Ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter

The sensitivity of soil organic carbon (Corg) and microbial carbon (Cmic) measurements, and the Cm~,/Co,, ratio, to reflect climatic, vegetation, cropping and management history was investigated using a range of topsoils in New Zealand. The Cmic generally comprised 1-4% of Corg, with the proportion being consistently greater under pastures, than the equivalent soil under native forest, exotic forest or arable cropping. However, absolute values differed markedly between soils and were greatly influenced by texture, mineralogy and the Corg content. The Cmic recovered more rapidly than Corg on returning to pasture following cropping. There was a generally greater Corg content in those soils from the areas with higher precipitation, but the precipitation-evaporation quotient proposed by Insam et al. (Soil Biol. Biochem. 1989, 21, 211-21) to predict the relationship between Cmic and Corg, greatly underestimated the Cmic content of New Zealand soils and there was too great a scatter in the data to derive a revised regression formula. The Cmic and the Cmic/Cor, ratio are useful measures to monitor soil organic matter and both provide a more sensitive index than COrg measured alone. However, under typical climatic and land use conditions in New Zealand, the values do not appear readily transferrable between soils. To ascertain whether a soil has achieved equilibrium in organic matter status, it will be necessary to establish reference values to which a tested soil can be compared.