The fate of urea 15N in a subtropical rain-fed maize system : Influence of organic amendments

Context: Organic amendments (OAs) from agricultural and non-agricultural industries provide an alternative source of nitrogen (N) and other nutrients for crop production, especially with increasing costs of synthetic fertilisers. Aims: This study examined the fate of urea in combination with OA in the form of raw mill mud, composted mill mud and high N compost in a maize field experiment on a sandy loam soil in a rain-fed subtropical environment. Methods: OAs (346 kg N ha-1) were incorporated at sowing with 15N labelled urea (200 kg N ha-1) applied 28 days later, including a no OA control. Key results: Grain yield, aboveground biomass and roots were not significantly different across all treatments, averaging 8.1, 17.4 and 1.3 t ha-1 at harvest. Total N uptake was 249.1 kg N ha-1 (on average) with fertiliser, native soil N and OAs sources accounting for 49.5, 44 and 6.5% of the total N uptake respectively. There was no significant difference in N fertiliser recovery between the OA treatments and the unamended control, with 61.7 and 3.7% of the applied N recovered in the plant and soil (to 100 cm), respectively, at harvest. Nitrogen fertiliser losses were equivalent to 33.6% of the applied N and were attributed to N leaching after extensive rainfall events late in the growing season. Conclusions: OAs with high mineral and/or organic N content at time of application can supply significant amounts of plant available N but high levels of soil mineral N at sowing may hinder their full potential. Implications: Reducing N loss and cost, and improving overall soil fertility by replacing synthetic N fertilisers with OAs may contribute to more environmentally sustainable crop production.