Nitrogen cycling in brigalow clay soils under pasture and cropping

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 15 N-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 15 N in the cropping and pasture systems. Under sorghum, 60% of the applied 15 N 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 15 N. During the second season, most of the 15 N was stabilised in soil organic matter and clay (maximum 42%). A significant proportion of the 15 N remained in the soil inorganic pool over the 2 seasons. Under green panic, 82% of the 15 N left the soil inorganic pool within 4 days and entered the microbial biomass, soil organic matter, and the plant. Uptake and re-release of 15 N 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 15 N, two-thirds of which was re-released in the second season. Stabilised N represented up to 62% of the applied 15 N, and was consistently greater under green panic than under sorghum. After 2 seasons, 15 N 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 15 N 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. Keywords: immobilisation, N availability, N balance, plant N, soil N. Australian Journal