Fundamentals of Soil Science

Soil Organic Matter

Lecture 5 Creating SOM

Learning Objectives

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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

Lecture 5 Topics

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Factors that control litter decay, nutrient mineralization, and humus formation

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Factors that control soil organic matter stabilization

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Major soil carbon pools

Review - What is Soil Organic Matter?

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What is soil organic matter?

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Living biomass (plant tissues, animal tissues and microorganisms)

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Dead roots and dead plant residues or litter

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Mixture or organic substances no longer identifiable as tissues

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Some carbon from decomposition process is converted to soil humus

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 Incorporation Soil humus (15-35 g) 1 year later

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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: CH 2 O + O 2 CO 2 + H 2 O + energy (478 kJ mol -1 C) In Aerobic soil activities of soil organisms create:

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Carbon dioxide, water, energy and decomposer biomass

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Release of essential nutrient elements such as nitrogen, phosphorus and sulfur and inorganic ions such as ammonium, nitrate an sulfate

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Compounds resistant to microbial action

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Mineralization – process that releases elements from organic compounds to produce inorganic forms

By-products of decomposition in anaerobic soil

Methanogenic bacteria and archaea reaction

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Anaerobic: 4C 2 H 5 COOH + 2H 2 O slow 4CH 3 COOH + CO 2 + CH 4 In anaerobic soil decomposition activities are very Wet, anaerobic soils accumulate large amounts of organic matter in partially decomposed condition.

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Alcohols and methane gas contain energy

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Foul odor and plant inhibitors

Controlling the Rate of Decomposition

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Environmental conditions in the soil

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Moisture

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Air

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Temperature Residues as food source for soil organisms.

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Physical location

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Surface

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Incorporated in soil by root deposition, faunal action, tillage

− Particle size − Carbon/Nitrogen Ratio •

Older plants higher proportion of slow decomposing lignin and cellulose

• • • •

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

Significance of C/N Ratio

Microbial activity, CO 2 evolved Soluble N level in soil Nitrate depression period Residues added Residues added (a) Time Microbial activity, CO 2 evolved Soluble N level in soil C/N ratio of residues C/N ratio of residues (b) Time 60 40 20 0 80 60 40 20 0

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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.

Mechanisms for SOM Stabilization

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Protection within soil aggregates Organo-mineral interaction (bound organic matter to mineral surfaces)

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Recalcitrance (intrinsic chemical resistance to decay)

(Sollins et al. 1996, Geoderma)

Protection

Root Microaggregates Plant and fungal debris Silt sized microaggregate Clay microstructure Particulate OM with hyphae Hyphae Pore space Interaggregate binding agents

Macroaggregate >250µm

(from Jastrow and Miller 1998) Soil Processes and the Carbon Cycle, CRC Press.

Organo-mineral interaction

Mineral Organic Matter Exchangeable

Hydrophilic functional groups Hydroxylated mineral surface Hydrophobic structures Electrostatic Interaction with soluble ions Direct bond with surface metal cation

(Kleber et al. 2007, Biogeochemistry)

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Organo-mineral interaction (cont.)

Hydrophobic Compounds COCH 3 HC=CHCO 2 H C/N means lots of C, little N

Phenolic groups = lignin

CH 3 O OH OCH 3 OH OCH 3 OH OH COOH COOH

Waxy, long chain fatty acids = cutin and suberin

OH OH

• Polar side chains for solubility, but will bind to minerals,

other organic matter, each other preferentially

• Very important role in ORGANO-MINERAL interactions

Recalcitrance

Phospholipids Simple sugars Starches Hemicellulose Peptides and AAs Cellulose Polyphenols Complex proteins Lipids Lignin Cuticular waxes Black carbon

Autofluorescence microsopy of pine wood

0.001

0.01

0.1

1 10 100 Mean Residence Time (y) 1000 10000 Free compounds

Recalcitrance (cont.)

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Stabilized in Soil LMW acids Phospholipids Simple sugars Starches Hemicellulose Peptides and AAs Cellulose Polyphenols Complex proteins Lipids Lignin Cuticular waxes Black carbon

0.001

0.01

0.1

1 10 100

Mean Residence Time (y)

Free compounds STABILIZED in the soil matrix 1000 10000

Pools of SOM

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

CO 2 CO 2 Active SOM

1-2 years C/N = 15-30

CO 2 CO 2 Slow SOM

15-100 years C/N = 10-25 • • • •

Small % of residue is retained Offset by slow decomposition Often in equilibrium in mature ecosystems Disturbance can cause drastic change Passive SOM

500-5000 years C/N = 7-10

CO 2

Pools of Soil Organic Matter (cont.)

SOM Active Pool

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Active Pool - 10-20% of SOM – labile materials with half-lives of only a few days to a few years.

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Provides most of the accessible food for soil organisms and most of the readily mineralizable nitrogen.

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Beneficial effects on structural stability that lead to enhanced infiltration of water, erosion resistance, ease of tillage.

SOM Passive Pool

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Passive Pool – 60-90 % of SOM – materials remaining in soil for hundreds or thousands of years.

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Material physically protected in clay-humus complexes

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Responsible for cation exchange and water holding capacities contributed to soil by organic matter

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Composed of humic substances

SOM Slow Pool

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Slow Pool – Between Active and Passive pools

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Particulate matter high in lignin and other slowly decomposable and chemically resistant components. (Half-lives in decades)

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Source of mineralizable N, P, and S

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Important source of mineralized nitrogen and provides food source for k-strategist microbes.