Seasonal fluctuations in gross N mineralisation, ammonium consumption, and microbial biomass ina Western Australian soil under different land uses

Abstract

A field experiment was conducted to study the seasonal variation in gross N mineralisation, NH₄⁺ consumption (immobilisation and nitriflcation), potentially available N, and microbial biomass-N.Measurements were made during the wheat growing season in Western Australia under continuouswheat, during the wheat phase of a 1 year lupin : 1 year wheat rotation, during the wheat phaseof a 2 year pasture : 1 year wheat rotation, and under a subterranean clover pasture. The accuracyof gross N mineralisation and NH₄⁺ consumption within intact soil cores was reduced by the largespatial variation in the size of the soil NH₄⁺ pool. Calculated daily rates of gross N mineralisation inthe 0-5 cm soil layer ranged from 0·0 to 1·0 kg N/ha·day in the continuous wheat, 0·1 to 0·8 kgN/ha·day in the lupin{wheat rotation,- 0·1 to 1·3 kg N/ha·day in the pasture-wheat rotation, and-0·1 to 2·5 kg N/ha·day in the pasture treatment. Gross N mineralisation in the 5-10 cm soil layerunder wheat followed the same range observed in the 0-5 cm layer; in continuous pasture, lower rates were measured in the 5-10 cm layer compared with 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 mineralisation,precluding accumulation of NH₄⁺ in soil when considered over a season. Gross N mineralised in the0-10 cm soil layer was equivalent to 10-19% of the total soil N in this layer. Net N mineralised,determined from the difierence between gross N mineralisation and gross immobilisation, was estimatedto be about half of the gross N mineralised during the wheat growing season. Plant uptake wasestimated to be 13-37% of the total gross N mineralised (0-10 cm) during the field season and wasgreater in the wheat after legume compared with continuous wheat. Potentially available N, measured by anaerobic incubation, declined 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 kgN/ha in the lupin-wheat rotation, 73 kg N/ha in the pasture-wheat rotation, and 99 kg N/ha underpasture. Only half of these quantities of microbial biomass were detected by the end of the season. Microbial biomass-N was concentrated in the surface soil layer with <25 kg N/ha in the 5-10 cmsoil layer under each land use. A reasonable estimate of gross N mineralisation was obtained in the continuous wheat and legume-wheat rotations by using a simple empirical relationship based on thesize and activity of the microbial biomass, and functions to describe the efiect of temperature andwater on microbial activity. However, the pattern of gross N mineralisation in the pasture treatment could not be explained using this approach.