Temperate Forest Soils Sequester as much Carbon as Trees in Response to Nitrogen Deposition

The terrestrial biosphere sequesters up to a third of annual anthropogenic carbon dioxide emissions, offsetting a substantial portion of greenhouse gas forcing of the climate system.  While a number of factors are responsible for this terrestrial carbon (C) sink, atmospheric nitrogen (N) deposition is thought to play an important role by enhancing tree productivity and promoting C sequestration in tree biomass. However, recent work indicates that the tree response may be less than predicted and that forest soils may represent an important and understudied C sink. Here, we examine the relative contribution of trees versus soil to total ecosystem C storage at the Harvard Forest LTER, and we investigate the mechanisms by which soils accumulate C under elevated N conditions. We find that 20 years of N fertilization of a temperate forest (hardwood and pine stands) resulted in an 11-38% increase in ecosystem C storage in the hardwood stand, representing an accumulation of 20-30 kg C kg^-1 N added. More than half (59-87%) of this C storage was attributable to an accumulation of soil organic matter, indicating that the soil has been more responsive to N additions than tree growth.  In the pine stand, a 9-18% loss of total standing tree biomass due to high tree mortality was offset by a 6-10% increase on soil C, resulting in no net change in ecosystem C balance.  We conclude that temperate forest soils represent a sink for atmospheric C under current and future scenarios of N deposition that are of equal or greater magnitude than that sequestered in trees.  We do not have strong evidence for a significant stimulation of plant C inputs to soil via litterfall or root production.  However, soil C accumulation was associated with reduced microbial biomass, enzyme activity, litter/wood decay, increased lignin concentrations, and a change in fungal community structure and function, suggesting that the observed soil C sequestration is primarily due to a suppression of organic matter decomposition.