Chapter 11 Soil Organic Matter

Main Objectives • Comprehend the role that soils play in the global carbon

cycle and their linkage with global environmental change; • Capable of describing soil organic matter in terms of its

composition, general chemical properties, and importance for soil functions;

• Gain thorough knowledge on decomposition processes; • Clearly understand all relevant factors that influence the

level of soil organic matter in a particular soil.

Key terms and Concepts

• Carbon cycle • Components of SOM (biomass, detritus, humus) • C:N ratio and effect on decomposition rate • Priming effects • Soil Management and SOM

Effects of increased SOM on soil functions Physical - enhance:

- water holding capacity - cation exchange capacity - aggregate stability - porosity (decreased bulk density)

Chemical - mineralization of nutrients to plant available forms (and direct uptake of organic forms)

- slow release fertilizer - increased pH buffering

- form organo-mineral complexes

Biological - increased microbial biomass and activities

- disease resistance

Atmospheric increase

= Emissions from fossil fuels

+ Net emissions from changes in land use

- Oceanic uptake

- Residual carbon sink

4.1(±0.04) = 7.7 (±0.4) + 1.5 (±0.7) - 2.3 (±0.4) - 2.8 (±0.9)

From: Woods Hole Research Center http://www.whrc.org/global/carbon/index.html

1850 2005

8

Figure 11.15 Vertical distribution of SOM

New finding: ~50% of SOC is in subsoil and is surprisingly responsive to land use change (but often ignored in models)

1. Composition of SOM Soil organic matter includes all the organic components of a soil: (1) Biomass

– Living – Soil organisms

(2) Detritus

– identifiable dead tissues – Plant material, dead microorganisms, etc

(3) Humus

– Still figuring this out... – Much more resistant to decomposition than 1 or 2

Elemental Composition: (by dry weight) Carbon = 45-50% Nitrogen = 3-7% P, S, micronutrients = trace amounts (but often very important)

Decomposition of Plant Detritus Rapid decomposition 1 – Sugars, starches, simple proteins 2 – Crude proteins 3 – Hemicellulose 4 – Cellulose 5 – Fats and waxes Slow decomposition 6 – Lignins and phenolic compounds starch (glucose polymers, amylose and amylopectin, a -1-4 or a -1-6) Cellulose (ß-1-4 glucose polymer) Hemicellulose [polymer of several pentose (xylose, arabinose), hexose (mannose, glucose, galactose)] Chitin: Chitin is an acetylated amino sugar polymer mostly found in fungal cell wall materials. Lignin: The most complex component, consists of aromatic rings and polymers (C-C or C-O-C ether). Simplified Decomposition Reaction R-COH + 2 O2 → CO2 + 2 H2O + Energy (note: this is respiration)

Figure 11.3 Typical composition of representative green-plant materials. The major types of organic compounds are indicated at left and the elemental composition at right. The ash is considered to include all the constituent elements other than carbon, oxygen, and hydrogen (nitrogen, sulfur, calcium, etc.).

Carbon : Nitrogen Ratio (C:N) - primary predictor of decomposition rate of plant material - microorganisms involved in decomposition require a balance of carbon to nitrogen (approximately 24:1) - if plant residue has high C:N, microorganisms become nitrogen limited and scavenge nitrogen from the soil to compensate (soil nitrogen availability decreases temporarily N-immobilization) - if plant residue has low C:N, microorganisms are carbon-limited and available nitrogen in the soil increases (residual nitrogen plus nitrogen released from decomposition)

Figure 11.5

Figure 11.6 N release and C/N ratio

Soil Humus - a complex array of substances, often secondary molecules - longer turnover time than biomass or detritus (sometimes stable for 1000s of years) - colloidal, amorphous, polymeric, dark brown material - traditionally separated into several factions based on solubility But, recent results seriously challenges this “old paradigm.”

Soil

Extract with Alkali (0.1 M NaOH)

insoluble soluble

Humin Treat with acid to pH=2

participate solution

Humic acid Fulvic acid

Reason for stability Chemical (internal) - large and complex molecules

Biophysical (external) - protected by clay associations - spatially unavailable

From: Schmidt et al. 2011. NATURE Vol. 478:49-56

Emerging Paradigm (Schmidt et al. 2011. NATURE Vol. 478:49-56)

1 - molecular structure not a good predictor of SOM decomposition rate 2 - large and complex humic substances does not exist (with exception of black C) 3 – but black carbon, which is complex, is not necessarily stable 4 – roots play a large role in SOM decomposition (“rhizosphere priming effect”) 5 – physical protection is important, i.e., microorganisms are not able to access all SOM equally 6 – soil microorganisms are hugely important in decomposition, and their byproducts are a large part of soil humus External (biophysical) factors rather than internal (chemical) seem to control SOM decomposition rates!

2. Factors influencing the level of SOM

What is in the INPUT category?

What is in the OUTPUT category?

What is in the Biotic group of factors?

What is in the Abiotic group?

How does management change SOM levels?