Research · Terrestrial Ecology

Forest Soil Carbon Dynamics in the Southern Appalachian Mountains

Goal

The goal of this research is to further the understanding of forest soil C and N dynamics in a changing climate, particularly the dynamics of labile soil C stocks in mountainous terrain (see photo). Quantification of soil C stocks in mountainous areas was identified as a knowledge gap in the North American Carbon Program (NACP). In order to predict future forest soil C stocks, we must understand how different environmental factors (temperature, N availability, litter chemistry) affect input and output processes that control amounts of forest soil organic matter (SOM). This study provides empirical data, for use in mathematical models, on amounts of forest soil C in mountainous terrain and factors affecting forest soil C dynamics.

Objective

The research objectives are directed at supplying answers to the following questions that are relevant to DOE’s Terrestrial Carbon Processes Program: 1) how do forest soil C and N stocks vary along elevation gradients in the southern Appalachian Mountains? 2) how do different environmental factors (e.g., soil temperature and litter chemistry) affect C apportionment among different soil pools? 3) what is the effect of C apportionment on soil C dynamics (i.e., soil C turnover times)? 4) how do changes in temperature and litter chemistry interact to influence measured and predicted soil C stocks? Each of foregoing questions is related to how forest ecosystems will respond to regional and global climate change. Although the research involves modeling, these questions cannot be resolved solely through the use of C cycle models, but require field studies to verify the effect of a changing climate on the apportionment of C among different soil pools, the effect of that apportionment on soil C storage and turnover, and the effect of N availability and litter chemistry on forest soil C dynamics.

Approach

The approach involves studies of amounts and forms of soil C and N at multiple forest sites along an elevation gradient, the use of ¹⁵N and ¹³C enriched isotopes as tracers, measurements of N and C isotopes at natural abundance levels, and modeling. Environmental changes along elevation gradients in the southern Appalachians include varying regimes of temperature and precipitation, N availability, and litter chemistry. Although it is widely recognized that many environmental factors vary with elevation, the use of altitudinal gradients as a resource in climate change research has not received much attention. One primary benefit of this approach is that elevation gradients reflect numerous interacting abiotic and biotic factors and, importantly, the long-term effect of climate and ecosystem processes on soil C storage. Field studies began in 1995. Past work has utilized stable C isotope techniques to ascertain differences in SOM decomposition rates and their relationship to N availability. Overall, measurements of amounts and forms of soil C have been made at ten forest sites. The data have been used in models of soil C dynamics to predict changes in labile and mineral-associated organic matter as a consequence of regional climate change.

Results to Date

Early field work funded under this research project demonstrated that forest ecosystems in the southern Appalachian Mountains contain relatively large stocks of labile soil C, both in the forest floor and as particulate organic matter in the mineral soil. There is a strong inverse relationship between the calculated turnover time of labile soil C and mean annual temperature along the elevation gradient. Modeling of soil C dynamics, based partly on field measurements, indicated that regional warming in the southeastern US, as a part of global climate change, could result in substantial losses of soil C from both low- and high-elevation forest ecosystems.

More recently, studies of natural ¹³C-abundance have been used to investigate the relative importance of factors controlling SOM decomposition and forest soil C turnover along the elevation gradient. Carbon-13 abundance increases with soil depth and decreasing C concentrations along a continuum of decomposition from fresh litter inputs to more decomposed soil constituents. The extent of change in ¹³C-abundance from forest litter inputs to deep mineral soil C is significantly associated with elevation and temperature. Vertical changes in soil ¹³C-abundance, C concentrations, and C:N ratios are interrelated through climate controls on decomposition. Temperature and litter quality appear to control the extent of isotopic fractionation during SOM decomposition. Loss rates of ¹³C-labelled glycine in soil are also inversely related to elevation and directly related to temperature. Different patterns of decomposition and potential net soil N mineralization can be explained within a framework of climate, substrate chemistry, and coupled soil C and N stocks. Although less SOM decomposition is indicated at cool, high-elevation forests, low substrate C:N ratios in these N-rich systems result in more N release (N mineralization) for each unit of C converted to CO₂ by soil microorganisms.

