IMPROVING WHEAT SIMULATION CAPABILITIES IN AUSTRALIA FROM A CROPPING SYSTEMS PERSPECTIVE - WATER AND NITROGEN EFFECTS ON SPRING WHEAT IN A SEMIARID ENVIRONMENT

Systems approaches can help to evaluate and improve the agronomic and economic viability of nitrogen application in the frequently water-lim ited environments. This requires a sound understanding of crop physiol ogical processes and well tested simulation models. Thus, this experim ent on spring wheat aimed to better quantify water x nitrogen effects on wheat by deriving some key crop physiological parameters that have proven useful in simulating crop growth. For spring wheat grown in Nor thern Australia under four levels of nitrogen (0 to 360 kg N ha(-1)) a nd either entirely on stored soil moisture or under full irrigation, k ernel yields ranged from 343 to 719 g m(-2). Yield increases were stro ngly associated with increases in kernel number (9150-19950 kernels m( -2)), indicating the sensitivity of this parameter to water and N avai lability. Total water extraction under a rain shelter was 240 mm with a maximum extraction depth of 1.5 m. A substantial amount of mineral n itrogen available deep in the profile (below 0.9 m) was taken up by th e crop. This was the source of nitrogen uptake observed after anthesis . Under dry conditions this late uptake accounted for approximately 50 % of total nitrogen uptake and resulted in high (>2%) kernel nitrogen percentages even when no nitrogen was applied,Anthesis LAI values unde r sub-optimal water supply were reduced by 63% and under sub-optimal n itrogen supply by 50%. Radiation use efficiency (RUE) based on total i ncident short-wave radiation was 1.34 g MJ(-1) and did not differ amon g treatments. The conservative nature of RUE was the result of the cro p reducing leaf area rather than leaf nitrogen content (which would ha ve affected photosynthetic activity) under these moderate levels of ni trogen limitation. The transpiration efficiency coefficient was also c onservative and averaged 4.7 Pa in the dry treatments. Kernel nitrogen percentage varied from 2.08 to 2.42%. The study provides a data set a nd a basis to consider ways to improve simulation capabilities of wate r and nitrogen effects on spring wheat. (C) 1997 Elsevier Science B.V.