Investigations of belowground carbon dynamics in east coast salt marshes, USA

Salt marsh, a productive transition zone between terrestrial and marine environments, is a key component of the carbon cycle in estuarine ecosystems. The goal of this study is to investigate salt marsh belowground carbon dynamics, specifically to distinguish root system respiration (RSR) from soil organic matter (SOM) decomposition. Two types of salt marshes were chosen for investigation. The first type is a mineral salt marsh located in North Inlet Estuary (NIE), South Carolina, where marsh accretion is dominated by mineral input. The other type of salt marsh is a peat salt marsh located in Plum Island Estuary (PIE), Massachusetts, where marsh elevation is maintained by organic matter accumulation.;A dieback site located in Sixty Bass (NIE) was used as a control and compared to other healthy salt marsh sites. Total organic carbon, total organic nitrogen, C/N ratios and carbon isotope composition of SOM, and pore water samples were measured as a function of depth in the two types of salt marshes to evaluate if the marsh plants are the major organic matter source to SOM pool. In vitro and monthly in situ soil incubations were conducted to trap soil-respired CO2. The paired carbon isotope composition of SOM and soil-respired CO2 were used to estimate the contribution of RSR to total soil respiration (TSR), and to investigate how its contribution varies over time. Temperature and root growth rate were used to evaluate their influence on the variations in the contribution of RSR to TSR and soil respiration rate. In addition, CO2 efflux was measured monthly to estimate the annual CO2 emission from RSR and SOM decomposition.;Results from this study indicate that belowground production of marsh plants is the dominant organic matter source in salt marsh soils. RSR is significant in mineral and peat salt marshes, accounting for 19 to 90% of TSR. Furthermore, RSR and soil respiration rate vary over time as a function of temperature and root growth rate. These two factors account for 44% and 58% of the observed variations, respectively. Our findings imply that RSR changes temporally and spatially, and accounts for a significant fraction of TSR. RSR plays an important role in salt marsh belowground carbon dynamics. Our study will help better quantify salt marsh carbon budget and better understand the influence of climate change and sea-level rise on soil respiration and carbon sequestration in salt marsh ecosystems.