Decreased N2O reduction by low soil pH causes high N2O emissions in a riparian ecosystem

Quantification of harmful nitrous oxide (N2O) emissions from soils is essential for mitigation measures. An important N2O producing and reducing process in soils is denitrification, which shows deceased rates at low pH. No clear relationship between N2O emissions and soil pH has yet been established because also the relative contribution of N2O as the denitrification end product decreases with pH. Our aim was to show the net effect of soil pH on N2O production and emission. Therefore, experiments were designed to investigate the effects of pH on NO3- reduction, N2O production and reduction and N-2 production in incubations with pH values set between 4 and 7. Furthermore, field measurements of soil pH and N2O emissions were carried out. In incubations, NO3- reduction and N-2 production rates increased with pH and net N2O production rate was highest at pH 5. N2O reduction to N-2 was halted until NO3- was depleted at low pH values, resulting in a built up of N2O. As a consequence, N2O:N-2 production ratio decreased exponentially with pH. N2O reduction appeared therefore more important than N2O production in explaining net N2O production rates. In the field, a negative exponential relationship for soil pH against N2O emissions was observed. Soil pH could therefore be used as a predictive tool for average N2O emissions in the studied ecosystem. The occurrence of low pH spots may explain N2O emission hotspot occurrence. Future studies should focus on the mechanism behind small scale soil pH variability and the effect of manipulating the pH of soils.