Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest

Overwinter and snowmelt processes are thought to be critical to controllers of nitrogen (N) cycling and retention in northern forests. However, there have been few measurements of basic N cycle processes (e.g. mineralization, nitrification, denitrification) during winter and little analysis of the influence of winter climate on growing season N dynamics. In this study, we manipulated snow cover to assess the effects of soil freezing on in situ rates of N mineralization, nitrification and soil respiration, denitrification (intact core, C₂H₂ -based method), microbial biomass C and N content and potential net N mineralization and nitrification in two sugar maple and two yellow birch stands with reference and snow manipulation treatment plots over a two year period at the Hubbard Brook Experimental Forest, New Hampshire, U.S.A. The snow manipulation treatment, which simulated the late development of snowpack as may occur in a warmer climate, induced mild (temperatures > -5°C) soil freezing that lasted until snowmelt. The treatment caused significant increases in soil nitrate (NO₃^-) concentrations in sugar maple stands, but did not affect mineralization, nitrification, denitrification or microbial biomass, and had no significant effects in yellow birch stands. Annual N mineralization and nitrification rates varied significantly from year to year. Net mineralization increased from ∼12.0 g N m^-2 y^-1 in 1998 to ∼22 g N m^-2 y^-1 in 1999 and nitrification increased from ∼8 g N m^-2 y^-1 in 1998 to ∼13 g N m^-2 y^-1 in 1999. Denitrification rates ranged from 0 to 0.65 g N m^-2y-1. Our results suggest that mild soil freezing must increase soil NO₃^- levels by physical disruption of the soil ecosystem and not by direct stimulation of mineralization and nitrification. Physical disruption can increase fine root mortality, reduce plant N uptake and reduce competition for inorganic N, allowing soil NO₃^- levels to increase even with no increase in net mineralization or nitrification.