Will more nitrogen enhance carbon storage in young forest stands in central Appalachia?

Abstract

Many temperate deciduous forests in the Eastern US are secondary, regrowing forests and have experienced decades of elevated inputs of acidic compounds and biologically available nitrogen (N) from the atmosphere. These young forests play an important role in the global carbon (C) cycle as C sinks, and it is possible that acidic deposition will influence the strength and longevity of this sink. We used the Fernow Experimental Forest Long Term Soil Productivity (LTSP) experiment near Parsons, WV to evaluate how 13years of experimental N additions has affected ecosystem C storage and stand level dynamics in a young temperate deciduous forest. Specifically we examined whether N additions: (1) directly increased aboveground growth of regenerating trees but did so in a way that was independent of the indirect effects of soil acidification; (2) directly (independent of acidification effects) increased forest floor and soil C pools, and decreased the fine root C pool; and (3) lowered stand density and diversity. We also tested whether tree species were differentially affected by either N fertilization or soil acidification.Thirteen years of ammonium sulfate additions to a regenerating deciduous forest stimulated C storage by 22% – even in a region with historically high levels of atmospheric N deposition. This response was driven primarily by a 27% increase in C storage in aboveground biomass and to a lesser extent by a 35% increase in C stored in the smaller forest floor pool. Despite the dominance of a single tree species (Prunus pensylvanica), the overall response may have been tempered by reductions in stand density and was only detectable when the changes in all species were included, rather than when only the changes in the dominant species were examined. Indirect acidification effects were found to increase C accumulation in the forest floor and decrease the number of different tree species.In the short-term it is likely that N deposition will stimulate forest growth and C storage in young temperate deciduous forests. However, given the differential responses observed for longer-lived tree species versus the positive response for short-lived species, it appears that the long-term effects of N deposition on C storage in temperate deciduous forests may be different than the short-term effects, and may even be negative.