Soil microbial biomass—what do the numbers really mean?

Abstract

Summary. Soil microbial biomass comprises less than 5% of organic matter in soil. However, it performs at least 3 critical functions in soil and the environment. It is a labile source of carbon, nitrogen, phosphorus, and sulfur; it is an immediate sink of carbon, nitrogen, phosphorus and sulfur; and it is an agent of nutrient transformation and pesticide degradation. In addition, microorganisms form symbiotic associations with roots, act as biological agents against plant pathogens, contribute towards soil aggregation, and participate in soil formation. Critical evaluation of the significance of soil microbial biomass is hampered by the reliable measurement of microbial biomass, and simultaneous partitioning of its 3 major functions in soil. For comparative purposes, soil microbial biomass and its derived indices have been successfully used to measure early changes induced by land use practices, zero tillage, crop rotations and other cultural practices, nutrient cycling, land disposal of sewage sludge, and applications of herbicides and insecticides. However, as a routine analytical tool, it is limited by the cumbersome and time consuming measurements, lack of benchmarking values and interpretation, ambiguous relationship with productivity, and cost-effectiveness. With increasing demand to monitor soil quality and protection of the environment, improved and rapid techniques, including molecular biotechniques, will be required to measure soil microbial biomass for its size of sink, source and rates of turnover. Eventually, agricultural science will benefit and utilise soil microbial biomass as an analytical tool to produce abundant, economical, and clean food and fibre.