A Multi-scale Study on Respiratory Processes in a Lower Coastal Plain Forested Wetland in the Southeastern United States

Carbon cycling in wetlands is an important component of the carbon budget in terrestrial ecosystems. Environmental change and land conversion for agricultural use have been affecting wetlands in recent decades and might change their role from carbon sinks to sources. However, carbon dynamics in wetlands are less-well investigated than in upland systems and still absent in most regional- or global-scale studies. This research, as a part of a comprehensive study on carbon dynamics of a lower coastal plain forested wetland in the Southeastern USA, focused on respiration - a key determinant affecting the role of an ecosystem as carbon source or sink.;Ecosystem respiration, soil respiration and decomposition of coarse woody debris were investigated by quantifying CO2 emissions of each in response to temperature and water table fluctuation. Soil respiration was intensively measured with an automated system to obtain high-frequency (30 minutes) temporal data as well as a survey system to characterize spatial variation. Stable isotope composition (13C) of soil-respired CO2 was also studied to understand the components of soil respiration. Ecosystem respiration was measured by an automated eddy-covariance flux system. Decomposition of coarse woody debris was measured monthly across the same survey area as soil respiration. To account for effects of microtopography on respiration and decrease up-scaling uncertainties, an ancillary study on microtopographic variation was also conducted.;Respiratory components under non-flooded conditions responded similarly to temperature between this forested wetland and upland ecosystems in an exponential pattern and with comparable temperature sensitivities. Hydrology had additional effects on soil respiration and ecosystem respiration, with the relationship resembling a saturating pattern of Michaelis-Menten reaction; that is, the rate increased with the water table drawdown and was nearly constant at deeper water table depths. Soil respiration responded rapidly to flooding and decreased significantly, whereas ecosystem respiration responded slowly because the plants were tolerant to flooding.;Based on the exponential pattern of response to temperature and Michaelis-Menten pattern of response to water table depth, models were developed to estimate soil respiration and ecosystem respiration. Ecosystem respiration was estimated at 1331 to 1731 g CO2-C m-2 yr-1, and soil respiration ranged 593 to 1082 g CO2-C m-2 yr-1 during the study period (2009- 2011). The contribution of soil respiration to ecosystem respiration increased with the water table drawdown, with an annual average relative contribution of 0.67+/-0.11 (mean+/-SD) during non-flooded periods and 0.24+/-0.06 when flooded. Decomposition of coarse woody debris contributed much less to ecosystem respiration, but with large uncertainty related to biomass estimation.;13C composition of soil-respired CO2 (delta 13CRs) varied seasonally and exhibited a significant relationship with hydrology and microtopography. The delta13CRs at mound microsites was depleted during summer and enriched in spring and autumn, whereas the delta13CRs at low-lying microsites exhibited the opposite pattern (i.e. enriched during summer and depleted in spring). This suggests that under warmer and drier conditions the relative contribution of root respiration to soil respiration increased at mound microsites, but soil organic carbon decomposition had a higher relative contribution at low-lying microsites. Accounting for microtopographic difference and hydrologic effects will improve methods partitioning soil respiration and differentiating fast and slow turnover carbon pools.