Soil freezing alters fine root dynamics in a northern hardwood forest

Abstract

The retention of nutrients within an ecosystem depends on temporal and spatial synchrony between nutrient availability and nutrient uptake, and disruption of fine root processes can have dramatic impacts on nutrient retention within forest ecosystems. There is increasing evidence that overwinter climate can influence biogeochemical cycling belowground, perhaps by disrupting this synchrony. In this study, we experimentally reduced snow accumulation in northern hardwood forest plots to examine the effects of soil freezing on the dynamics of fine roots (< 1 mm diameter) measured using minirhizotrons. Snow removal treatment during the relatively mild winters of 1997-1998 and 1998-1999 induced mild freezing temperatures (to -4°C) lasting approximately three months at shallow soil depths (to -30 cm) in sugar maple and yellow birch stands. This treatment resulted in elevated overwinter fine root mortality in treated compared to reference plots of both species, and led to an earlier peak in fine root production during the subsequent growing season. These shifts in fine root dynamics increased fine root turnover but were not large enough to significantly alter fine root biomass. No differences in morality response were found between species. Laboratory tests on potted tree seedlings exposed to controlled freezing regimes confirmed that mild freezing temperatures (to -5°C) were insufficient to directly injure winter-hardened fine roots of these species, suggesting that the marked response recorded in our forest plots was caused indirectly by mechanical damage to roots in frozen soil. Elevated fine root necromass in treated plots decomposed quickly, and may have contributed an excess flux of about 0.5 g N/m2·yr, which is substantial relative to measurements of N fluxes from these plots. Our results suggest elevated overwinter mortality temporarily reduced fine root length in treatment plots and reduced plant uptake, thereby disrupting the temporal synchrony between nutrient availability and uptake and enhancing rates of nitrification. Increased frequency of soil freezing events, as may occur with global change, could alter fine root dynamics within the northern hardwood forest disrupting the normally tight coupling between nutrient mineralization and uptake.