Impact of microorganisms on chemical transformations in soil

Abstract

Microorganisms (e.g. bacteria, fungi, actinomycetes, microalgae) play a key role in organic matter decomposition, nutrient cycling and other chemical transformations in soil. In fact general measurements of microbial activity in soil are synonymous with the breakdown of organic matter. Decomposition of organic matter is usually controlled by heterotrophic microorganisms and leads to the release and cycling of nutrients (especially nitrogen (N), sulphur (S) and phosphorus (P)). Microorganisms also immobilise significant amounts of carbon (C) and other nutrients within their cells. The total mass of living microorganisms (the microbial biomass) therefore has a central role as source, sink and regulator of the transformations of energy and nutrients in soil (Table 1). The vast diversity of microbial species, and their ability to break a wide range of chemical bonds, means that they are responsible for many key soil functions including: i) Decomposition of soil organic matter and plant/animal residues with subsequent release of nutrients. ii) Transformation of compounds between chemical forms; often leading to the formation of more reactive or gaseous compounds which can be lost from the soil. iii) Degradation of synthetic compounds such as pesticides and herbicides. iv) Production of antibiotics, which can aid the suppression of soil borne diseases. v) Production of soil cementing agents, which may aid aggregation. vi) Production and degradation of hydrophobic waxy compounds which can lead to water repellence. vii) Plant nutrient acquisition through symbiotic associations (see Chapter 6 on rhizobia and Chapter 7 on mycorrhizas). (table 1 presented) Land management practices have considerable impact on the size and dynamics of microbial populations. Intensification of agriculture has focussed on the use of chemical and mechanical inputs, often at the expense of biologically mediated processes. However, even in fertilised systems, microbial processes can play an important role in nutrient supply to plants (Table 2). Where purchased inputs are either costly or unobtainable, microorganisms have a critical role in maintaining soil fertility and crop health (Giller et al. 1997). This chapter therefore examines our fundamental understanding of how agricultural management practices and soil amendments influence soil microbial biomass and its activity, and consequently key chemical transformations in soil. (table 2 presented). © 2007 Springer.