Chapter 3 Soil Organic Matter

A picture of your favorite forest soil again! This soil contains approximately 200 Mg of organic-C per ha! Soil OM serves as a source of humus, holds water, and supplies soil organisms with food, nutrients and energy as it becomes decomposed. Some soluble organic compounds also have the ability to bind metals like Al and Cu into very insoluble complexes.

Carbon is worth money! Carbon Credits…

Carbon Sequestration in Soils! Great current political and governmental interest in building soil C pools via reforestation and minimum tillage. If documented, soil C sequestration may be used to offset C emissions from power plants and vehicles. These “C credits” may be traded nationally and internationally once a regulatory structure and market is established.

3.1 Global Carbon Cycle Autotrophs take in CO2 from the atmosphere. Through photosynthesis, the energy of sunlight is trapped in the carbon-to-carbon bonds of organic molecules. Some of these organic molecules are used as a source of energy (via respiration) by the autotrophs themselves (especially by the plant roots). Other organic molecules are used by heterotrophs that consume autotrophs. The carbon being returned to the atmosphere as CO2 in either case. The remaining organic materials (OM) are eventually added to the soil through incorporation and decomposition of the dead organisms. Yet there is more CO2 released to the atmosphere than stored in the soil. Why do you think that is? *

GLOBAL N CYCLE * Note: larger export from soil than inputs! Could that be from agriculture and methane efflux? Could that be from agriculture increasing the decomp. of C in long-term storage and methane efflux from wetland soils and melting permafrost? Methane is 25X as effective as a greenhouse gas than CO2 over a 100 yr lifespan.

Table 3.1 * * * * *

3.2 Decomposition of OM in Soils The rate of decomposition or “digestibility” of a given plant litter or root depends largely on what it’s made of. *

Figure 3.4 * SOC = carbon component of SOM (variable, ~0.4 to 0.6), often calculated as OM/1.7 = OC Ash = other elements

Decomposition under oxidized (aerobic) soil conditions Decomposition under oxidized (aerobic) soil conditions. Note major losses of CO2 and H2O … (C, O, H make up 92% of OM). *

Important Aspects of Aerobic Decay Soil respiration rate increases dramatically as fresh residues are decomposed, releasing CO2. The spurt in microbial decomposition will also attack some of the more resistant humus compounds – priming effect. Soil humus is the final product after multiple cycles of digestion/excretion/decay. Mineralization is the release of inorganic minerals from decomposed organic matter.

Figure 3.5 Management affects equilibrium levels Incr. microbial activity accelerates tissue decomp.

Anaerobic decomposition Anaerobic decomposition. This is a much slower set of microbially-processed reactions that generates a mix of CO2 and CH4 as metabolic gasses. As discussed earlier, the lower the redox potential in soil, the higher the ratio of CH4 to CO2 emitted.*

3.3 Factors Affecting Decomposition and Mineralization Rates In general: The time needed to complete the processes of decomposition and mineralization may range from days to years, depending mainly on two broad factors: The environmental conditions in the soil, and The quality of the added residues as a food source for soil organisms * *

Specific Factors Affecting Decomposition Rates * Physical location: on the soil or in it Particle Size: smaller particles are more rapidly shredded and decomposed C/N Ratio: Very high C/N ratio residues will be limited in decay rate by available N (Note: Also P may be limiting (required by microbes for energy and cell components). *

Table 3.2

Figure 3.8a. Changes in microbial activity, C/N ratio of added residues, and nitrate levels in soils over time as freshly added high-C residues are decomposed.

Figure 3.8b. Changes in microbial activity, C/N ratio of added residues, and nitrate levels in soils over time as freshly added low-C residues are decomposed.

Litter Quality * C/N Ratio: Usually, low C/N ratio (< 40) materials are higher in proteins and are more palatable (RE: microbes 5:1 to 10:1) Polyphenolic materials directly inhibit microbial decomposers and some shredders Lignin in very resistant to microbial degradation, mostly broken by aerobic fungi *

Which has Higher Lignin?

Figure 12.10 Temporal patterns of nitrogen mineralization or immobilization with organic residues differing in quality based on their C/N ratios and contents of lignin and polyphenols. Lignin contents greater than 20%, polyphenol contents greater than 3%, and C/N ratios greater than 30 would all be considered high in the context of this diagram, the combination of these properties characterizing litter of poor quality—that is, litter that has a limited potential for microbial decomposition and mineralization of plant nutrients. (Diagram courtesy of R. Weil)

3.4 Genesis of Soil Organic Matter (SOM) and Humus All living biomass + Dead roots and other recognizable macro-detritus + Soil humus – processed, stable colloid material that is typically analyzed and reported as “soil organic matter” in lab tests. *

Figure 12.11 Classification of soil organic matter components separable by chemical and physical criteria. Although surface residues (litter) are not universally considered to be part of the soil organic matter, we include them because they are the principal components of the O horizons in soil profiles. Fig 12.11

Humus - Two Constituents Humic substances – 60 to 80% of SOM; stable, dark, high molecular weight, resistant to microbial attack. Lignins and polyphenols are major types. Non-humic substances – Other biomolecules like polysaccharides, polymers, etc. Remember the list? Sugar, starch, proteins, cellulose. These are less resistant to decay. Clay/humus complexes – Very strong binding to or encapsulation of humus by clays (Fe oxides) in micropores protects humus and N from further microbial attack. Possibly true of metal-organic compounds also. All of these are colloids! * *

