EVALUATION AND CALIBRATION OF BIOCHEMICAL METHODS TO MEASURE MICROBIAL BIOMASS-C AND BIOMASS-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 A ustralia. The FI method appeared to underestimate microbial C when the C-flush was calculated using a non-fumigated control, whereas microbi al C appeared to be overestimated when no correction was applied. Corr ection 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 Fl methods, but revised relationships: microbial C = 40(NPC) and microbial N = 6.5(NPC) are c onsidered appropriate for fumigations of 24 h. An overall estimate of microbial C and N in each soil was obtained by combining the results f rom the FI and NPC methods. These combined estimates were used to cali brate the SIR method and to calculate k(EC)- and k(EN)-factors for the FE technique. Microbial C ranged from 50 to 421 mug g-1 and microbial N from 7 to 65 mug g-1. The SIR method showed reasonable agreement wi th the other techniques and a revised calibration of Microbial C = 23. 5(mul CO2 g-1 soil h-1) was obtained for the SIR response on these aci d sandy soils when estimated by infra-red gas analysis. The FE methods showed greater variability than the Fl or NPC methods, with the k(EC) and k(EN)-factors differing between soil types. Overall k(EC) and k(E N)-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 ar e obtained by cross-comparison with other techniques.