Nitrogen use and losses in agriculture in subtropical Australia

This review examines the use of nitrogen (N) fertilizer on sugar-cane, summer and winter grain crops, cotton, tropical fruit crops and pastoral areas in the four subtropical zones in eastern Australia. The pathways for N loss from the various crops grown in these zones are also examined and estimates of N loss given. Sugar-cane is the most important crop grown in the subtropical humid northern and southern zones, using 77% of all N fertilizer applied in 1988-89. Urea is the most widely used form of N fertilizer with about 50% of the applied N often lost via ammonia volatilization, denitrification and leaching. Losses of N via ammonia volatilization can be reduced by either irrigating after application, applying urea in subsurface bands or delaying application until after canopy development. Denitrification losses of 20% of applied N have been measured on clay soils in sugar-cane areas while leaching losses may occur by movement of solutes down preferential pathways (e.g. soil fauna, root channels and structural weaknesses in the soil profile). Tropical fruit crops also make a significant contribution to the economy of the humid northern and southern zones. The livestock industry is well established in the subtropical northern zones, with beef and dairy production relying on leguminous as well as N fertilized pastures. Urea is again the most widely used form of N and is susceptible to large losses via ammonia volatilization. Over a 12 month period, losses of between 9% and 42% of the N applied were recorded from a subtropical pasture. Wheat is the major winter crop of the sub-humid northern and southern zones with grain sorghum the main summer crop. Urea is the principal form of N fertilizer applied to both crops and is essential for increasing or maintaining economic yields from both regions. This decrease in soil fertility in grain producing areas is due mainly to a decrease in the amount of soil organic matter available for mineralization. Cotton is another major crop of both areas and relies heavily on N fertilizer application. Nitrogen fertilizer losses have been recorded from all cropping areas, although nitrification inhibitors such as wax coated calcium carbide and 2-ethynylpyridine have reduced denitrification losses from soils growing wheat and cotton respectively. Subtropical agriculture relies heavily on N fertilizer, principally urea, to maintain and increase crop yields. Losses of N from soils sown to crops and from native and sown pasture occur although management practices are being developed to help minimize this loss.