The fate of urea [sup.15]N 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.sup.-1]) were incorporated at sowing with [sup.15]N labelled urea (200 kg N [ha.sup.-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.sup.-1] at harvest. Total N uptake was 249.1 kg N [ha.sup.-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. Keywords: [sup.15]N, compost, fertiliser, leaching, maize, mill mud, nitrogen, organic wastes.
Introduction The ever-increasing cost of synthetic nitrogen (N) fertiliser and the demand for more sustainable production systems have promoted the use of organic amendments (OAs) from agricultural (e.g. crop residues, [...]