Effect of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N-turnover, the N(2)O reductase-gene nosZ and N(2)O:N(2) partitioning from agricultural soils

Nitrification inhibitors (NIs) have been shown to reduce emissions of the greenhouse gas nitrous oxide (N(2)O) from agricultural soils. However, their N(2)O reduction efficacy varies widely across different agro-ecosystems, and underlying mechanisms remain poorly understood. To investigate effects of the NI 3,4-dimethylpyrazole-phosphate (DMPP) on N-turnover from a pasture and a horticultural soil, we combined the quantification of N(2) and N(2)O emissions with (15)N tracing analysis and the quantification of the N(2)O-reductase gene (nosZ) in a soil microcosm study. Nitrogen fertilization suppressed nosZ abundance in both soils, showing that high nitrate availability and the preferential reduction of nitrate over N(2)O is responsible for large pulses of N(2)O after the fertilization of agricultural soils. DMPP attenuated this effect only in the horticultural soil, reducing nitrification while increasing nosZ abundance. DMPP reduced N(2)O emissions from the horticultural soil by >50% but did not affect overall N(2) + N(2)O losses, demonstrating the shift in the N(2)O:N(2) ratio towards N(2) as a key mechanism of N(2)O mitigation by NIs. Under non-limiting NO(3)(−) availability, the efficacy of NIs to mitigate N(2)O emissions therefore depends on their ability to reduce the suppression of the N(2)O reductase by high NO(3)(−) concentrations in the soil, enabling complete denitrification to N(2).