OCH 3 OH HC=CHCO 2 H OH OHOH COOH OH COOH Phenolic groups = lignin C/N means lots of C, little N Waxy, long chain fatty acids = cutin and suberin Polar side chains for solubility, but will bind to minerals, other organic matter, each other preferentiallyPolar side chains for solubility, but will bind to minerals, other organic matter, each other preferentially Very important role in ORGANO-MINERAL interactionsVery important role in ORGANO-MINERAL interactions

16 Cellulose Polyphenols Complex proteins Lipids Lignin Cuticular waxes Black carbon Hemicellulose Starches Simple sugars Phospholipids Peptides and AAs Recalcitrance Autofluorescence microsopy of pine wood Mean Residence Time (y) Free compounds

17 LMW acids Cellulose Polyphenols Complex proteins Lipids Lignin Cuticular waxes Black carbon Hemicellulose Starches Simple sugars Phospholipids Peptides and AAs Stabilized in Soil Stabilized in Soil Recalcitrance (cont.) Mean Residence Time (y) Free compounds STABILIZED in the soil matrix

18 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= Metabolic C low lignin, high N year C/N=10-25 Slow SOM years C/N = Active SOM 1-2 years C/N = Passive SOM years C/N = 7-10 CO 2

19 Pools of Soil Organic Matter (cont.)

20 SOM Active Pool Active Pool % of SOM – labile materials with half-lives of only a few days to a few years. Active 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 Pool Passive Pool – % of SOM – materials remaining in soil for hundreds or thousands of years. Passive Pool – % 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

22 SOM Slow Pool Slow Pool – Between Active and Passive pools 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.


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