Nitrogen budget of wheat growing on a Riverine clay soil

Abstract

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/m² (fertilised crop). The total N supply to the fertilised crop was 29 g N/m²—8 g N/m² of mineral N in the soil at sowing, net mineralisation of 5.3 g N/m², and fertiliser inputs of 1.7 and 14 g N/m². The corresponding value for the non-fertilised crop was 15 g N/m².

The urea application produced a 50% increase in above-ground biomass (1521 and 1008 g/m² dry matter at harvest) and a 1.8-fold increase in grain yield (692 and 384 g/m²). 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/m²), even though ≈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/m² 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/m², 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 ¹⁵N tracer analysis. The N balance was closed for the non-fertilised crop, but a discrepancy of 2.8 g N/m² remains for the fertilised crop. The uncertainty of ≈10% of the fertilised treatment may possibly be due to ammonia volatilisation following topdressing with urea.