Examination of opportunity for compost to reduce synthetic nitrogen fertiliser requirements of sugarcane and nitrous oxide emissions

A replicated trial was conducted in sugarcane at Maroochy River (SE Queensland) over three years to assess the potential for partially replacing use of synthetic nitrogen (N) fertiliser with compost and a biological N fixing product. Agronomic indices and nitrous oxide (N2O) emissions were measured. Banded application of compost above or below the sugarcane trash blanket can partially substitute the use of synthetic fertiliser. Treatments where compost-derived N (mineral N at time of application plus mineralisable N) reduced annual use of synthetic N fertiliser by 35 to 43 kg N/ha, showed identical yields to treatments that received only synthetic fertiliser. However, surface application of compost with reduced N rates was an unattractive agronomic and economic strategy. Use of a biological N fixing product did not show any N effects within the timeframe of this project. There was no consistent significant impact of compost in reducing emissions of N2O. In fact, emission rates were enhanced by the presence of compost in the 2014–15 crop. Soil data at the time of high daily emissions showed higher levels of nitrate (NO3–) N in the row where compost was applied. Conversely ammonium (NH4+) N levels were higher than NO3––N in the row where compost was not applied. Avoiding application of N in excess of recommended rates was more effective than partially substituting mineral N fertiliser with compost in minimising N2O emissions. Beneficial impact of compost on total carbon and N in soil was confined to the row zone and largely the 0–5 cm depth.