Colorado mountains

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

Poster Number: 
Presenter/Primary Author: 
Serita Frey
Richard Bowden*, Eddie Brzostek, Andrew Burton, Bruce Caldwell, Susan Crow, Christine Goodale, Stuart Grandy, Adrien Finzi, Marc Kramer, Kate Lajtha, Mary Martin, Bill McDowell, Rakesh Minocha, Knute Nadelhoffer, Scott Ollinger, Pam Templer, and Kyle Wicking

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.

Background Photo by: Nicole Hansen - Jornada (JRN) LTER