Nitrogen budget of wheat growing on a Riverine clay soil

The fate of nitrogen in wheat grown on a Mesotrophic, Red Kandosol near Wagga Wagga was studied in the 1993 growing season, which had above-average rainfall: 417 mm (31 May-30 November 1993) compared with an average (June-November) of 289 mm. Nitrogen supply (fertiliser and mineralisation) was partitioned between crop uptake, gaseous and leaching losses, and residual mineral N in the soil profile. The study plots were 2 adjacent 5-ha areas. At stem elongation (Zadock's decimal code 31), one area was topdressed with urea at 14 g N/m(2) (fertilised crop). The total N supply to the fertilised crop was 29 g N/m(2)-8 g N/m(2) of mineral N in the soil at sowing, net mineralisation of 5.3 g N/m(2), and fertiliser inputs of 1.7 and 14 g N/m(2). The corresponding value for the non-fertilised crop was 15 g N/m(2). The urea application produced a 50% increase in above-ground biomass (1521 and 1008 g/m(2) dry matter at harvest) and a 1.8-fold increase in grain yield (692 and 384 g/m(2)). The proportion of the total N supply recovered in the crops was similar (55% and 60% for the non-fertilised and fertilised treatments, respectively). Leaching losses were low (0.4 and 0.5 g N/m(2)), even though approximate to 100 mm drained beyond the root-zone (equivalent to 24% of the seasonal rainfall). The periods of saturated soil required to generate drainage also caused denitrification losses of 1.7 and 3.4 g N/m(2) for the non-fertilised and fertilised treatments, respectively. Increased net mineralisation and reduced crop N uptake that began a month prior to anthesis were responsible for the substantial amounts of mineral N remaining in the soil after harvest (4.7 and 4.3 g N/m(2), respectively). The low NO3 leaching loss associated with high drainage was explained by displacement flow mechanics operating in soil that has a high water retention capacity, which is confirmed by Br and N-15 tracer analysis. The N balance was closed for the non-fertilised crop, but a discrepancy of 2.8 g N/m(2) remains for the fertilised crop. The uncertainty of approximate to 10% of the fertilised treatment may possibly be due to ammonia volatilisation following topdressing with urea.