The amplitude of soil freeze-thaw cycles influences temporal dynamics of N2O emissions and denitrifier transcriptional activity and community composition

In agricultural fields at high and mid latitudes, a large percentage of the annual emissions of the greenhouse gas nitrous oxide (N₂O) can occur during freeze-thaw (FT) cycles. The objective of the study was to determine the effects of FT cycles of different amplitudes on N₂O emissions, denitrifier transcriptional activity, and the abundance and composition of present and active denitrifier communities. Soil microcosms amended with NO₃ ⁻ (N) and/or NO₃ ⁻ plus organic C, i.e., red clover residues (N + RC), were subjected to freezing at −5 °C followed by thawing at either +4 or +15 °C. Peaks of N₂O emission rates following FT were ∼2-fold greater in N + RC than in N amended soils. In N + RC amended soils, the maximum rates following FT were similar at +4 and +15 °C. However, thawing at +4 °C resulted in a delay in the peak of emissions, suggesting that denitrification enzymatic activity was reduced at colder temperatures. In general, the abundance and the number of transcripts of nirS and nirK were similar over time and among treatments with a few exceptions, including, for example, fewer nirS transcripts under the cooler thawing temperature. Temperature regimes had a significant effect on the compositions of the present and active nirS and present nirK denitrifier communities during FT. Changes in these community compositions were correlated with changes in temperature and N₂O emissions, and these variables explained 3.7 to 4.9 % of the changes in community composition. Results indicated that addition of electron donor, i.e., organic C, increased N₂O emissions. Furthermore, the composition of active nirS community changed during the peak of N₂O emissions following FT events but not the active nirK community. This finding indicated that active nirS community better adapted to the changes in conditions has established suggesting a greater role of this group in N₂O production.