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

Abstract

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.