Delaying nitrogen fertiliser application improves wheat 15N recovery from high rainfall cropping soils in south eastern Australia

Improving nitrogen (N) fertiliser uptake of crops growing in soils susceptible to waterlogging could potentially reduce fertiliser input costs and harmful losses of N to the surrounding environment. The fate of N-15 labelled urea applied to wheat cv. Bolac was studied on brown chromosol soils at Hamilton and Tarrington, in the high rainfall zone of south western Victoria, in south eastern Australia. Wheat was fertilised with N-15-urea solution, either deep banded 0.1 m below the seed at sowing or top-dressed with or without the nitrification inhibitor DMPP (3,4-dimethylpyrazole phosphate or ENTECA (R)) later in the crops development. Daily temporal topsoil (0-0.1 m) water was monitored, throughout the growing season, and at physiological maturity the recovery of N-15-urea in straw, grain and soil (to 0.4 or 0.6 m depth) was measured. Delaying untreated N-15-urea application until top-dressing at mid-tillering to first node stage of growth resulted in significantly (P < 0.001) greater recovery of applied N than when deep banded at sowing or top-dressed shortly after crop emergence. However, top-dressing with DMPP did not significantly improve crop recovery of N-15-urea compared with untreated urea, except when top-dressed early in the growing season. Across all sites, between 64 and 84 % of the applied N-15-urea was recovered in the plant and soil at maturity when top-dressed at mid tillering to first node, compared with 7-42 % when N-15-urea was either deep banded at sowing or top-dressed shortly after crop emergence. The poor recovery of N-15-urea when applied around sowing appeared to result from wet to waterlogged soil and subsequent gaseous or drainage losses before wheat reached peak growth and demand for N in spring. Despite, the poor recovery from N-15-urea applied early in the growing season, wheat grain yields were the same as those top-dressed with N-15-urea; the former treatment compensating for low fertiliser recovery by sourcing more N from the soil. All sites had high concentrations of topsoil organic C (> 2.8 %) and the potential for large rates of mineralisation during the growing season.