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Decomposition | Forms and Function | Management of Soil Organic Matter | Carbon Cycle

Soil organic matter decomposition is a key part of the soil organic carbon cycle. Green plant material is the initial source of organic matter, and when it dies it is progressively decomposed by a range of organisms. Decomposition is brought about by relatively complex biological processes.

Soil organisms continually decompose organic matter from one form to another. In consuming and utilising the organic matter they create by-products, wastes and cell tissue for other organisms to use. Eventually as the energy contained within the progressively decomposed organic matter becomes harder and harder to extract, it accumulates as soil humus and plays its role in stabilising soil structure.

Freshly added organic matter residue dominates at the surface where decomposition and incorporation is just beginning. Subsequent decomposition by soil organisms distributes by-products through the soil and can increases the accumulation of humus.

Decomposition results in:

  • Breakdown of organic residues -
    Soil organisms breakdown organic matter to obtain energy and nutrients along with the release of carbon dioxide (respiration). In doing so they progressively convert the organic matter to simpler materials. These simpler materials may be available for further breakdown, or they may become humus
  • Nutrient mineralisation -
    Soil organisms in breaking down organic material will produce water soluble compounds and nutrients
  • Transfer of organic carbon and nutrients between organic matter types -
    Soil function is much dependant on the incorporation of plant materials and the maintenance of the energy flow into the soil and the maintenance of humus content
  • Release of carbon dioxide via respiration -
    Respiration converts sugars to carbon dioxide and water with the subsequent release of useable energy

Decomposition rates
The level of soil carbon in soil is determined by the balance between inputs of organic matter, and its subsequent rate of decomposition and loss.
Rates of decomposition are affected by:

1. Soil practices
  • Tillage type and frequency
  • Species choice (C:N ratio)
  • Stubble handling
  • Manure inputs
2. Environment
  • Temperature
  • Rainfall (frequency, intensity and amount)
  • Water and atmospheric balances – aerobic or anaerobic systems
3. Soil characteristics
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