Microbial Indicators of Soil Quality in Upland Soils

Abstract

The concept of soil quality is very difficult to satisfactorily define. Possibly a definition based upon the suitability of a soil to perform the purpose for which it is required, i.e. function, while having limitations, is suitable in most circumstances. Certainly, some of the most important determinants of soil quality include the ability of a soil to decompose plant and animal residues, maintain adequate nutrient and soil organic matter pools and to act as a filtering system to supply pure water to our rivers, lakes and ground waters. The activities of the soil micro-organisms (i.e. the fungi, bacteria, protozoa, yeasts etc.) drive these processes. Any factors which inhibit their activities therefore have large effects on soil fertility, and therefore soil quality. Linking the activities of one, or at best a few, species of soil micro-organism to soil quality is normally an impractical proposition. Another problem is that soil microbial activity, resulting for example, in carbon dioxide evolution from soil, is the net result of complex substrate-microbial interactions, again involving many different microbial populations. To overcome this, we have developed a number of methods to measure the soil micro-organisms as a single, undifferentiated, unit, or ‘black box’ – termed the microbial biomass. Other methods can indicate the activity of this biomass. While having obvious limitations, this approach has enabled us to detect the direction of change (i.e. decrease or increases) in soil organic matter and in soil ecosystem functioning due, for example to increases in heavy metal concentrations, changes in pH or to inputs of crop residues. The value of the microbial biomass as an indicator, or ‘early warning’ of changing soil conditions, or changing soil quality, will be illustrated and discussed.