TIME-COURSE OF THE SOIL MICROBIAL BIOMASS UNDER WHEAT - A ONE-YEAR FIELD-STUDY

The fumigation-extraction (FE) method, applied after extraction of inorganic N and sieving was used to measure soil microbial biomass C, N and carbohydrate C contents at a depth of 10 cm under wheat every week in a field study lasting from October 1990 to November 1991. A minimum of 180 mug microbial biomass C g-1 soil and a maximum of 363 mug g-1 were recorded, the corresponding values for biomass N being 35 mug g-1 soil (=53 kg ha-1) and 66 mug g-1 soil (=99 kg ha-1). An increase was observed from December 1990 to November 1992, with minor fluctuations during periods with strong variations in weather conditions: cold and very wet in winter, extremely dry in summer. All three biomass components measured in the 0.5 M K2SO4 extracts of the FE procedure were significantly correlated. The intercepts of the linear relationships between microbial biomass C, N and carbohydrate C were non-significant, indicating constant component ratios. The amount of carbon additionally made extractable by fumigation consisted of 17% carbohydrate C. The biomass C:N ratio fluctuated closely around a mean value of 5.5.