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 (C(org)) and microbial carbon (C(mic)) measurements, and the C(mic)/C(org) ratio, to reflect climatic, vegetation, cropping and management history was investigated using a range of topsoils in New Zealand. The C(mic) generally comprised 1-4% of C(org), 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 C(org) content. The C(mic) recovered more rapidly than C(org) on returning to pasture following cropping. There was a generally greater C(org) 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 C(mic), and C(org), greatly underestimated the C(mic) content of New Zealand soils and there was too great a scatter in the data to derive a revised regression formula. The C(mic) and the C(mic)/C(org) ratio are useful measures to monitor soil organic matter and both provide a more sensitive index than C(org) measured alone. However, under typical climatic and land use conditions in New Zealand, the values do not appear readily transferable 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.