J.F. Angus1 and P. R. Grace2
1CSIRO Agriculture and Food, GPO Box 1600, Canberra 2601, ACT, Australia and EH Graham Centre, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia
2Queensland University of Technology, 2 George St, Brisbane, Queensland 4000, Australia
Farms producing crops and animal products occupy 14% of the Australian land mass. Within this agricultural land, 7% consists of intensive industries (dairy, horticulture and viticulture, sugar cane, cotton, irrigated cereals and feedlots) for which the input of fertiliser nitrogen (N) is typical of such industries worldwide. The sugar and dairy industries are adjacent to populated and environmentally fragile water bodies where nitrate (and phosphate) runoff and leaching contributes to water pollution. The nitrogen use efficiency (NUE) of these industries is low but NUE for the inland irrigated rice and cotton industries are relatively high. The remaining 93% of agricultural land grows dryland crops and animal products (wheat, coarse grains, canola, grain legumes, cattle meat, sheep meat, and wool) partly from continuous crops, partly permanent pasture and partly from phased crop-pasture systems. Until the mid-1990s the source of most of the N in dryland crops was from mining the soil organic matter and increasingly, since the 1950s, from N built up from biological N-fixation by pastures grown in phased rotation. Export of N in products from dryland farms exceeded the input from N fertiliser. Since the mid 1990s N fertiliser input increased to an average of about 45 kg N ha-1, only about half of which is taken up by crops. Of the rest, most is retained in the soil after harvest and about one quarter is lost from denitrification, ammonia volatilisation and leaching. Overuse of N fertiliser in dryland farming is rare because neither products nor fertiliser are subsidised. Arid and semi-arid land occupies 86% of the continent, half of which is not used for production and the other half produces cattle meat, sheep meat and wool with no fertiliser input. The source of N is rain, biological N fixation and redistribution from dust, the amounts of which are greater than the controlled N inputs in the agricultural regions. The feature of N cycling in Australia that distinguishes it from other developed countries is the importance of natural N sources, reflecting the extensive and relatively young agricultural system.