SOIL MICROBIAL BIOMASS ESTIMATED BY THE FUMIGATION INCUBATION PROCEDURE - SEASONAL FLUCTUATIONS AND INFLUENCE OF SOIL-MOISTURE CONTENT

The effects of season and soil moisture content on fumigation flushes of CO2-C and mineral-N, and the kC and kN values of five added organisms (three fungi and two bacteria), [Fusarium oxysporum, Penicillium novae-zeelandiae, Agaricus bisporus, Bacillus sp., and Pseudomonas sp.] were determined by the fumigation-incubation procedure with four soils from introduced pastures. These properties all varied seasonally, with CO2-C flush and kC 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 CO2-C and mineral-N, or kC and kN values. Values of these properties were, however, significantly lower in smeared and compacted wet soil, generally because of impeded mineraliztion in fumigated soil. Flushes of CO2-C and mineral-N, and kC and Kn values, were also lowered by prior incubation of soil at 60% of water-holding capacity for 7 days at 25.degree. C. "Fumigation" biomass C values, calculated with these seasonally determined kc 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.