Evaluation and calibration of biochemical methods to measure microbial biomass C and N in soils from western australia

A comparison of the fumigation-incubation (FI), fumigation-extraction (FE), release of ninhydrin-positive compounds (NPC), and the substrate-induced respiration (SIR) methods to estimate soil microbial C and N, was made on a range of predominantly sandy, acid soils from Western Australia. The FI method appeared to underestimate microbial C when the C-flush was calculated using a non-fumigated control, whereas microbial C appeared to be overestimated when no correction was applied. Correction for basal respiration using a fumigated soil ‘control’ appears suitable for these soils, and gave similar estimates of microbial C to those obtained from the N-flush. The NPC method correlated well with the microbial biomass C and N estimated by the FI methods, but revised relationships: microbial C = 40(NPC) and microbial N = 6.5(NPC) are considered appropriate for fumigations of 24 h. An overall estimate of microbial C and N in each soil was obtained by combining the results from the FI and NPC methods. These combined estimates were used to calibrate the SIR method and to calculate k EC- and k EN-factors for the FE technique. Microbial C ranged from 50 to 421 μg g −1 and microbial N from 7 to 65 μg g −1. The SIR method showed reasonable agreement with the other techniques and a revised calibration of Microbial C = 23.5( μ1 CO 2g −1soil h −1) was obtained for the SIR response on these acid sandy soils when estimated by infra-red gas analysis. The FE methods showed greter variability than the FI or NPC methods, with the k EC and k EN-factors differing between soil types. Overall k EC and k EN-factors were 0.30 ± 0.18 and 0.38 ± 0.14, respectively. The FE and NPC methods appear suitable to estimate microbial C and N in soils from Western Australia provided the appropriate conversion factors are obtained by cross-comparison with other techniques.