The Interaction between Nitrogen Fertiliser and Leaf Litter Application Drives Nitrous Oxide Emission from Tropical Mango Orchards

Intensification of agriculture in the Tropics is likely to increase reactive N losses in the form of nitrous oxide (N2O), a powerful greenhouse gas contributing to global climate change. However, drivers on the magnitude and temporal variability of N2O emissions from tropical agricultural systems are poorly understood. This study investigated the effect of leaf litter, urea fertiliser and their interaction on annual N2O emissions from two mango orchards in a humid and semi-humid tropical climate.Annual N2O emissions were measured in two orchards in the 'wet-dry' tropical region of Australia. Treatments included urea or leaf litter at a rate 15 t ha-1 of litter dry matter and 600 g urea N tree-1, their combined application and untreated control. N2O was measured using automated and manually operated chambers.Up to 0.9 ± 0.1 kg N-N2O ha-1 were lost within the first two weeks after treatment application, accounting for more than 60 % of annual emissions. Average daily emissions ranged from 1.3 ± 0.8 to 14.7 ± 4.2 g N-N2O ha-1 and followed the sequence leaf litter+urea > urea > control > leaf litter at both sites. Soil mineral N, DOC and DON content peaked within a month of treatment application and were highest in the leaf litter+urea treatment. Of the applied urea N, 0.25 ± 0.05 % were emitted as N2O from the urea only treatment, while 0.62 ± 0.05 % was emitted from leaf litter+urea, demonstrating an interaction between N fertiliser and leaf litter.The initial pulse of N2O fluxes reflects increased N availability and high soil moisture as conducive for N2O production. Differences between treatments are consistent with soil mineral N, DON and DOC concentrations, explaining the high N2O fluxes in the leaf litter+urea treatment as the effect of C and N supply for denitrification. This indicates the C:N ratio driving N mineralisation and immobilisation, determining N availability in the soil for N uptake and loss pathways.The initial pulse of N2O identifies the period after N fertilisation as a critical window for N2O emissions from these tropical soils. The fertiliser/leaf litter interaction demonstrates that denitrification is both N and C limited in tropical soils. This interaction needs to be considered when developing litter and fertiliser management to reduce N2O emissions while optimising nutrient supply in these tropical soils.