Fungi are an important class of decomposers due to their abundance and ability to decompose rather recalcitrant organic material. In fact, fungi are able to secrete enzymes that are capable of breaking down virtually all classes of plant compounds. Thus, fungi can decompose substrates such as fresh plant litter and some structural materials (e.g., lignin, chitin, and keratin) that are initially almost inaccessible to other decomposers. Moreover, fungi account for a large fraction of the soil microbial biomass, as they contribute to about 60-90% of the microbial biomass in forest soils and to 50% in grassland soils. Fungi have extensive hyphae networks, which make it possible to acquire carbon (e.g., from forest litter) and nutrients (e.g., from mineral soil) from different locations. Mycorrhizae are symbiotic associations between plant roots and fungi. Based on conservative estimates, approximately 95% of all vascular plant species have the potential to form this mutualistic association with mycor-rhizal fungi. Approximately 70% of all plant families include species, which develop specialized endomycorrhizae called vesicular arbuscular mycorrhizae (VAM) or just arbuscular mycorrhizae (AM). In these associations, plants receive nutrients from fungi - especially the less mobile groups (e.g., phosphates), while providing, in return, fungi with carbohydrates. Mycorrhizal fungi play a role in the decomposition process by breaking down proteins into amino acids. Under certain conditions, mycorrhizal fungi have been found to turn into aggressive decomposers capable of decomposing humus that used to be considered stabilized.

Bacteria are another major group of decomposers. Like fungi, bacteria spores are ubiquitous in air, water, and both dead and live organic matter. There is a wide range of types of soil bacteria. Recent studies have shown that bacteria are able to degrade cellulose/hemi-cellulose, lignin, and even intact fiber walls. Due to their small size and large surface to volume ratio, bacteria are able to rapidly absorb soluble substrates and to reproduce quickly in substrate-rich conditions. In substrate-rich environments such as the rhizosphere or dead animal carcasses, bacteria tend to undergo population 'explosions' and thereby become the dominant decomposers. These populations collapse as the freely available resources are consumed. Bacteria decomposition seems to be more common in situations where fungi are under stress. Bacteria have also been found to degrade substrates resistant to fungal decay.

Soil fauna can be classified into three categories based on the size: microfauna (less than 0.1 mm), mesofauna (between 100 mm and 2 mm), and macrofauna (between 2 mm and 20 mm), though some researchers adopt slightly different size criteria. Soil fauna used to be considered an important contributor to litter decomposition; however it was later recognized that, soil microorganisms such as fungi and bacteria are the dominant functional groups of decomposers. In fact, complex organic polymers, such as lignin, can be degraded exclusively by these microorganisms. Soil fauna affect decomposition through the processes of litter fragmentation, bacteria/fungi grazing, and soil structure alteration. They graze either directly on microorganisms or on dead organic matter inhabited by bacteria and fungi. Moreover, soil fauna spread the populations and increase the turnover rate of microbial communities, thus enhancing the rates of organic matter decomposition. Therefore, despite its limited direct participation in the decomposition process, the overall influence of soil fauna on the turnover of soil organic matter should not be underestimated especially in warm environments.

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