Effects of temperatures near the freezing point on N2O emissions, denitrification and on the abundance and structure of nitrifying and denitrifying soil communities.

Climate warming in temperate regions 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 N(2)O emissions, denitrification, and on the abundance and structure of soil nitrifiers and denitrifiers. 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. Among microcosms amended with residues, N(2)O emission and/or denitrification increased with increasing temperature on Days 2 and 14. Interestingly, N(2)O emission and/or denitrification after Day 14 were the greatest at -1 °C. Substantial N(2) O emissions were only observed on Day 2 at +2 °C and +5 °C, while at -1 °C, N(2)O emissions were consistently detected over the duration of the experiment. 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 denitrifiers and Nitrobacter-like bacteria shifted between below-zero and above-zero temperatures. Structure of AOA and AOB communities also changed but not systematically among frozen and unfrozen temperatures. Results indicated shifts in some nitrifier and denitrifier communities with freezing and a surprising stimulation of N(2)O emissions at -1 °C when NO3 - and C are present.