Flooding-induced N2O emission bursts controlled by pH and nitrate in agricultural soils

• Flooding induced pulse of N2O emission in the range of annual emission measurements.
• Following flooding, a burst in N2O conc. and emissions lasting only days are seen.
• N2O emissions are positively correlated with NO3− content.
• Liming highly fertilized fields may reduce N2O emissions.

Agricultural soils are a major source of the greenhouse gas nitrous oxide (N2O) to the atmosphere. Increasing frequency and severity of flooding as predicted for large intensively cropped areas may promote temporary denitrification and N2O production but the effect of flooding events on N2O emissions is poorly studied for agricultural systems. The overall N2O dynamics during flooding of an agricultural soil and the effect of pH and NO3− concentration has been investigated based on a combination of the use of microsensors, stable isotope techniques, KCl extractions and modelling. This study shows that non-steady state peak N2O emission events during flooding might potentially be at least in the order of reported annual mean N2O emissions, which typically do not include flood induced N2O emissions, and that more than one-third of the produced N2O in the soil is not emitted but consumed within the soil. The magnitude of the emissions are, not surprisingly, positively correlated with the soil NO3− concentration but also negatively correlated with liming (neutral pH). The redox potential of the soil is found to influence N2O accumulation as the production and consumption of N2O occurs in narrow redox windows where the redox range levels are negatively correlated with the pH. This study highlights the potential importance of N2O bursts associated with flooding and infers that annual N2O emission estimates for tilled agricultural soils that are temporarily flooded will be underestimated. Furthermore, this study shows that subsurface N2O reduction is a key process limiting N2O emission and that a reduction in N2O emissions is achievable if highly fertilized N-rich soils are limed.