Improved prediction of farm nitrous oxide emission through an understanding of the interaction among climate extremes, soil nitrogen dynamics and irrigation water.

Reducing nitrous oxide (N ₂ O) emissions from agriculture soils is crucial, as it accounts for 5.6-6.8% of global anthropogenic emissions. This study aims to understand the interaction among climate, soil nitrogen (N) and applied N on N ₂ O emissions from the irrigated cotton farming system and its implications on farm economics. We conducted simulations for 116 years (1900-2015) and assessed the effect of different N-fertiliser application rates, initial soil nitrate (NO ₃ ) N levels and rainfall conditions on N ₂ O emissions, N ₂ O emission factors (EFs) and financial returns (with and without N ₂ O costs). Results showed the following. 1) The proportional impact of higher N fertiliser rates on soil N ₂ O emissions was greater when initial soil N level was lower (5 mg NO ₃ kg ^-1 ) than higher (35 mg NO ₃ kg ^-1 ). However, the volume of impact was greater under higher initial soil N levels. 2) The relationship between N fertiliser rates and the EFs (range 0.03-7.2%) was not linear but bell-shaped. 3) Fertiliser N requirements increased with rainfall and decreased with initial soil N. Accordingly, the cotton returns for the driest rainfall condition (<10th percentile) were maximum at 300, 250 and 150 kg N ha ^-1 for initial soil N of 5, 20 and 35 mg NO ₃ kg ^-1 . For the wettest rainfall condition (>90th percentile), these rates were 50 kg ha ^-1 higher across the initial soil N conditions. Any additional application of N-fertiliser above these rates was counterproductive. 4) Inclusion of N ₂ O cost into farm economics reduced the annual returns by up to $39 ha ^-1 , but the optimal fertiliser application rates remain the same. 5) Optimising N fertiliser rates to soil N and rainfall conditions increased the annual returns by up to $303 ha ^-1 , with a further increase of $15 ha ^-1 from fertiliser use efficiency when the Australian Government incentives under the $2.55 billion dollar Emission Reduction Fund program was considered. These findings suggest that N-fertiliser application rates and N ₂ O emission mitigation strategies need further refinements specific to prevailing soil and climate variabilities.
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