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 uncertain. 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.