Chapter three Soil Organic Matter Soil Organic

Chapter three Soil Organic Matter

Soil Organic Matter (SOM) affects soil properties • SOM-source of humus, holds water, • provides substrate for micobes, and • provides soil nutrients as it decomposes • Soluble organic compounds bind Fe and Al into insoluble complexes

SOM Soil Organic matter encompasses all organic components of a soil: ◦ Fresh residues ◦ Decomposing organic matter ◦ Stable organic matter ◦ Living organisms

Soil Organic Matter Soil organic matter = ◦ all living organisms (microorganisms, earthworms, etc), ◦ fresh residues (old plant roots, crop residues, recently added manures), ◦ well-decomposed residues (humus). The SOM content of agricultural topsoil is usually in the range of 1 to 6%. This amount is the result of all additions and losses of SOM that have occurred over the years. Non-cultivated soils will have SOM ranges between 3 -10%

Fresh Residues Up to 15% of organic matter is fresh residue (usually <10) Comprised mainly of litter fall Many of the different types of plant litter can be recognized.

Carbon sequestration in soils Current trends are for building soil C pools Organic matter is linked with emmision of green house gases Soil C sequestration may be used to offset C emissions from burning of fossil fuels Autotrophs take in CO 2 from the atm thru photosynthesis Sunlight energy is trapped in C-C bonds of organic molecules Some of these copds are used as a source of energy by autotrophs themselves esp plant roots

The global carbon cycle C stored in c-C bonds is transformed into CO 2 thru metabolic processes Heterotrophs that consume autotrophs use other organic molecules also releasing CO 2 to the aatm The remaining organic molecules are eventually added to the soil through incorporation and decomposition of the dead organisms Eventually morre CO 2 is released to the atm than is added to the soil

Glogal carbon cycle

Components of soil organic matter

Decompositon of Organic Matter in soils Plant material - transformed from one org copd to another mainly by soil organisms Organisms create by-products, wastes, and cell tissue Compounds released as waste by one organisms can often be used as food by another The rate of decompostion or ‘digestibility of given plant litter or root depends largely on composition decomposition rateof org copds is as follows: 1. sugars, starches, simple proteins > 2. crude protein > 3. hemicelluloses > 4. cellulose > 5. fats, waxes > 6. lignins

SOM management affects eqm levels Fig 12. 5 handout

Aerobic decomposition R—(CH 2)2 OH +2 O 2 enzy mes hydrocarbon CO 2 + 2 H 2 O + energy 478 k. J/mol C) Important aspects of aerobic decay There are major losses of C, O and H as CO 2 and H 2 O during aerobic decompostion (constitute 92% of OM) Soil respiration rate increaases dramatically as fresh residues are decomposed releaseing CO 2 The spurt in microbial decomposition will also attack some of the more resistant humuus compounds This is called the priming effect Soil humus is the final product after multiple cycles of digestion/excretion/decay Mineralization is the release of inorganiic minerals from decomposed residues

Anaerobic decomposition 4 C 2 H 5 COOH + 3 H 2 O propionate enzy mes 4 CH 4 COOH + CO 2+ 3 CH 4 acetate CH 3 COOH CO 2+ 3 CH 4 CO 2 + 4 H 2 H 2 O + carbon dioxide methane CH 4 Anaerobic decomposition is much slower Generates a miix of carbon dioxide and methane Predominate in soils with low redox potentials ie under anaerobic conditions The more anoxic the soil conditions the higher the ration of CH 4 to CO 2

SOM Readily decomposable SOM is labile* -it can decline rapidly if the soil environment changes renewable -it can be replenished by inputs of organic material to the soil. * Labile = Constantly or readily undergoing chemical, physical, or biological change or breakdown; unstable.

Adequate levels of SOM can be maintained with: ◦ proper fertilization, ◦ crop rotations, and tillage practices ◦ Returning crop residues to the soil.

Factors affecting decomposition and mineralization rates In general the time needed to complete the process of decomposition and mineralisation may range from days to years depending on two broad factors: 1. Environmental condns in the soil 1. The quality of the added residues added as a substrate for soil microbes

Environmental factors affecting decomposition rates Physical location- is OM on the soil or in it Particle size: smaller particles are more rapidly shredded and decomposed Soil Redox potential or aeration status: are soil condns aerobic or anerobic Management: tillage , erosion, pesticides C/N ratio: high C/N ratio residues will be ltd in decay rate by availability of N 9 and also P in some instances-important in cell components synthesis

Factors Controlling SOM levels in soils 1. Kind of parent materials (texture, clay content), soil moisture, slope, and management practices 2. Climate: rainfall (affects leaching), and areas where temperature and water are adequate will have high SOM. 3. Management practices: crop biomass prodn (water, ferts, cv, tillage, litter) affect SOM content. 4. As dm prodn increases, SOM increases. 5. However, only biomas that remains after harvest & root biomass influences long-term SOM content.