In FY 2004, we completed an analysis of patterns in forest soil C stocks and predicted turnover times with increasing altitude. Using field measurements in mature and undisturbed forests and Monte Carlo type calculations, the turnover time of soil C was calculated for different sites over a 1.3 km rise in elevation. Soil C stocks (to a 30 cm soil depth) ranged from 4.4 to 12.2 kg C m-^-2. The calculated median turnover time of soil C ranged from 11 to 31 years. Both forest soil C stocks and the predicted turnover time of soil C increase with altitude (see Figure). The analysis indicated that altitudinal differences in soil C stocks arise from changing rates of SOM decomposition rather than changes in soil C inputs. Soil C storage and turnover times increase from warmer, drier, less N-rich forests to colder, wetter, more N rich forests. Predicted turnover times are significantly correlated with vertical changes in soil ¹³C-abundance indicating that measurements of stable C isotopes are related to forest soil C dynamics.

A preliminary analysis of data from the first 3 years of a soil transplant experiment designed to disentangle the effects of altitudinal trends in temperature and N availability (litter quality) on soil C storage and dynamics were also completed in FY 04. Reciprocal transplants between forest sites with similar C:N ratios (approx. 27) did not produce a change in soil C concentrations, even when cool, C-rich (6.5%) soils were transplanted to warmer sites. Reciprocal transplants between forest sites with different C:N ratios caused a decline in soil C and N, and increased net soil N mineralization, when soils from cool, high-elevation forests with high quality organic matter were moved to warmer sites. The results indicate (1) an interaction between climate and litter quality in soil organic matter decomposition and (2) regional warming will accelerate C and N loss, and net soil N mineralization, in high-elevation forest soils more than in low-elevation forest soils. In FY 2004, we initiated an integration of field and modeling studies of ORNL’s Global Carbon Cycle Studies FWP by partial parameterization the ORNL LoTEC ecosystem model to predict soil C stocks along the elevation gradient in the southern Appalachian Mountains. Preliminary data sets were compiled for ten forest sites. There are two research objectives associated with this task: 1) determine if LoTEC can reproduce observed changes in soil C stocks along the gradient of varying climate and litter quality in mountainous terrain, and 2) utilize LoTEC to sort out controls of various interacting environmental factors on soil C dynamics along the elevation gradient.

Significant Publications

Garten Jr. CT, Post III WM, Hanson PJ, Cooper LW. 1999. Forest soil carbon inventories and dynamics along an elevation gradient in the southern Appalachian Mountains. Biogeochemistry 45: 115-145.

Hanson PJ, Edwards NT, Garten Jr. CT, Andrews, JA. 2000. Separating root and soil microbial contributions to soil respiration: a review of methods and observations. Biogeochemistry 48: 115-146.

Garten Jr. CT, Cooper LW, Post III WM, Hanson, PJ. 2000. Climate controls on forest soil C isotope ratios in the southern Appalachian Mountains. Ecology 81: 1108-1119.

Garten Jr. CT. 2002. Soil carbon sequestration beneath recently established tree plantations in Tennessee and South Carolina, USA. Biomass & Bioenergy 23: 93-102.

Garten Jr. CT. 2004. Potential net soil N mineralization and decomposition of glycine-¹³C in forest soils along an elevation gradient. Soil Biology and Biochemistry 36: 1491-1496.

Garten Jr. CT. 2004. Soil Carbon Dynamics Along an Elevation Gradient in the Southern Appalachian Mountains (ORNL/TM-2004/50). Oak Ridge National Laboratory, Oak Ridge, TN.

Garten Jr. CT. Hanson PJ. Forest soil C amounts and turnover along an elevation gradient. Geoderma (submitted).

For more information, contact:
C. T. Garten, Jr. (gartenctjr@ornl.gov, 865-574-7355)

Revised: 8/03/05