Measurement of microbial biomass in rewetted air-dried soil by fumigation-incubation and fumigation-extraction techniques

Abstract

The quantity of microbial biomass was measured in a range of soils by the fumigation-incubation (FI) and fumigation-extraction (FE) techniques. The air-dried soils were rewetted and incubated before the measurements were made. The FE method for biomass C was calibrated from the relationship between the flush of extractable organic C ( E c ) and microbial-C, as estimated by the FI technique. In the case of N, the size of the flush of total extractable N ( E N ) was compared to the flush of mineralization N ( F N ) by FI. Two different soils were chosen for further calibration by labelling ( 14C) microorganisms in situ ( k EC factor). Biomass C (FE) accounted for 1% of soil C in the finer soils and only 0.4% of soil C in the coarser soils. Biomass content was related to soil texture, presumably because texture influences the ability of soils to preserve microbial biomass. The FI method appeared to underestimate biomass C (relative values) in the coarser soils. The FE technique gave higher biomass C estimates in these soils. The relationship between the flush ( E C ) and soil biomass C (FI) forced through zero was: biomass C = (2.42 ± 0.164) E C (r = 0.90) . Somewhat different results were obtained when the FE method was calibrated by in situ labelling. A weaker relationship was obtained for the flush ( E N ) and the flush ( F N ) by FI ( r = 0.80).