Simulated rainfall on agricultural soil reveals enzymatic regulation of short-term nitrous oxide profiles in soil gas and emissions from the surface

Many microbial species use nitrate to support respiration under oxygen limiting conditions via the process of denitrification, which generates the gaseous products nitric oxide (NO), nitrous oxide (N₂O), and dinitrogen (N₂). Denitrifying bacteria reduce NO to N₂O, which is a potent greenhouse gas, to maintain intercellular concentrations below cytotoxic levels. The enzymes that reduce N₂O to N₂ play a crucial role in restricting N₂O emissions from the surface. Laboratory studies have demonstrated that accumulation of N₂O results from unbalanced rates of the sequence of denitrification reactions, which is ascribed to enzyme kinetics and sequential gene expression. However, the same enzymatic regulation of N₂O accumulation in soil during short periods of anoxia has not been observed in the field. Here, we investigated the role of enzymatic regulation on the accumulation of N₂O during a transition from oxic to anoxic conditions that was induced by a simulated rainfall in the field. Distinct regulation regimes for activities of pre- and de novo synthesized denitrification enzymes were observed. The activity of N₂O reductase played a crucial role in regulating N₂O emissions. Dynamics of N₂O mixing ratios in soil gas and emissions from the surface were in excellent agreement with simulations using a one-dimensional, diffusion–reaction equation with explicit representations of denitrification enzyme kinetics. A more explicit representation of the regulatory biology of denitrification in current biogeochemical models, like the approach developed in the subject study, is a promising strategy for improving predictions of episodic emissions of N₂O from soil.