The interactive effects of soil freezing and nitrogen deposition on plant productivity, bacterial and fungal biomasses, and soil N losses
In northern temperate regions, climate warming is projected to decrease the portion of precipitation that falls as snow. Reduced snow cover can increase the occurrence of soil freezing, which can damage plant roots and cause microbial lysis, increasing N leaching over the growing season. These N losses may be exacerbated by future increases in N deposition. However, it is unclear to what extent N losses in response to freezing may only be transient. The objective of this study was to investigate the interactive effects of soil freezing and N deposition on plant productivity, bacterial and fungal biomass and soil N losses in a grass-dominated old field. We established plots with combinations of snow removal and N addition. Snow removal was discontinued after year one, but monitoring continued to assess the multi-year effects of enhanced freezing. We predicted that soil freezing would decrease plant productivity, lower fungal biomasses and increase soil N losses, with N losses being particularly high under N addition, but these effects would dissipate after one year.
Snow removal reduced plant productivity, increased soil N losses, but had no effect on bacterial and fungal biomass during the first growing season. N addition interacted with soil freezing to enhance plant productivity in the snow removal plots, but had no effect on N losses or bacterial and fungal biomass. During the second growing season, plant productivity was still reduced in the snow removal plots; however, soil N losses and bacterial and fungal biomass were unaffected. N addition increased plant productivity in year two, but unlike the first year its effects were additive with snow removal effects. Our results suggest that episodes of soil freezing can decrease plant productivity over multiple growing seasons, but these decreases can be offset by the effects of increased N deposition. Furthermore, although soil freezing may increase N losses in the short term, these effects may not persist across multiple years despite lasting effects on plant biomass.