Snow Depth, Soil Freezing and Nitrogen Losses In Northern Hardwood Forest Soils.

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 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.  Snow manipulation studies have shown that decreases in snow depth induce soil freezing and marked increases in nitrate and phosphate leaching and nitrous oxide flux.  The increase in losses appears to be driven by decreases in plant uptake caused by freezing-induced root mortality.  Freeze effects on nutrient losses in the snow manipulation studies have varied with the overstory tree composition in experimental plots and with dissolved organic carbon response to the snow removal treatment.  Natural gradient studies have supported the idea that increases in mean annual temperature will lead to increases in soil freezing during winter and 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.