A rapid chloroform-fumigation extraction method for measuring soil microbial biomass carbon and nitrogen in flooded rice soils

A chloroform-fumigation extraction method with fumigation at atmospheric pressure (CFAP, without vacuum) was developed for measuring microbial biomass C (C-BIO) and N (N-BIO) in water-saturated rice soils. The method was tested in a series of laboratory experiments and compared with the standard chloroform-fumigation extraction (CFE, with vacuum). For both methods, there was little interference from living rice roots or changing soil water content (0.44-0.55 kg kg(-1) wet soil). A comparison of the two technique(s) showed a highly significant correlation for both CBIO and N-BIO (P<0.001) suggesting that the simple and rapid CFAP is a reliable alternative to the CFE. It appeared, however, that a small and relatively constant fraction of well-protected microbial biomass may only be lysed during fumigation under vacuum. Determinations of microbial C and N were highly reproducible for both methods, but neither fumigation technique generated N-BIO values which were positively correlated with C-BIO. The range of observed microbial C:N ratios of 4-15 was unexpectedly wide for anaerobic soil conditions. Evidence that this was related to inconsistencies in the release, degradation, and extractability of N-BIO rather than C-BIO came from the observation that increasing the fumigation time from 4 h to 48 h significantly increased N-BIO but not C-BIO The release pattern of C-BIO indicated that the standard fumigation time of 24 h is applicable to water-saturated rice soils. To correct: for the incomplete recovery of C-BIO, we suggest applying the k(C) factor of 2.64, commonly used for aerobic soils (Vance ct al. 1987), but caution is required when correcting N-BIO data. Until differences in fumigation efficiencies among CFE and CFAP are confirmed for a wider range of rice soils, we suggest applying the same correction factor for both methods.