SEASONAL FLUCTUATIONS IN GROSS N MINERALIZATION, AMMONIUM CONSUMPTION, AND MICROBIAL BIOMASS IN A WESTERN-AUSTRALIAN SOIL UNDER DIFFERENT LAND USES

A field experiment was conducted to study the seasonal variation in gr oss N mineralisation, NH4+ consumption (immobilisation and nitrificati on), potentially available N, and microbial biomass-N. Measurements we re made during the wheat growing season in Western Australia under con tinuous wheat, during the wheat phase of a 1 year lupin:1 year wheat r otation, during the wheat phase of a 2 year pasture:1 year wheat rotat ion, and under a subterranean clover pasture. The accuracy of gross N mineralisation and NH4+ consumption within intact soil cores was reduc ed by the large spatial variation in the size of the soil NH4+ pool. C alculated daily rates of gross N mineralisation in the 0-5 cm soil lay er ranged from 0.0 to 1.0 kg N/ha.day in the continuous wheat, 0.1 to 0.8 kg N/ha.day in the lupin-wheat rotation, -0.1 to 1.3 kg N/ha.day i n the pasture-wheat rotation, and -0.1 to 2.5 kg N/ha.day in the pastu re treatment. Gross N mineralisation in the 5-10 cm soil layer under w heat followed the same range observed in the 0-5 cm layer; in continuo us pasture, lower rates were measured in the 5-10 cm layer compared wi th the 0-5 cm layer. the range in daily rates of NH4+ consumption in a given treatment was similar to the range in daily rates of gross N mi neralisation, precluding accumulation of NH4+ in soil when considered over a season. Gross N mineralised in the 0-10 cm soil layer was equiv alent to 10-19% of the total soil N in this layer. Net N mineralised, determined from the difference between gross N mineralisation and gros s immobilisation, was estimated to be about half of the gross N minera lised during the wheat growing season. Plant uptake was estimated to b e 13-37% of the total gross N mineralised (0-10 cm) during the field s eason and was greater in the wheat after legume compared with continuo us wheat. Potentially available N, measured by anaerobic incubation, d eclined by about one-third during the season. At the beginning of the season, microbial biomass-N in the 0-5 cm soil layer contained 61 kg N /ha in continuous wheat, 68 kg N/ha in the lupin-wheat rotation, 73 kg N/ha in the pasture-wheat rotation, and 99 kg N/ha under pasture. Onl y half of these quantities of microbial biomass were detected by the e nd of the season. Microbial biomass-N was concentrated in the surface soil layer with <25 kg N/ha in the 5-10 cm soil layer under each land use. A reasonable estimate of gross N mineralisation was obtained in t he continuous wheat and legume-wheat rotations by using a simple empir ical relationship based on the size and activity of the microbial biom ass, and functions to describe the effect of temperature and water on microbial activity. However, the pattern of gross N mineralisation in the pasture treatment could not be explained using this approach.