Do “hot moments” become hotter under climate change? Soil nitrogen dynamics from a climate manipulation experiment in a post-harvest forest

Whole-tree forest harvest can increase soil nitrous oxide (N2O) effluxes and leaching of nitrogen (N) from soils. These altered N dynamics are often linked to harvesting effects on microclimate, suggesting that this “hot moment” for N cycling may become hotter with climate change. We hypothesized that increases in temperature and precipitation during this post-harvest period would increase availability of soil mineral N and soil-atmosphere N2O efflux. To test this hypothesis we implemented a climate manipulation experiment after a forest harvest, and measured soil N2O fluxes and inorganic N accumulating on ion exchange resins. Climate treatments were: control (A, ambient), heated (H, +2.5 °C), wetted (W, +23 % precipitation), and a two-factor treatment (H+W). For all treatments, the first year after harvest had highest N2O efflux and resin N. Wetting significantly increased cumulative soil N2O fluxes, but only when soils were not heated too. The cumulative soil-to-atmosphere N2O efflux from W (5.8 mg N2O–N m−2) was significantly higher than A (−1.9 mg N2O–N m−2), but H+W (~0 mg N2O–N m−2) was similar to A. Regardless of wetting, heating increased resin N, but only on certain dates. Cumulative resin N was on average 125 % greater in the H plots than non-heated plots. Thus, changes in temperature and precipitation each impart distinct changes to the soil N cycle. Heating increased resin N regardless of water inputs, while wetting increasing N2O but not when combined with heating. Our results suggest that climate change may exacerbate soil N losses from whole-tree harvest in the future, but the form and quantity of N loss will depend on how the future climate changes.