Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest

Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N 2 O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N 2 O, methane (CH 4 ) and carbon dioxide (CO 2 ) in field chambers monthly (bi‐weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO 2 and CH 4 showed a strong seasonal pattern, with low rates in winter, but N 2 O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N 2 O flux and decreased CH 4 uptake in both treatment years, especially during winter. Annual N 2 O fluxes in sugar maple treatment plots were 207 and 99 mg N m −2  yr −1 in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N 2 O or CO 2 fluxes, but CH 4 uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N 2 O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N 2 O fluxes from northern hardwood forests.