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RESEARCH ARTICLE

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

D. V. Murphy, G. P. Sparling and I. R. P. Fillery

Australian Journal of Agricultural Research 49(3) 523 - 536
Published: 1998

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.

Keywords: N transformations, legume rotation, wheat N

https://doi.org/10.1071/A97096

© CSIRO 1998

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