Soil Freezing and the Acid-Base Chemistry of Soil Solutions in a Northern Hardwood Forest

Reductions in the depth and duration of snow cover under a warmer climate may cause soil freezing events to become more frequent, severe, and spatially extensive in northern temperate forest ecosystems. In this experiment, snow cover was manipulated to simulate the late development of snowpack and to induce soil freezing at sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) stands at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire. The objective of this manipulation was to elucidate the effects of soil freezing on the concentrations and fluxes of soil solution H+, Ca2+, Mg2+, K+, and Na+, as well as values of acid neutralizing capacity (ANC). Mild soil freezing events (soil temperatures never decreased below -5(degree)C) resulted in pronounced acidification of soil solutions, driven primarily by nitrification, in the forest floor of sugar maple stands during the growing season. This mobilization of NO-3 from the forest floor of maple stands was accompanied by the leaching of Ca2+ and Mg2+ in Oa horizon solutions. Responses of soil solution acid-base chemistry to soil freezing were not evident in yellow birch stands or in the Bs horizon of either vegetation type, emphasizing the importance of vegetation type and the mineral soil in determining the effects of climatic disturbance on drainage water chemistry and nutrient loss. These results suggest that models of soil biogeochemistry in temperate forest ecosystems should consider soil-freezing events when simulating the acid-base chemistry of soil solutions and the translocation of nutrient base cations between soil horizons.