Threshold-like effect of soil NO₃^- concentrations on denitrification product N₂O/(N₂O+N₂) ratio is mediated by soil pH

The interactive effects of soil pH and nitrate (NO3-) availability on denitrification-derived nitrous oxide (N2O) and dinitrogen (N-2) emissions, as well as N2O/(N2O + N-2) product ratio (N(2)Oi) remain difficult to evaluate. Here, we consecutively monitored denitrification-derived N2O and N-2 emissions with different soil pH and NO3- concentrations by using the artificial N-15 gas flux method in combination with a global meta-analysis of 458 observations to illustrate the interactive effects of soil pH and NO3- availability on N(2)Oi. Isotopic data indicated that pH adjustment through liming significantly and nonlinearly reduced N(2)Oi and increased cumulative N-2 emissions. Liming decreased cumulative N2O emissions by approximately threefold in low NO3- treatments (p < 0.01), while at high NO3- treatments liming showed no effect. The meta-analysis revealed a logarithmic relationship between N(2)Oi and increasing soil NO3- concentrations across different soil pH levels, indicating a threshold-like effect of NO3- concentrations on the reduction of N2O during denitrification. The threshold NO3- concentration tends to raise with increasing soil pH. Hence, there were significant interaction effects of soil pH and soil NO3- concentration on the N(2)Oi value. The present study emphasizes the need to consider soil pH and NO3- concentration interaction effects on denitrification for proposing the effective mitigation practices of denitrification-derived soil N2O emissions.