A rapid chloroform-fumigation extraction method for measuring soil microbial biomass carbon and nitrogen in flooded rice soils |
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Authors: | C Witt J L Gaunt C C Galicia J C G Ottow H-U Neue |
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Institution: | International Rice Research Institute (IRRI), Soil and Water Sciences Division, MCPO Box 3127, 1271 Makati City, The Philippines e-mail: c.witt@cgiar.org, PH IACR-Rothamsted, Harpenden, Herts, AL5 2JQ, UK, GB Institut für Angewandte Mikrobiologie, Justus-Liebig-Universit?t Giessen, Senckenbergstrasse 3, D-35390 Giessen, Germany, DE UfZ-Center for Environmental Research, Department of Soil Sciences, Hallesche Strasse 44, D-06246 Bad Lauchstaedt, Germany, DE
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Abstract: | A chloroform-fumigation extraction method with fumigation at atmospheric pressure (CFAP, without vacuum) was developed for
measuring microbial biomass C (CBIO) and N (NBIO) 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 techniques showed a highly significant correlation for both CBIO and NBIO (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 NBIO values which were positively correlated with CBIO. 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 NBIO rather than CBIO came from the observation that increasing the fumigation time from 4 h to 48 h significantly increased NBIO but not CBIO. The release pattern of CBIO indicated that the standard fumigation time of 24 h is applicable to water-saturated rice soils. To correct for the incomplete
recovery of CBIO, we suggest applying the k
C factor of 2.64, commonly used for aerobic soils (Vance et al. 1987), but caution is required when correcting NBIO 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.
Received: 1 June 1999 |
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Keywords: | Microbial biomass Fumigation extraction method Flooded soil Lowland rice Microbial C : N ratio |
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