Factors Affecting Organic Matter Decomposition
The following are the major factors affecting organic matter decomposition;
1. Climate
Climate, i.e. precipitation and temperature, influence the amount and type of vegetation as well as the rate of decomposition. The organic matter content of the soil increases with increasing decomposition up to the limit set by the temperature.
In soils, every 100C increase in mean annual temperature results in the organic matter content being reduced by about 1/3 to 1/2, if all other factors are constant.
Generally, the cold and arid climate tends to slow down the microbial processes within the soil and thus decomposition and mineralization. Also, the warmer the climate, the higher the rates of microbial process, i.e., the lower the organic matter content in those soils.
The soil moisture content also has a remarkable effect on soil organic matter decomposition and accumulation. Waterlogged soils tend to accumulate organic matter because the microbial processes, in particular decomposition and mineralization, are slowed down.
In aquic moisture regimes, the drainage and soil aeration is poor thereby creating anaerobic conditions. Anaerobic oxidation of organic residues is less efficient than aerobic oxidation. If organic matter is accumulated the soil development is towards organic soils (Histosols).
2. Vegetation
The rate of soil organic matter accumulation depends largely on the quantity and quality of organic matter input. Under tropical conditions, applications of readily degradable materials with low C: N ratios, such as green manure and leguminous cover crops, favor decomposition and a short-term increase in the labile nitrogen pool during the growing season.
On the other hand, applications of plant materials with both large C: N ratios and lignin contents such as cereal straw and grasses generally favor nutrient immobilization, organic matter accumulation, and humus formation, with increased potential for improved soil structure development.
Generally, when the C: N ratio is > 25, net immobilization occurs, whereas at ratios < 25 net mineralization is likely.
Read Also: Components of Soil Organic Matter
3. Soil Structure
Soil organic matter tends to increase as the clay content increases. This increase depends on two mechanisms. First, bonds between the surface of clay particles and organic matter retard the decomposition process. Second, soils with higher clay content increase the potential for aggregate formation.
Macro aggregates physically protect organic matter molecules from further mineralization caused by microbial attacks (Rice, 2002).
For example, when earthworm casts and the large soil particles they contain are split by the joint action of several factors (climate, plant growth, and other organisms), nutrients are released and made available to other components of soil microorganisms.
4. Salinity and Acidity
Salinity, toxicity, and extremes in soil pH (acid or alkaline) result in poor biomass production and, thus reduced additions of organic matter to the soil.
For example, pH affects humus formation in two ways: decomposition and biomass production. In strongly acid or highly alkaline soils, the growing conditions for microorganisms are poor, resulting in low levels of biological oxidation of organic matter (Primavesi, 1984).
Soil acidity also influences the availability of plant nutrients and thus regulates indirectly biomass production and the available food for soil biota. Fungi are less sensitive than bacteria to acid soil conditions.
5. Topography
Organic matter accumulation is often favored at the bottom of hills. There are two reasons for this accumulation: conditions are wetter than at mid- or upper-slope positions, and organic matter is transported to the lowest point in the landscape through runoff and erosion.
Similarly, soil organic matter levels are higher on north-facing slopes (in the Northern Hemisphere) compared with south-facing slopes (and the other way around in the Southern Hemisphere) because temperatures are lower (Quideau, 2002).
In conclusion, soil organic matter plays a vital role in nutrient retention and availability in the soil. This is a result of the decomposition of plant and animal matter brought about by soil organisms, the physical environment, and the quality of the organic residue. There are several factors, however, that affect the decomposition of the organic residue in the soil.
Read Also: Organic Matter Decomposition in Soil
