Effects of Contrasting Cold Temperatures On N2o Emissions, Denitrification and On the Abundance and Structure of Nitrifying and Denitrifying Soil Communities.

Climate warming in northern latitudes, such as in eastern Canada, may lead to decreased soil temperatures over winter as a result of reduced snow cover. We examined the effects of temperatures near the freezing point on N₂O emissions, denitrification, and on the abundance and structure of soil nitrifiers and denitrifiers. In addition, the influences of NO₃^- and clover residue amendments to the soils at these temperatures were assessed. Soil microcosms supplemented with NO₃^- and/or NO₃^- plus red clover residues were incubated for 120 days at -4°C, -1°C, +2°C or +5°C, representing contrasting soil temperatures encountered during winter in eastern Canada. Addition of red clover residues increased N₂O emissions and denitrification at all temperatures except at -4°C. Among microcosms amended with residues, N₂O emission and denitrification increased with increasing temperature at the start of the incubation (i.e. on Days 2 and 14). Interestingly, N₂O emission and/or denitrification after day 14,  and cumulated denitrification losses over 120 days were greater at -1°C probably due to the presence of more free water compared with -4°C and more anoxic microsites as soil water is freezing compared with +2°C and +5°C. Abundances of ammonia oxidizing bacteria (AOB) and archaea (AOA), Nitrospira-like bacteria and nirK denitrifiers were the lowest in soils at -4°C, while abundances of Nitrobacter-like bacteria and nirS denitrifiers did not vary among temperatures. Community structures of nirK and nirS denitrifers and Nitrobacter-like bacteria shifted between below-zero and above-zero temperatures. Structure of AOB also changed but not systematically among frozen and unfrozen temperatures, whereas AOA structure was globally not affected by temperatures. Results suggest that should climate change result in colder soil temperatures over winter, i.e. about -1°C, N₂O emission and denitrification may increase in presence of C and NO₃^- combined with shifts in soil nitrifier and denitrifier communities.