Freezing effects on carbon and nitrogen cycling in northern hardwood forest soils

We evaluated the effects of freeze-thaw events on soil respiration, nitrogen mineralization, nitrification, and nitrous oxide production in soils from a northern hardwood-dominated forest in New Hampshire, USA. Soil samples from three horizons (O_e, O_a, A) from nearly monospecific stands of sugar maple (Acer saccharum Marshall) and yellow birch (Betula alleghaniensis Britton) were placed in 200-mL incubation vessels (microlysimeters), subjected to severe (-13°C) and mild (-3°C) freeze treatments for 10 d, and then incubated at laboratory temperature (20-25°C) for 3 wk. Evolution of CO₂ and N₂O and concentrations of leachable NH₄⁺ and NO₃^- were measured at weekly intervals. Freezing increased rates of C and N cycling in these soils, but the effects varied with species, horizon, and freeze treatment. Whereas severe freezing stimulated respiration, N₂O flux, and mineralization, mild freezing had very few effects. Nitrification did not appear to be affected by either freeze treatment, but increases in denitrification may have masked freeze effects on this process. Freeze effects were much more marked in maple than in birch soils and in the O_a and A horizons. Maple consistently had higher rates of nitrification and N₂O production than did birch. The species and horizon differences were likely driven by higher levels of available C in the birch soils and O_e horizon at both sites. These results suggest that changes in climate and snow cover that influence soil freezing could increase N and C losses from northern hardwood forest ecosystems with potential effects on soil fertility and carbon storage, receiving water quality, and atmospheric chemistry.