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 (N2O)) 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 N2O, methane (CH4) and carbon dioxide (CO2) 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 CO2 and CH4 showed a strong seasonal pattern, with low rates in winter, but N2O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N2O flux and decreased CH4 uptake in both treatment years, especially during winter. Annual N2O 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 N2O or CO2 fluxes, but CH4 uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N2O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N2O fluxes from northern hardwood forests.