Combined Effects of Soil Freezing and N Addition on Losses and Interception of N Over Winter and Summer

Climate warming is expected to decrease snow cover in many northern temperate regions, potentially increasing the severity of soil freezing. Severe soil freezing can enhance nitrogen (N) losses, but it remains unclear which N pools contribute to these losses, how this may change seasonally and how freezing damage might interact with increased atmospheric N deposition. We added 15N to grass-dominated old field plots treated with combinations of snow removal and N addition, then resampled the plots at the end of the winter and the next fall to assess N losses from the soil, litter, roots and shoots. In addition, we added 15N to separate treated plots immediately after snowmelt and at peak biomass and sampled these plots after 1 week to assess the potential for interception of N deposition. Snow removal increased soil and root losses of the 15N label over the winter, but it had no effect on 15N losses from these pools over summer. N addition had no effect on 15N losses from these pools over winter or summer, but N addition decreased the interception of 15N added at snowmelt. Snow removal decreased plant biomass over summer and reduced the interception of 15N, whereas N addition enhanced plant biomass and increased the interception of 15N at this time. Our results confirm that severe soil freezing can both enhance soil and root N losses over winter and reduce the interception of N deposition over the growing season, although the latter can be counteracted by increased plant biomass in response to increased N.