AN EVALUATION OF METHODS FOR MEASURING THE MICROBIAL BIOMASS IN SOILS FOLLOWING RECENT ADDITIONS OF WHEAT STRAW AND THE CHARACTERIZATION OF THE BIOMASS THAT DEVELOPS

Four methods for measuring soil microbial biomass in soils containing recently-added wheat straw were tested: fumigation-incubation (FI). substrate-induced respiration (SIR), fumigation-extraction (FE) and soil adenosine 5'-triphosphate (ATP). The measurements were made at 13 and 35 days after addition of wheat straw (2% w/w) to a sandy loam (10.4% clay) and a non-calcareous clay (39% clay) soil. FI was unsuitable for measuring soil biomass C and N in straw-amended soils-in some cases, negative biomasses were measured. Biomass C measured by SIR was closely correlated with ATP but the intercept of the regression equation was significantly different from zero because of soil type, not straw amendment. Biomass C, N and ninhydrin N (all measured by FE) and ATP were all closely linearly correlated, irrespective of whether the soils were amended and with intercepts not significantly different from zero. It was concluded that biomass could be reliably estimated by cither FE or ATP in soils which have recently received decomposable substrates (e.g. straw). Straw addition roughly doubled the amount of biomass in the sandy loam soil (from ca 160 to 300μCg-1 soil) and increased it by ca 50% in the clay soil (from ca 620 910μg Cg-1 soil). The new biomass that developed had the same ATP concentration, C/N ratio and biomass C/ninhydrin N ratio as the original biomass. Meancd over the two soils and two treatments, the biomass had an ATP concentration of 9.2μmol ATPg-1 biomass C, a C/N ratio of 6.8 and a biomass C/ninhydrin N ratio of 31, all values similar to those that have been reported for unamended soils. The biomass of the sandy soil, whether unamended or not, had a higher specific respiration rate (mg CO2-C evolved g-1 biomass C day-1) than that in the clay soil, despite the similarity in the ATP concentration of the biomass in the two soils. Most of the increase in biomass following straw addition was found in the soil and not in the straw, although the concentration of biomass (nmol ATP g-1) was higher in the straw. © 1990.