Clay soils previously under native brigalow
(Acacia harpophylla) forest are highly productive under
annual cropping in central and southern Queensland. Grass pastures sown on
these soils are initially productive, but deteriorate after several years
because of N-stress (rundown). The aim of this work was to compare the
patterns of N cycling in these pasture and cropping systems, in order to
understand the rundown of the pastures. A small pulse of
15N-labelled ammonium sulfate was applied in the field
to sites cropped with sorghum (Sorghum bicolor) and
under green panic (Panicum maximum var.
trichoglume) pasture, and its movement through the soil
and plant pools was followed over 2 growing seasons.
There were large differences in the cycling of 15N in
the cropping and pasture systems. Under sorghum, 60% of the applied
15N was immobilised by microorganisms after 4 days,
after which it was re-mineralised. Plant uptake and stabilisation in soil
organic matter and clay were relatively slow. The first sorghum crop
assimilated 14% of the applied 15N. During the
second season, most of the 15N was stabilised in soil
organic matter and clay (maximum 42%). A significant proportion of the
15N remained in the soil inorganic pool over the 2
seasons.
Under green panic, 82% of the 15N left the soil
inorganic pool within 4 days and entered the microbial biomass, soil organic
matter, and the plant. Uptake and re-release of 15N were
most rapid in the microbial biomass (maximum uptake 34% of applied
after 4 days). Microbial immobilisation and re-mineralisation were, however,
slower under green panic than under sorghum. The pasture plant accumulated
32% of the applied 15N, two-thirds of which was
re-released in the second season. Stabilised N represented up to 62% of
the applied 15N, and was consistently greater under
green panic than under sorghum. After 2 seasons, 15N was
released from the stabilised N pool in both systems, at approximately the same
rate as it had been stabilised. At the end of the experiment, 40% of
the applied 15N was unaccounted for in the pasture
system, and 66% in the crop system.
The reduced N availability in the pasture system was attributed to
immobilisation of N in soil organic matter and clay, plant material, and, to a
lesser extent, soil microbial biomass. This immobilisation resulted from the
large accumulation of carbonaceous plant residues.
SOIL MICROBIAL BIOMASS NITROGEN IN RELATION TO PLANT NITROGEN UPTAKE UNDER PADDY SOILS
