Nitrogen cycling in brigalow clay soils under pasture and cropping

Abstract

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.