Quantifying SOM contentd SOM is usually measured in the laboratory as organic carbon, Soil organic matter is estimated to contain 50% organic carbon (varies from 40 to 70%) with the rest of the SOM comprising of other elements (eg, 5% N, 0. 5% P and 0. 5% S). A conversion to SOM from a given organic carbon analysis requires that the organic carbon content be multiplied by a factor of 2. 00(1. 00/0. 50). Thus, 2% SOM is about 1 % organic carbon.

Typical C/N ratios, %C and %N values for common organic materials Org material %C %N C/N ratio News paper 39 0. 3 120 Wheat straw 38 0. 5 80 Maize stover 40 0. 7 57 Decomposed farmyard manure 41 2. 1 20 Broccoli residues 35 1. 9 18 Soil microorganisms -bacteria 50 10. 0 5 50 5. 0 10 50 0. 05 600 -A horizon 50 2. 6 20 -B horizon 46 5. 1 9 40 3. 0 13 - fungi Sawdust pine SOM-forest Young alfafa hay

Changes in microbial activity , C/N ratio of added residues, & nitrate levels in soils over time as freshly added high-C residues decompose

Changes in microbial activity , C/N ratio of added residues, & nitrate levels in soils over time as freshly added low-C residues decompose

Low quality litter C/N Ratio: Usu low C/N ratio (<40) materials are higher in proteins and are more palatable (czz microbes C/N= 5: 1 to 10: 1) Polyphenolic materials; directly inhibit microbial decomposers and some shredders Lignin: v. resistant to microbial degradation, mostly broken by aerobic fungi

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Active Fraction 10 to 30% of the soil organic matter (active fraction) is responsible for maintaining soil microorganisms. The active fraction of organic matter is most susceptible to soil management practices. (Inactive = humus) ACTIVE

Adding Fresh OM In a soil which at first has no readily decomposable materials, adding fresh tissue under favorable conditions: 1) immediately starts rapid multiplication of bacteria, fungi, and actinomycetes, 2) which are soon actively decomposing the fresh tissue. ADDED

Fresh SOM as most readily available energy sources are used up, microorganisms again become relatively inactive, leaving behind a dark mixture usually referred to as humus – a stable organic compound

Stable Organic Matter & Humus SOM- all living biomass in soil as well as dead biomass (roots, detritus, organisms) Part of the soil fraction ie passes the 2 mm sieve Thus, soil organic compounds become stabilized and resistant to further changes by microorganisms Stabilized organic matter acts like a sponge and can absorb six times its weight in water – colloidal in nature

Soil Organic matter Living or gsms BIOMASS Identifiable dead tissue DETRITUS NON LIVING NON TISSUE HUMUS HUMIC SUBSTANCES NON HUMIC SUBSTABCES CLASSIFICATION OF SOIL ORGANIC MATTER COMPONENTS SEPARABLE BY CHEMICAL AND PHYSICAL MEANS. Although surface residues are not universally considered to be part of SOM included bcz it is considered as POM which the principal component of

HUMUS-Major constituents Humivcsubstances- 60 t 0 80% of SOM, stable, dark, high Mr Wt, resistant to microbial attack. Lignins and polyphenols are major types Non-humic substances- 20 -30% OF SOM, biomolecules like polysaccharides, polymers, etc 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.

After -1 year IN the soil

Function of Humus holds water and nutrients; sticks together & helps establish and maintain a strong crumb structure & thus reduce soil erosion provides some nutrients (N & P) as it is slowly decayed by microbial activity, Buffers effects of pesticides humus decomposes at the rate of 2. 5% per year Creates good soil “ Tilth” Coates the sand, silt, clay particles making them dark and the darker the color, the greater the amount of soil Humus = High Medium Low humus present.

