Mitigation of nitrous oxide emissions with nitrification inhibitors in temperate vegetable cropping in southern Australia

Intensive vegetable production in southern Australia is characterised by high inputs of nitrogen (N) fertiliser, water, and occasionally animal manures, which creates the potential for high nitrous oxide (N2O) emissions. Three field experiments were conducted to investigate the effects of the nitrification inhibitors 3,4-dimethylpyrazole phosphate (DMPP), 3-methyl pyrazole plus 1H-1,2,4 triazole (3MP+TZ), and dicyandiamide (DCD) on N2O emissions and yields in broccoli (Brassica oleracea), lettuce (Lactuca sativa) and cauliflower (Brassica oleracea) crops in southern Australia. The inhibitor treatments on fertilisers and poultry manure were compared with standard commercial practice for vegetable crops in this region, and N2O emissions were measured using manual chambers through to harvest. Daily fluxes ranged from 0.81 g N2O-N ha(-1) day(-1) for untreated soil to 11.65 g N2O-N ha(-1) day(-1) for manure treated soil. Extrapolation of these results translate to annual emissions of 0.30 kg N2O-N ha(-1) year(-1) to 4.24 kg N2O-N ha(-1) year(-1), respectively. Cumulative soil N2O fluxes from the manure treatments were similar to 4-fold greater than the standard inorganic fertiliser program for a given crop. Nitrous oxide direct emission factors were in the range 0.02-0.16% for inorganic fertilisers and from 0.19% to 0.43% for poultry manure. The greatest decrease in N2O emissions occurred when DMPP or a combination of 3MP+TZ were added to poultry manure (62% and 66% decrease, respectively). Decreases in N2O emissions from nitrification inhibitors were smaller and less consistent when used with inorganic fertilisers, but DMPP decreased emissions in two out of three trials, with a maximum decrease of 32% observed in the broccoli trial. DCD proved ineffective for mitigating N2O emissions in all trials.