Soil microbial biomass estimated by the fumigation-incubation procedure: Seasonal fluctuations and influence of soil moisture content

Abstract

The effects of season and soil moisture content on fumigation flushes of CO 2-C and mineral-N. and the k c and k N values of five added organisms (three fungi and two bacteria), were determined by the fumigation-incubation procedure with four soils from introduced pastures. These properties all varied seasonally, with CO 2-C flush and k c values being negatively related to field-moisture content. Laboratory adjustment of soil moisture content (up to 60% of water-holding capacity) at the time of fumigation had generally no influence on flushes of CO 2-C and mineral-N, or k C and k N values. Values of these properties were. however, significantly lower in smeared and compacted wet soil. generally because of impeded mineralization in fumigated soil. Flushes ofCO 2-C and mineral-N, and k C and k N values, were also lowered by prior incubation of soil at 60% of water-holding capacity for 7 days at 25°C. “Fumigation” biomass C values, calculated with these seasonally determined k C factors, also varied seasonally, and were generally lowest in the wettest samples. The seasonal “fumigation” biomass C values were not related significantly to other concurrently-determined indices of microbial biomass and activity (viz. biomass C estimated by substrate-induced respiration, bacterial biomass C estimated by direct observation, and dehydrogenase activity), or to mineral-N flush values, which were all significantly inter-related. The fumigation-incubation procedure is, therefore, considered unreliable for determining microbial biomass C in wet compacted soils, and an alternative procedure is then needed.