Combined effect of nitrogen fertiliser and leaf litter carbon drive nitrous oxide emissions in tropical soils

Intensification of agriculture in the tropics is likely to increase reactive nitrogen (N) losses in the form of nitrous oxide (N2O), however, drivers of emissions from tropical soils remain poorly understood. This study investigated the effect of leaf litter and urea fertiliser on N2O emissions from two Australian tropical mango orchards. Treatments included urea (25 g N m−2), leaf litter (1500 g m−2 dry matter), their combined application, and untreated control. Up to 80.5 ± 8.4 mg N2O-N m−2were lost within two weeks of treatment application, accounting for more than 60% of annual N2O emissions. Indirect emissions ' 30 mg m−2 d−1 were recorded at one site, potentially resulting from groundwater transported N. Highest annual N2O emissions were observed from Litter + Urea with 130.4 mg N2O-N m−2y−1, exceeding those from Urea-Only by a factor of two and those from Litter-Only by a factor of four. This resulted in residue and fertiliser emission factors (EF) of ' 0.01–0.37%, well below the IPCC and Tier 2 defaults, with whole orchard losses equivalent to 0.19–0.66 kg N2O-N ha−1. The findings suggest N2O is N limited when no N fertiliser is applied, even when applying large amounts of litter-N, and by carbon (C) in the absence of litter. The combined effect of Litter + Urea on N2O emissions surpassed the effect of the sole application of litter and urea, demonstrating a critical interaction between both substrates. This interaction effect needs to be considered when developing management strategies aimed at increasing soil C in tropical soils.