Abstract
Nitrification inhibitors (NIs) have been shown to reduce emissions of the greenhouse gas nitrous oxide (N2O) from agricultural soils. However, their N2O 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 N2 and N2O emissions with 15N tracing analysis and the quantification of the N2O-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 N2O is responsible for large pulses of N2O after the fertilization of agricultural soils. DMPP attenuated this effect only in the horticultural soil, reducing nitrification while increasing nosZ abundance. DMPP reduced N2O emissions from the horticultural soil by >50% but did not affect overall N2 + N2O losses, demonstrating the shift in the N2O:N2 ratio towards N2 as a key mechanism of N2O mitigation by NIs. Under non-limiting NO3− availability, the efficacy of NIs to mitigate N2O emissions therefore depends on their ability to reduce the suppression of the N2O reductase by high NO3− concentrations in the soil, enabling complete denitrification to N2.
Highlights
Microbial metabolic pathways can contribute via a wealth of different processes to N2O production and consumption, i.e. the reduction to N2 in soils
Gross N transformation rates were quantified with a 15N tracing model (Fig. 1) and differed markedly between soils when N-fertilizer was applied without the Nitrification inhibitors (NIs) dimethylpyrazole phosphate (DMPP), referred to as the fertilizer only treatment (Table 2)
mineralization rates (Mtot) was dominated by the mineralization of labile N (MNlab), while the mineralization of recalcitrant organic N (Mrec) dominated in the sandy CL
Summary
Microbial metabolic pathways can contribute via a wealth of different processes to N2O production and consumption, i.e. the reduction to N2 in soils. A reduction of N2O emissions by NIs can be attributed to (a) reduced N2O production via nitrification mediated pathways, (b) reduced N2O production via denitrification (c) increased consumption of N2O via denitrification, i.e., a shift in the N2O:N2 ratio towards N2. As these effects may overlap, a mechanistic understanding of the effects of NIs on N2O production and consumption processes needs to be based on N2O source partitioning, and the direct quantification of N2. We combined a 15N tracing analysis with the direct quantification of N2 and N2O emissions using the 15N gas flux method, complemented with the quantification of the nosZ gene via quantitative polymerase chain reaction (qPCR) in a soil microcosm study to constrain factors determining the efficacy of the NI DMPP to mitigate N2O emissions from agricultural soils
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