Soil freezing perturbation to nitrogen cycling in the northern hardwood forest

Background/Question/Methods

Winter climate is changing markedly in the northern hardwood forests of northeastern North America.  Of great interest are changes in snow depth that influence soil freezing, which can be a significant disturbance of soil nutrient cycling processes.  In ongoing research at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA we have used snow manipulation (shoveling) plot and natural elevation gradient studies to evaluate the effects of changes in snow depth on soil freezing and nitrogen losses.  The manipulation studies involved removal of snow by shoveling in early winter and compared overstory tree composition (sugar maple versus yellow birch) and a range of soil freezing intensity by locating plots along the elevation gradient at the HBEF.  Analyses have also exploited natural variation in snow depth and freezing along this gradient.  Response variables have included nitrate leaching, nitrous oxide flux, potential and in situ net nitrogen mineralization and nitrification, microbial biomass carbon and nitrogen content and respiration, and denitrification. 

Results/Conclusions

Snow manipulations have consistently produced deep and persistent soil freezing, but effects on hydrologic and gaseous losses of nitrogen have varied markedly between studies.  Relationships between frost depth, frost heaving and nitrate leaching and nitrous oxide flux have been weak.  Comparison of studies suggests that dissolved organic carbon, which sometimes responds to the manipulation and sometimes does not, may be a key regulator of the nitrogen response to soil freezing i.e., soil freezing that mobilizes available carbon may stimulate retention of nitrogen and prevent losses. While results support the hypothesis that climate change resulting in less snow and more soil freezing will increase N losses from northern hardwood forests, they also suggest that ecosystem response to soil freezing disturbance is affected by multiple factors that must be reconciled in future research. Natural gradient studies have supported the idea that increases in mean annual temperature will lead to increases in soil freezing during winter and to drier soils in summer.  These changes increase nitrogen losses and decrease available nitrogen supply by mineralization and suggest that climate change may exacerbate nitrogen limitation and reduce the potential for nitrogen saturation in northern hardwood forests.