SOM Maintains soil “Tilth” aiding infiltration of air and water promoting water retention reducing erosion BMI

Amt and quality of OM-SOM POOLs - CO 2 is given off as SOM moves from stage to stage shoown Active pool-10 -20% of SOM Easily and readily decomposed non- humic substances Incluudes tiny pieces of detritus called POM Half life of days to a few years Slow pool- Up to 20%. Mainly humic substances. Half life is decades Passive Pool- 60 -90% of SOM. Mostly humus and clay-humus complexes (all colloids) Includes humus locked up inside stable aggregates –t 1/2

Rel turnover rates of various SOM pools in the soil 60 -80% C lost as CO 2 in 1 st yr Plant residues Structural C Highlignin, low N 2 -4 yrs C/N=`100 -200 Metabolic C Low lignin, high N 2 -4 yrs C/N=`100 -200 Active SOM 2 -4 yrs C/N=`100 -200 Slow SOM 2 -4 yrs C/N=`100 -200 Passive SOM 2 -4 yrs C/N=`100 -200

Effects of long term 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% Carbon balance - when gains >losses short term SOM increases -there are short term SOM decreases when losses>gains

Carbon balance: Factors affecting the balance between gains and losses of OM in soils: Factors promoting gains Green manures/cover crops Conservatio tillage Return of plant residues Low temperature & shading Controlled grazing High soil moisture Surface mulches Applicn of composts/manures Appropriate N levels High plant productivity High plant shoot: root ratio factors promoting losses eroson Intensive tillage Whole plant removal High Temps and exposing soil overgrazing low soil moisture fire Applcn of inorganic ferts only Excessive mineral nitrogen low plant productivity low lant to shoot ratio

What controls SOM levels? Basically the same factors thar control decomposition rates Climate Texture Drainage(oxidation) Management TEXTURE -sandy soils are well drained, allow losses of CO 2 and are low in clay annd stable aggregates. Low clay allows OC to eluviate to thee sub-soil in sandy soils -Clayey soils are high in clay and stable aggregates, retain Humus thru clay-aggr associatiations. SOM increases with clay ctnt

Texture- soils high in silt and clay 0 1 OC % 2 3 4 5 7 tend to contain high levels of OC 10 20 30 40 50 60 Silt and clay content % 70 80

Drainage Histosols are thick OM dominated soils that form primarily in wetland due to very slow OM decomposition under anaerobic conditions Due to their very high levels of chargee, nutrients , whc, etc. , organic soils tend to be very productive when they are drained and used for agriculture for agriicculture or forestry produuctiion Poorly drained mineral soils are not well oxidised and also retain SOM due to the slow nature anaerobic decomposition

Distribution of OC in well drained soils climate plowing Texture is sandy

Soils and clmate change Biofuel growth processing and production ü It is unclear whhethere is net increase oe removal of CO 2 and CH 4 - use of maize stover for ethanol production is probably a bad idea for the soil and the environment ü Use of byproducts from biofuels for soil ammendment is probably a small positive despite the –ve effects of using biomass for biofuels Draining or filling up of wetlands ü it is unclear whether the high sequestration rate and storage of C offsets the natural emissions of green house gasses CO 2 and CH 4 (also N 2 O) ffrom wetlands

Humus vs Composts Natural humus is colloidal and quite high in p. H-dependent charge, up to 150300 cmolc/kg on a weighht basis Composting is in increasingly popular way of generating a humus like product via accelerated self heating microbial decompositiion ◦ can generate a more stable N resource and less toxic prdt than the original precursor waste products (sludge, biosolidds, manure) ◦ Gets the C/N ratio down pretty rapidly.

Long term control of SOM level Any soil has an eqm level of SOM that is dictated by texture , climate, and current mgmt practice that limits the amt of SOM that it can retain over time Changes in any of the five soil forming factors causes a shift in SOM levels back to the eqm level ◦ Best mgmt practices inc. OM inputs and minimises excess OM decomosiition annd erosion losses ◦ Annual OM additions are critical for any soil ; to keep the orgsm fat and happy, improve soil quality and sustainability of yiields

Organic Matter Management Guidelines ◦ Plant growth should be encouraged for residues and N is required for organic matter accumulation. ◦ Conservation tillage reduces the rate of organic matter decomposition and residues at the soil surface reduce erosion The amount of org matter in any soil is controlled by the interaction of climate, vegetation, management, practices like tillage and crop removal, ddrainage, textture and overall levels of biotic activity As we change the inputs or levels of any of those factors, we shld expect a corresponding chang to the soil OM level

Use of Soil Quality 1) Match use and management of land to soil capability, because improper use of a soil can damage it and the ecosystem. 2) Establish a baseline understanding about soil quality so that we can recognize changes as they develop. 3) Use baselines to determine if soil quality is deteriorating, stable, or improving. Thus soil quality becomes a good indicator of the health of an ecosystem. Nature. Watch

Soil Quality Soil quality is the capacity of soils within landscapes to sustain biological productivity, maintain environmental quality, and promote plant and animal http: //www. directs eed. org/soil_qualit y. htm health. Protecting soil quality like protecting air quality and water quality should Poor Good be fundamental goal of our Nation’s Environmental Policy http: //www. nrsl. umd. edu/research/NRSLResearch. Area. Info. cfm? ID=14