Importance of soil water content when estimating soil microbial C, N and P by the fumigation-extraction methods

Abstract

The influence of soil water content on the estimation of microbial C, N and P by the fumigation-extraction (FE) method and microbial C and N by the fumigation-incubation (FI) method was investigated using a range of air-dry soils. The estimates of microbial C were compared with those obtained by the substrate-induced respiration (SIR) method. Soils were fumigated overnight with CHCl 3 when either air-dry, or rewetted to 50% w/w water content immediately before fumigation. The presence of water during fumigation greatly increased the C and N extracted by 0.5 M K 2SO 4 compared with soils fumigated while air-dry. Overnight rewetting of non-fumigated (control) soils decreased extractable-C but the effect on extractable-N was variable. Rewetting before fumigation also increased CO 2-C production and net N-mineralization (during subsequent incubation) compared to soils fumigated while air dry. However, because of high variability the increases were often not significant. The flushes of extractable-C and N (the difference between fumigated and non-fumigated soils) were calculated in three ways. Comparison with the biomass C estimated on air-dry soils by the SIR method suggests the most appropriate way to estimate the flush is: flush = (extractable-C from wetted, fumigated soil) — (extractable-C from air-dry, non-fumigated soil). Estimates of microbial C varied by up to 5-fold depending on how the flush was calculated. The release of inorganic P (P 1) by fumigation of air-dry soil was generally increased by rewetting. Releases were greatest from rewetted soil fumigated with CHCl 3 vapour, lower when using liquid CHCl 3 and usually lowest when air-dry soil was fumigated with CHCl 3 vapour. On two soils, gradually air-dried in the laboratory, the estimates of microbial C by the FE method were affected by rewetting (to 50% w/w water content) once the soils had dried below 20% w/w water content, and the rewetting effects were highly significant at < 10% w/w water content. To estimate microbial biomass C, N and P of air-dry or moisture-stressed soils when using the FE or FI methods we recommend the soils be rewetted immediately before fumigation.