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 i nfluence 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 ra tes of N mineralization, nitrification and soil respiration, denitrificatio n (intact core, C2H2 - based method), microbial biomass C and N content and potential net N mineralization and nitrification in two sugar maple and tw o yellow birch stands with reference and snow manipulation treatment plots over a two year period at the Hubbard Brook Experimental Forest, New Hampsh ire, U.S.A. The snow manipulation treatment, which simulated the late devel opment of snowpack as may occur in a warmer climate, induced mild (temperat ures > -5 degreesC) soil freezing that lasted until snowmelt. The treatment caused significant increases in soil nitrate (NO3-) concentrations in suga r maple stands, but did not affect mineralization, nitrification, denitrifi cation or microbial biomass, and had no significant effects in yellow birch stands. Annual N mineralization and nitrification rates varied significant ly from year to year. Net mineralization increased from similar to 12.0 g N m(-2) y(-1) in 1998 to similar to 22 g N m(-2) y(-1) in 1999 and nitrifica tion increased from similar to8 g N m(-2) y(-1) in 1998 to similar to 13 g N m(-2) y(-1) in 1999. Denitrification rates ranged from 0 to 0.65 g N m(-2 ) y(-1). Our results suggest that mild soil freezing must increase soil NO3 - levels by physical disruption of the soil ecosystem and not by direct sti mulation of mineralization and nitrification. Physical disruption can incre ase fine root mortality, reduce plant N uptake and reduce competition for i norganic N, allowing soil NO3- levels to increase even with no increase in net mineralization or nitrification.