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 pasto ral areas in the four subtropical zones in eastern Australia. The path ways for N loss from the various crops grown in these zones are also e xamined and estimates of N loss given. Sugar-cane is the most importan t crop grown in the subtropical humid northern and southern zones, usi ng 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 a pplication, applying urea in subsurface bands or delaying application until after canopy development. Denitrification losses of 20% of appli ed N have been measured on clay soils in sugar- cane areas while leach ing 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 t o the economy of the humid northern and southern zones. The livestock industry is well established in the subtropical northern zones, with b eef and dairy production relying on leguminous as well as N fertilized pastures. Urea is again the most widely used form of N and is suscept ible to large losses via ammonia volatilization. Over a 12 month perio d, 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 n orthern and southern zones with grain sorghum the main summer crop. Ur ea is the principal form of N fertilizer applied to both crops and is essential for increasing or maintaining economic yields from both regi ons. This decrease in soil fertility in grain producing areas is due m ainly 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 inhibito rs such as wax coated calcium carbide and 2-ethynylpyridine have reduc ed denitrification losses from soils growing wheat and cotton respecti vely. Subtropical agriculture relies heavily on N fertilizer, principa lly urea, to maintain and increase crop yields. Losses of N from soils sown to crops and from native and sown pasture occur although managem ent practices are being developed to help minimize this loss.