Although temperate ecosystems are projected to experience less severe winter climate warming than high latitude systems over the next century, it has been hypothesized that the nitrogen dynamics of temperate ecosystems may be highly sensitive to winter warming as a result of an increased frequency and intensity of soil freeze-thaw cycles.  In addition, many temperate systems will simultaneously experience large increases in atmospheric nitrogen deposition, and the extent to which these potentially influential global change factors may interact is unclear.  We explored the effects of winter warming and soil freezing on the soil nitrogen dynamics of a temperate old field using a combination of laboratory soil incubations and temperature manipulations of plant-soil mesocosms.  In addition, we examined changes in net nitrogen mineralization, nitrogen leached below the rooting zone and the recovery of added ¹⁵N tracer in response to simulated warming and nitrogen deposition in a field experiment.

Results/Conclusions

In the laboratory soil incubations, only extreme freeze-thaw cycles increased soluble nitrogen availability.  This result was consistent with the lack of a response of extractable soil nitrogen to temperature manipulations of plant-soil mesocosms; however, freeze-thaw cycles coupled with simulated melt events increased the quantity of nitrogen leached from the bottom of the mesocosms.  In the field experiment, net nitrogen mineralization in the rooting zone and nitrogen leaching below the rooting zone did not respond significantly to warming over winter.  Similarly, winter warming did not affect the recovery of the ¹⁵N label in plant tissue or soil one year after addition, and ¹⁵N retention in plants was instead increased by warming over summer.  Overall, our results indicate that changes to soil freezing dynamics caused by projected increases in mean air temperature over winter may not substantially alter soil nitrogen dynamics in our system.  Nevertheless, there is evidence for potential responses of soil nitrogen dynamics to more extreme, acute climate events over winter.