Short-term effect of the nitrification inhibitor DMPP on N-turnover and denitrification losses from two agricultural soils in subtropical Australia

Intense wetting and drying cycles render agricultural soils in the subtropics prone to nitrogen (N) loss via denitrification, with large pulses of the greenhouse gas nitrous oxide (N2O) triggered by rainfall. The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) proved to be effective under subtropical conditions, demonstrating substantial reductions of N2O emitted from cropping soils. However, DMPP has consistently failed to reduce N2O emissions from subtropical pasture soils. The aim of this study was therefore to investigate (a) the response of N-transformations and N2O emissions from a subtropical pasture and a vegetable soil to DMPP, and (b) if the abundance of nosZ, the gene encoding the N2O reductase, can explain N2O emissions as affected by DMPP. Soil microcosms were established in centrifuge tubes and fertilised with ammonium nitrate (35 μg g-1 soil) with or without DMPP. Labelling either ammonium (NH4+), or nitrate (NO3-) with 15N at 10 atom% excess enabled the quantification of gross N-transformations using 15N tracer and pool dilution methods. Soil microcosms were incubated at 75% WFPS over two days, and gas samples were taken each day. Gas samples were analysed for 15N2O to split N2O production into the ammonia oxidation pathway and denitrification. Soil was extracted before and after the incubation for DNA, quantifying the response of nosZ abundance to DMPP. Denitrification was the main source of N2O production in both soils. The pasture soil emitted more than 1.5 μg N-N2O g-1 soil over two days, exceeding N2O emissions of the vegetable soil by a factor of 10. This trend was consistent with the high N-transformation rates in the pasture soil, exceeding those of the vegetable soil by a factor >10. DMPP reduced gross nitrification by 12 and 60% for the pasture and vegetable soil, respectively. However, DMPP reduced cumulative N2O emissions from the vegetable soil only. Fertilisation decreased nosZ abundance in the pasture soil, regardless of the treatment. The same trend was observed for the fertiliser only treatment from the vegetable soil. DMPP however increased nosZ abundance compared to the fertiliser only treatment in the vegetable soil. Gross N transformation rates identified the pasture soil as the more productive soil regarding soil mineral N supply and demonstrate the magnitude of N2O emissions as a function of N-turnover. The reduction of nosZ abundance after fertilisation in both soils reflects the stimulating effect of fertiliser and water addition on N turnover. Increased NO3- production suppresses nosZ activity, limiting further reduction of N2O to dinitrogen (N2). This mechanism was mitigated by DMPP in the vegetable soil, explaining the significant reduction of N2O emissions by DMPP. The high N turnover in the pasture soil and the resulting NO3- concentration is likely to limit the short-term efficacy of DMPP. The relationship between N2O emissions and nosZ abundance identifies the shift in the N2:N2O ratio to N2 as a key mechanism of N2O reduction by DMPP. This shift is however driven and limited by soil-intrinsic N-turnover, explaining differences in N2O reduction by DMPP observed for different soil types in the field