Decomposition

Decomposition Breakdown of organic matter, recycles nutrients. Important for recycling of C and energy, as well as all nutrient elements (N, P, K, etc.). Determines productivity and health of plants.

Biotic Decomposition Organisms most directly responsible for decomposition are bacteria and fungi.  Biotic processes: Aerobic respiration OM + O2  CO2, organic products. Anaerobic respiration Use of acceptors other than oxygen. Facultative or obligate anaerobes OM + CO2  CH4 + organic products.

Sequence – Biotic Decomposition Humus Inorganic compounds Living organisms Dead organisms Litter OM Detritus

Biotic decomposition Many organisms are very beneficial in ecosystems, participate in biotic decomposition.

Sequence – Biotic Decomposition Living organisms Dead organisms Litter OM Detritus Humus Inorganic compounds Soil Animals Bacteria, fungi Soil chemistry, mineralization

Sequence of Events in Decomposition Animals speed decomposition: Break litter into small pieces. Add products (excretion) to stimulate microbial growth. Feeding on fungi and bacteria often increases growth rate of these decomposers, since new surface area is exposed to decomposition. Predators regulate densities of animals that consume bacteria and fungi. Move decomposers (bacteria, fungi) to new locations.

Measuring Decomposition Litter bag method.

Decomposition Rates Decomposition rates vary depending on: Climate and temperature Soil organisms present Aeration of the soil Composition of the material (C:N ratio) Etc.– season, tillage

Decomposition Rates Decomposition rates vary depending on: Climate and temperature Soil organisms present Aeration of the soil Composition of the material (C:N ratio) Etc.– season, tillage Temperature affects decomposition rates (heat units). Less Organic Matter in warm climates.

Decomposition Rates Decomposition rates vary depending on: Climate and temperature Soil organisms present Aeration of the soil Composition of the material (C:N ratio) Etc.– season, tillage

Decomposition Rates Decomposition rates vary depending on: Climate and temperature Soil organisms present Aeration of the soil Composition of the material (C:N ratio) Etc.– season, tillage Season: spring = fresh green material; fall = tough residues. Tillage: aerates soil.

Decomposition Rates Decomposition rates vary depending on: Climate and temperature Soil organisms present Aeration of the soil Composition of the material (C:N ratio) Etc.– season, tillage

C:N ratio Most important factor influencing the rate of decomposition of OM. Ratio of amount of carbon to the amount of nitrogen in a substance. Slower decomposition if C:N ratio is high. Examples of C:N ratios…..

C:N Ratios of Selected Materials 20:1 for crops

Importance of C:N ratio With residues with a C:N ratio that is high, there is competition for the limited N available. If C:N ratio is high: Only some organisms can decompose (some types of bacteria and fungi, protozoan symbionts of termites). They use up N quickly, so N becomes tied up and unavailable (immobilized).

Adding organic N to soil Organic N materials decompose, release nitrate. Uptake, leaching, denitrification

Adding organic N to soil Organic N materials decompose, release nitrate. Nitrate used in decomposition process by bacteria, etc.

Adding organic N to soil Low C:N ratio material Decomposition and release of nitrate is fast, levels recover quickly.

Adding organic N to soil High C:N ratio material Nitrate release difficult, tied up for a long time. N rob = immobility of N in presence of residues with high C:N ratios.

Importance of C:N Ratio Bacteria free up N Organic N Inorganic N Mineralization Immobilization Bacteria take scarce N for themselves ! Nrob

Risky Mix of High and Low C:N High C:N residue + Inorganic N fertilizer = N from fertilizer immobilized by bacteria.

Importance of C:N ratio As C:N ratio goes lower (as it does during decomposition), more organisms (especially fungi, different bacteria types, even some insects) can join in, so decomposition proceeds much faster. Most insects and other animals join later in the process, as the C:N ratio of their food is low: E.g., fungi 10:1, decomposed animals 6:1, nematodes 10:1.

Composting Accelerates decomposition process. Reduces C:N ratio. High temperatures – 60-80oC (140-175oF). Thermophilic bacteria. Many recipes available.

Practical Application Farmer incorporates 2560 kg of OM to soil. 55% Carbon, C:N ratio = 20:1. Will there be any available for crop or all immobilized? 2560 X 0.55 = 1408 kg C 1408 / 20 = 70 kg N Microbes use 75% of C and incorporate 25% in new tissue. C:N ratio of microbe is 8:1, so they require 1 kg N to incorporate 8 kg C in new tissue. 1408 / 0.25 = 352 kg C used in new tissue. 352 / 8 = 44 kg N used in new tissue. 70 – 44 = 26 kg N leftover for uptake by plants.

References Text, pp. 68-73. Brady and Weil, 1996. The Nature and Properties of Soils. Prentice Hall, Upper Saddle River, NJ. Odum, 1983.