Greenhouse gas (N2O and CH4) fluxes under nitrogen-fertilised dryland wheat and barley on sub-tropical Vertosols: risk, rainfall and alternatives

Abstract

The northern Australian grains industry relies on nitrogen (N) fertiliser to optimise yield and protein, but N fertiliser can increase soil fluxes of nitrous oxide (N₂O) and methane (CH₄). We measured soil N₂O and CH₄ fluxes associated with wheat ('Triticum aestivum') and barley ('Hordeum vulgare') using automated (Expts 1, 3) and manual chambers (Expts 2, 4, 5). Experiments were conducted on subtropical Vertosol soils fertilised with N rates of 0-160kg Nha⁻¹. In Expt 1 (2010), intense rainfall for a month before and after sowing elevated N₂O emissions from N-fertilised (80kg Nha⁻¹) wheat, with 417g N₂O-N ha-1 emitted compared with 80g N₂O-N ha⁻¹ for non-fertilised wheat. Once crop N uptake reduced soil mineral N, there was no further treatment difference in N₂O. Expt 2 (2010) showed similar results, however, the reduced sampling frequency using manual chambers gave a lower cumulative N₂O. By contrast, very low rainfall before and for several months after sowing Expt 3 (2011) resulted in no difference in N₂O emissions between N-fertilised and non-fertilised barley. N₂O emission factors were 0.42, 0.20 and -0.02 for Expts 1, 2 and 3, respectively. In Expts 4 and 5 (2011), N₂O emissions increased with increasing rate of N fertiliser. Emissions were reduced by 45% when the N fertiliser was applied in a 50 : 50 split between sowing and mid-tillering, or by 70% when urea was applied with the nitrification inhibitor 3,4-dimethylpyrazole-phosphate. Methane fluxes were typically small and mostly negative in all experiments, especially in dry soils. Cumulative CH₄ uptake ranged from 242 to 435 g CH₄-C ha⁻¹ year⁻¹, with no effect of N fertiliser treatment. Considered in terms of CO₂ equivalents, soil CH₄ uptake offset 8-56% of soil N₂O emissions, with larger offsets occurring in non-N-fertilised soils. The first few months from N fertiliser application to the period of rapid crop N uptake pose the main risk for N₂O losses from rainfed cereal cropping on subtropical Vertosols, but the realisation of this risk is dependent on rainfall. Strategies that reduce the soil mineral N pool during this time can reduce the risk of N₂O loss.