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Fundamentals of Soil Science

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Presentation on theme: "Fundamentals of Soil Science"— Presentation transcript:

1 Fundamentals of Soil Science
Soil Organic Matter

2 Lecture 5 Creating SOM

3 Learning Objectives 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 soil organic matter stabilization Major 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 litter Mixture or organic substances no longer identifiable as tissues Some carbon from decomposition process is converted to soil humus

6 polysaccharides, polyuronides, acids, etc.
Humus Creation 60-80 g Organic C in residues 100 grams 3-8 g 3-8 g 10-30 g Biomass (soil organisms) polysaccharides, polyuronides, acids, etc. Complex compounds Soil 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 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

8 By-products of decomposition in anaerobic soil
Methanogenic bacteria and archaea reaction Anaerobic: 4C2H5COOH + 2H2O 4CH3COOH + CO2 + CH4 In anaerobic soil decomposition activities are very slow Wet, anaerobic soils accumulate large amounts of organic matter in partially decomposed condition. Alcohols and methane gas contain energy Foul odor and plant inhibitors

9 Controlling the Rate of Decomposition
Environmental conditions in the soil Moisture Air Temperature Residues as food source for soil organisms. Physical location Surface Incorporated in soil by root deposition, faunal action, tillage Particle size Carbon/Nitrogen Ratio 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 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 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
60 40 20 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. Microbial activity, CO2 evolved C/N ratio Soluble N level in soil C/N ratio of residues Nitrate depression period Residues added Time (a) 80 60 40 20 Microbial activity, CO2 evolved Soluble N level in soil C/N ratio C/N ratio of residues Residues added Time (b)

12 Mechanisms for SOM Stabilization
Protection within soil aggregates Organo-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 microaggregate Clay microstructure Particulate OM with hyphae Hyphae Macroaggregate >250µm Pore space Interaggregate binding agents (from Jastrow and Miller 1998) Soil Processes and the Carbon Cycle, CRC Press.

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

15 Organo-mineral interaction (cont.)
Hydrophobic Compounds OCH3 OH CH3O COCH3 OCH3 OH HC=CHCO2H C/N means lots of C, little N Phenolic groups = lignin O H C Waxy, long chain fatty acids = cutin and suberin Polar side chains for solubility, but will bind to minerals, other organic matter, each other preferentially Very important role in ORGANO-MINERAL interactions

16 Recalcitrance Mean Residence Time (y)
Cellulose Polyphenols Complex proteins Lipids Lignin Cuticular waxes Black carbon Hemicellulose Starches Simple sugars Phospholipids Peptides and AAs Autofluorescence microsopy of pine wood 0.001 0.01 0.1 1 10 100 1000 10000 Mean Residence Time (y) Free compounds

17 Recalcitrance (cont.) Stabilized in Soil Mean Residence Time (y)
LMW acids Cellulose Polyphenols Complex proteins Lipids Lignin Cuticular waxes Black carbon Hemicellulose Starches Simple sugars Phospholipids Peptides and AAs Free compounds STABILIZED in the soil matrix 0.001 0.01 0.1 1 10 100 1000 10000 Mean Residence Time (y)

18 Pools of SOM Small % of residue is retained
Plant residues Structural C high lignin, low N 2-4 years C/N= Metabolic C low lignin, high N year C/N=10-25 Small % of residue is retained Offset by slow decomposition Often in equilibrium in mature ecosystems Disturbance can cause drastic change CO2 CO2 Active SOM 1-2 years C/N = 15-30 CO2 Slow SOM years C/N = 10-25 CO2 Passive SOM years C/N = 7-10 CO2

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. 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. 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
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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