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.