Effect on Soil Microorganisms

Heterotrophic organisms in the soil are ultimately responsible for ensuring the availability of nutrients for primary production (Wardle 2002). Microorganisms play a very important role in many biogeochemical cycles in agroecosystems including organic matter decomposition, nutrient mineralization, and trace gas emission and consumption (Carney et al. 2004). The principal “players” in the decomposition process are microorganisms, i. e., bacteria, archaea, and fungi.

Bacteria are able to perform an extremely wide range of chemical transforma­tions, but are, however, only active over a very narrow range of environmental conditions (Lavelle and Spain 2001). As with all microorganisms, bacteria have a system of external digestion mediated through the production of extracell­ular enzymes, and some of the metabolites released by extracellular digestion may be used by other organisms, thus creating a trophic stimulus for opportunistic or cooperating microorganisms (Hattori 1973; Lavelle and Spain 2001). Until recently, archaea were considered to occur in extreme environments only, but their presence was also reported in numerous other habitats, including forest and agricultural soils, where their potential for ammonia oxidation was demonstrated (Bintrim et al. 1997; Pace 1997; Prosser and Nicol 2008). Bacteria and archaea, on the one hand, and fungi, on the other, differ biochemically and morphologically. Fungi are larger than bacteria and have hyphae that can grow into and explore distant microhabitats, and translocate carbon and nitrogen and other nutrients within the hyphal network. Thus, fungi are regarded as being more capable than bacteria and actinobacteria in degrading polysaccharides (Atlas and Bartha 1998; Lavelle and Spain 2001). The broad functions of fungal mycelium in soil and litter are decomposition and nutrient cycling. In contrast to bacteria, fungi can remain active in soils at very low water potential (—7,200 kPa) and are better suited than bacteria to exist in interpore spaces (Shipton 1986). These microorganisms influ­ence or control ecosystem processes and form mycorrhizal interactions with plants (Coleman 2001; Wardle 2002).

Soil microbial community diversity has been suggested as a way of assessing the “health” or “quality” of soils (Chapman et al. 2007). High biodiversity may be vitally important in structurally diverse ecosystems such as soil because it may promote productivity and stability of this environment (Grime 1997; van Bruggen and Semenov 2000). The biodiversity of fungal or bacterial populations in the rhizosphere is closely related to growth of crops; hence, crop yield may be used as an indicator of soil health associated with greater stability in productivity (Lynch et al. 2004).

The effect of different composts on the microbial biomass and diversity depends in part on the amount used and very strongly on the compost quality (Ros et al. 2006). Populations of rhizosphere microorganisms were reported to increase in relation to increasing inputs of composted organic matter to soil, and compost application has been found to enhance biomass nitrogen, carbon, and sulfur content and microbial activity over several years (Perucci 1990; Ros et al. 2006). Single and repetitive applications of different amounts of organic wastes signifi­cantly increase the amount of soil microbial biomass and enhance nitrogen mineralization potential, but excessive rates of application (100 t ha—1) reduce the functional diversity of the microbial community (Banerjee et al. 1997). Several studies have reported modification of both bacterial and fungal community structure following application of compost (Crecchio et al. 2004; Ros et al. 2006; Innerebner et al. 2006). Increases in dehydrogenase, p-glucosidase, urease, nitrate reductase, and phosphatase activities were observed 3 months after application of municipal solid waste compost (Crecchio et al. 2004). Some composts rich in heavy metals (Zn, Cu, and Pb) have been reported to decrease enzyme (phosphatase and urease) activities, whereas other enzymes (dehydrogenase, catalase, protease) were not affected (Garcia-Gil et al. 2000).