Net nitrogen mineralization and leaching in response to warming and nitrogen deposition in a temperate old field: the importance of winter temperature.

While climate warming can increase plant N availability over the growing season by increasing rates of N mineralization, increased N mineralization over winter at a time when plant roots are largely inactive, coupled with an increased frequency of soil freeze-thaw cycles, may increase soil N leaching losses. We examined changes in soil net N mineralization and N leaching in response to warming and N addition (6 g m(-2) year(-1)) in a factorial experiment conducted in a temperate old field. We used two warming treatments, year-round and winter-only warming, to isolate the effects of winter warming on soil N dynamics from the year-round warming effects. We estimated net N mineralization using in situ soil cores with resin bags placed at the bottom to catch throughput, and we measured N leaching using lysimeters located below the plant rooting zone at a depth of 50 cm. There were minor effects of warming on changes in soil extractable N and resin N in the soil cores over winter. Nevertheless, the overall effects of both warming and N addition on net N mineralization (the sum of changes in soil extractable N and resin N) were not significant over this period. Likewise, there were no significant treatment effects on the concentration of N in leachate collected below the plant rooting zone. However, in response to winter warming, net N mineralization over summer was approximately double that of both the ambient and year-round warming treatments. This result demonstrates a potentially large and unexpected effect of winter warming on soil N availability in this old field system.