Partitioning ecosystem respiration in lower coastal plain forests: Quantifying sensitivity of soil C and N cycling to variation in the water table

John S. King, Department of Forestry and Environmental Resources, North Carolina State University

Co-investigator: Asko Noormets, Department of Forestry and Environmental Resources, North Carolina State University

Abstract

Mechanistic understanding of ecosystem respiration (ERs) lags behind that of carbon (C) uptake through assimilation. The proposed work will reduce uncertainties associated with predicting ERs by generating data to improve regional C cycle model parameterization.

We hypothesize that nitrogen mineralization from decomposition of soil organic matter, which supports microbial and plant metabolic activity sustaining high rates of productivity and C turnover, is under strong climatic and hydrologic control in lowland settings. We propose to evaluate the mechanistic linkages between nitrogen mineralization, ecosystem carbon balance, the primary sources of respired carbon, and how they change with changes in the ground water table and climatic drivers, which will be used as proxies for predicted climate change and sea level rise.

The work will be conducted in forests of the lower coastal plain (LCP) of North Carolina and will complement two existing eddy covariance flux tower sites in 3- and 15-year old loblolly pine plantations. Sites were selected because (i) the LCP mixed forest province represents a 450,000 km² ecoregion that stores and cycles large quantities of C (soil stocks in excess of 100 t ha^-1 in the top 1 m), yet is severely under-represented in national monitoring networks, and (ii) studies of long-term changes in soil C stocks have detected the greatest losses in soils high in organic C (e.g. Bellamy et al. 2005).

To assess climatic and hydrologic sensitivity of partitioning ERs between heterotrophic and autotrophic sources, we will expand our ongoing studies in the LCP by establishing a new monitoring site in a mixed-native hardwood stand that has experienced minimal disturbance to vegetation, soils, and hydrology. The new site will help bracket the range of environmental conditions (soil types, vegetation, and land uses) that occur across the Southeast LCP. We will add new measurements at all sites to partition ER with eddy covariance, soil respiration, stable isotopes, and biometric and chemical analyses of plants and soil organic C.

The work will provide detailed information for our forest types of processes controlling soil carbon accumulation and turnover, but will also provide critical data to modeling syntheses aimed at improving regional C cycle models (e.g. Tian et al. 2006). It will also contribute to specific NICCR Focus 3 objectives, general goals of the USGCRP, and provide unique insights regarding the phenology of ERs to the developing National Phenological Network established to document ecosystem responses to climate change.