The Effects of Freeze-Thaw Intensity on Soil Microbial Denitrifying Communities as Related to N2O Emissions.

Nitrous oxide (N₂O) is both a greenhouse gas and a catalyst of stratospheric ozone depletion. Of the emissions associated with human activity, 60% come from agriculture. In many terrestrial systems, 50% to 66% of N₂O emissions occur in late winter/early spring during cycles in which the soil freezes and thaws. The phenomenon of N₂O emission peaks during freeze-thaw cycles has been attributed to a flush in available carbon substrate for microbial metabolism coupled with anaerobic conditions inducing denitrification.  While this may explain increases in gaseous N loss (N₂ and N₂O), it does not completely account for the magnitude of increase in N₂O emissions.  Emission of N₂O is often referred to as a “leak” in the denitrification pathway and understanding of the mechanism underlying the increase in this leak is needed.  Using laboratory simulations, Sharma et al. (2006) found that freeze-thaw cycles resulted in a change of denitrifier community activity. This change included very low or no expression of the gene responsible for reduction of N₂O to N₂ coinciding with increased N₂O fluxes.  Furthermore, research by Wagner-Riddle et al. (2007) has shown a correlation between accumulated degree hours below 0 ¢ªC and N₂O emissions (r² =.90).  In a five year study examining soil management practices they determined freezing degrees to be the most important factor explaining year to year variability and between treatment differences in winter and spring N₂O fluxes.  Our study seeks to connect the intensity of freezing with quantitative changes in denitrifier community activity and further correlate these changes with increases in N₂O emissions and N₂O/N₂ ratios.  We examine both an indoor freeze-thaw simulation and a natural spring thaw in a cornfield in which freeze intensity was controlled by field cover manipulation.