Organic Wastes Amended with Sorbents Reduce N2O Emissions from Sugarcane Cropping

Nutrient-rich organic wastes and soil ameliorants can benefit crop performance and soil health but can also prevent crop nutrient sufficiency or increase greenhouse gas emissions. We hypothesised that nitrogen (N)-rich agricultural waste (poultry litter) amended with sorbents (bentonite clay or biochar) or compost (high C/N ratio) attenuates the concentration of inorganic nitrogen (N) in soil and reduces emissions of nitrous oxide (N2O). We tested this hypothesis with a field experiment conducted on a commercial sugarcane farm, using in vitro incubations. Treatments received 160 kg N ha−1, either from mineral fertiliser or poultry litter, with additional N (2–60 kg N ha−1) supplied by the sorbents and compost. Crop yield was similar in all N treatments, indicating N sufficiency, with the poultry litter + biochar treatment statistically matching the yield of the no-N control. Confirming our hypothesis, mineral N fertiliser resulted in the highest concentrations of soil inorganic N, followed by poultry litter and the amended poultry formulations. Reflecting the soil inorganic N concentrations, the average N2O emission factors ranked as per the following: mineral fertiliser 8.02% > poultry litter 6.77% > poultry litter + compost 6.75% > poultry litter + bentonite 5.5% > poultry litter + biochar 3.4%. All emission factors exceeded the IPCC Tier 1 default for managed soils (1%) and the Australian Government default for sugarcane soil (1.25%). Our findings reinforce concerns that current default emissions factors underestimate N2O emissions. The laboratory incubations broadly matched the field N2O emissions, indicating that in vitro testing is a cost-effective first step to guide the blending of organic wastes in a way that ensures N sufficiency for crops but minimises N losses. We conclude that suitable sorbent-waste formulations that attenuate N release will advance N efficiency and the circular nutrient economy.