Abundance and gene expression in nitrifier and denitrifier communities associated with a field scale spring thaw N2O flux event

Agricultural soils are the main global source of nitrous oxide (N2O) emissions. Nitrous oxide is produced by nitrifying and denitrifying microbes in soils. In cold regions such as Canada, N2O emissions during spring thaw can account for the majority of annual emissions. The objectives of this study were: (i) to quantify gene expression in nitrifying and denitrifying communities by targeting key functional gene transcripts in the nitrogen cycle (amoA, nirS, nosZ), and, (ii) to compare microbial activity to timing of N2O flux in two conventionally tilled corn fields over spring thaw, one field with residues removed (−R), and one with residues returned (+R). In both fields, initial sampling contained populations with amoA and nirS transcripts, and these quantities did not significantly change over the spring thaw sampling period. In contrast, the –R field had a denitrifying population that transcribed significantly less nosZ, and abundance increased significantly over the sampling period, N2O flux over the spring thaw was inversely proportional to nosZ transcription. The combination of anaerobic soil conditions favoring denitrification, nutrient availability, and gene expression in both nitrifiers and denitrifiers present at the onset of the spring thaw provide evidence of de novo denitrification. To the best of our knowledge, ours is the first study to relate the analysis of the quantity and activity of N cycling soil microbes in situ to the timing of a spring thaw N2O emission event.