Effect of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on N-turnover, the NO reductase-gene nosZ and NO:N partitioning from agricultural soils.

Nitrification inhibitors (NIs) have been shown to reduce emissions of the greenhouse gas nitrous oxide (NO) from agricultural soils. However, their NO 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 and NO emissions with N tracing analysis and the quantification of the NO-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 NO is responsible for large pulses of NO after the fertilization of agricultural soils. DMPP attenuated this effect only in the horticultural soil, reducing nitrification while increasing nosZ abundance. DMPP reduced NO emissions from the horticultural soil by >50% but did not affect overall N + NO losses, demonstrating the shift in the NO:N ratio towards N as a key mechanism of NO mitigation by NIs. Under non-limiting NO availability, the efficacy of NIs to mitigate NO emissions therefore depends on their ability to reduce the suppression of the NO reductase by high NO concentrations in the soil, enabling complete denitrification to N.