OOS 19-6 - Denitrification phenotypes: A framework for linking soil moisture regime to nitrous oxide emissions

The capacity of microbial nitrous oxide (N2O) reduction to mitigate N2O emissions from soil depends on its rate and timing relative to N2O production, with a longer delay between peak rates of N2O production and reduction leading to a higher proportion of N2O emitted to the atmosphere. Soils can be characterized as having sequential or concurrent ‘denitrification phenotypes’ based on the delay between N2O production and reduction, but the factors that determine this distinction are not yet well-understood. We hypothesized that consistent exposure to anoxia would shape soil microbial communities to respond more quickly to anoxia, thus shortening the delay between N2O production and reduction. We predicted that N2O production and reduction would be sequential in soils exposed to transient flooding (i.e., time scale of hours) and concurrent in soils exposed to prolonged flooding (i.e., time scale of days).

To test our hypothesis, we collected soil cores from the well- and poorly-drained areas of a conventionally-managed Central Illinois maize field to represent transient and prolonged flooding regimes respectively. The soil cores were sieved, homogenized, repacked, and then pre-incubated at field capacity for 3 days. To disentangle the effects of long-term soil moisture regime and short-term antecedent soil moisture, additional soil cores from both drainage classes were incubated under native and non-native flooding treatments for 30 days. For all treatments, the relative rates and timing of N2O production and reduction of the soil cores following a small nitrate amendment were measured over ten days in a fully-anoxic headspace.

Our results supported our hypothesis, showing that the production and reduction of N2O in poorly-drained soil was concurrent whereas the well-drained soils exhibited sequential timing. N2O reduction rate did not exceed production rate in well-drained soil until > 20 hours, which is longer than the typical duration of anoxia in these soils. When well-drained soil was incubated under a prolonged flooding regime, the delay between N2O production and reduction was reduced significantly, indicating that antecedent soil moisture regime can also drive N2O reduction timing. Our results showed that exposure to prolonged flooding on both long and short timescales was associated with the concurrent ‘denitrification phenotype’, leading to low N2O emissions regardless of anoxia duration.