Substrate quality and quantity
Substrate quantity and quality are major biotic factors controlling the rates of decomposition. Litter quality is determined by three general characteristics: (1) the type of chemical bonds present in the organic compounds, (2) the amount of energy released by their decay, and (3) the size and structure of these compounds and their nutrient content. Glucose and other simple sugars have high carbon quality for microbial decomposer, followed by cellulose and hemicellulose. Lignin, a structural compound second only to cellulose in quantitative importance in most plant tissues, has a content ranging from 2% to more than 50% in plant dry weight. These polyphenols dramatically slow the decomposition and mineralization rates. The quality of organic matter is generally expressed (especially in biogeo-chemical models) in terms of the C/N ratios of litter, soil organic matter, and microbial biomass, though the mechanistic role of C/N ratios in the decomposition and mineralization processes is still not completely understood. Because microbes are generally more N limited than carbon limited, lower C/N ratios in plant residues usually lead to higher decomposition rates. The C/N ratios of plant residues range from 10:1 to 100:1. The C/N ratios of soil organic matter remain rather constant with a typical value of 10. The C/N ratios of microbial biomass range from 5 (e.g., tropical arable soil) to 10 (e.g., tropical dry forest). Microbial biomass is the most readily decomposable pool of organic material due to the simple structure and high quality of both carbon and nutrients. The microbial decomposition rates are followed by those of plant litter and soil organic matter. In most systems, if the C/N ratio of soil organic matter exceeds about 25:1, net N immobilization occurs instead of net mineralization.
The composition of microbial communities is another important factor determining the rates of decomposition due to the different types and rates of enzyme production in different microbial communities. These enzymes are major players in breakdown of different classes of substrates. In addition, different microbial decomposers have different tolerance to soil moisture and temperature conditions, with consequent effects on the rates of decomposition and mineralization. For example, because fungi are usually less sensitive to water stress than bacteria, they may play a more important role in organic matter decomposition in arid and semiarid environments.
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