Figure 3.12 After ~1 year IN the soil

3.5 Influence of OM on Plants See Table Direct influence: Soluble organic compounds (OC-N and OC-P) may be taken up by plants and microorganisms. Allelochemicals (chemical leachates and microbial metabolites) act as growth regulators and suppress germination of other plants and microbes. These are non-humic and short-term. *

Influence of OM on Plants * Indirect influence: Physical properties: Humus improves Aggregate formation and stability Water-holding capacity and retention Chemical properties: Humus improves Cation-exchange capacity Soil mineral weathering rate (through chelation) Aluminum toxicity (buffers pH and attaches to Al3+) Serve as a food source for fauna and microorganisms, supporting the food web

3.6 Amount and Quality of SOM SOM Pools – CO2 is given off as SOM moves from stage to stage. Active (labile) pool – 10-20% of total SOM. Easily and rapidly decomposed non-humic substances. Half-life of days to a few years. Contains particulate OM. Slow (stable) pool – Up to 20% of SOM. Mainly lignins and humic substances. Half-life is decades. Passive (long-term) pool - 60-90% of SOM. Mostly clay-humus complexes and colloids. Half-life of hundreds to thousands of years.

Figure 3.16 Relative turnover rates of various SOM pools in the soil.

So, what is the long term effect of intensive cultivation and crop removal on these various OM pools?

In general, tillage and crop residue removal cause rapid decreases in the active and slow SOM pools, with less of an effect on the passive pool. Oxidation, loss of aggregate stability, and erosion are factors that accompany tillage and result in lower OC%. Figure 3.17

3.7 Carbon Balance Rate of SOM increase or decrease depends on the balance between gains and losses Gains: Plant residues and OM additions (manures, sludge, biosolids, compost, etc.) Losses: Accelerated decomposition, respiration (CO2 loss), plant removal, erosion

Sugar Cane – high residue, adds OM Organic Soil forms – when undrained, low oxidation, low losses of CO2 Insert drainage canal – let the oxidation begin!

Histosols oxidize away when drained! This one was drained for 60 years.

3.8 What Controls SOM Levels? * Climate, Texture, Drainage (oxidation), Management: Climate = Temperature & Moisture Moist cool climates generate more OM inputs. Cool and/or wet soils limit microbial decomposition, leading to OM accumulation

3.8 What Controls SOM Levels? Texture: Sandy soils allow ready losses of CO2 and are low in clay, while clayey soils retain OM via humus-clay associations. So, SOM increases with clay content! Drainage: Poorly drained wet soils retain SOM due the slow nature of anaerobic decomposition.

Figure 12.22 Soils high in silt and clay tend to contain high levels of organic carbon. Charged particles attract charged particles, and form aggregates that protect OM from consumers and erosion.

Drainage Histosols are thick organic matter dominated soils that form primarily in wetlands due to very slow OM turnover under anaerobic conditions. Due to their very high levels of charge, nutrients, water holding, etc., organic soils tend to be very productive when they are drained and used for agriculture or forestry production.

12.19

Figure 12.23 Distribution of organic carbon in well and poorly drained soil Poor drainage results in higher organic carbon content, particularly in the surface horizon. The soil on the right probably represents a mineral wetland soil.

What Controls SOM Levels? Any soil has an equilibrium level of SOM that is dictated by texture, climate, and current management practice that limits the amount of SOM that it can retain over time. You can waste a lot of energy and money trying to “force” OM levels above their equilibrium point! Changes in any of the five soil forming factors causes an equilibrium shift in SOM levels. Annual OM additions are critical for any soil; to keep the organisms fat and happy!

Managing Organic Matter Best management practices increase OM inputs and minimizes OM decomposition and erosion losses. Reduced tillage temporarily increase the equilibrium level of OM in soils. The factors have been covered previously in this chapter.

12.9 Soils and Climate Change Biofuel growth, processing, and combustion Unclear whether there is a net increase or removal of CO2 and CH4. Draining/filling Wetlands Unclear whether the high sequestration rate and storage of C offsets the natural emissions of CO2 and CH4 (and N20).

Biological inputs of various greenhouse gases are shown in darker blue shades. White inputs from industry. Fig. 12.27

12.10 Humus vs. Composts Natural humus is colloidal and quite high in pH dependent charge; up to 150 to 300 cmolc/kg on a weight basis. Composting is an increasingly popular way of generating a humus-like product via accelerated self-heating microbial decomposition. This can generate a more stable (N) and less toxic organic product than the original precursor waste. Gets the C/N ration way down. See Table 12.2.

Compost generated from farm residues (above) and from a mixture of sewage biosolids and wood chips via the static aerated pile method (below). Box 12.4

~Fig. 12.33

So, in summary, the amount of organic matter in any soil is controlled by the interactions of climate, vegetation, management practices like tillage and crop removal, drainage, texture, and overall levels of biotic activity. As we change the inputs or levels of any of these factors, we should expect a corresponding change in the soil OM level. Organic carbon Semi-arid soil Inorganic carbon -white CaCO3