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.