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 (N₂) emissions, as well as N₂O/(N₂O + N₂) product ratio (N₂Oi) remain difficult to evaluate. Here, we consecutively monitored denitrification-derived N₂O and N₂ emissions with different soil pH and NO₃⁻ concentrations by using the artificial ¹⁵N 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 N₂Oi. Isotopic data indicated that pH adjustment through liming significantly and nonlinearly reduced N₂Oi and increased cumulative N₂ 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 N₂Oi 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 N₂Oi 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.