Soil Carbon Characteristics in Two Mississippi River Deltaic Marshland Profiles

Abstract

Current understanding of the soil organic matter (SOM) transformation pathways and mechanisms in coastal wetland ecosystems is limited. This study was carried out to characterize and compare chemical composition of belowground SOM from a freshwater marsh and a saline marsh in coastal Louisiana. Composite samples at three depths (0–50, 50–100, and 100–150 cm) from each site were collected and characterized by solid state 13C NMR spectroscopy. The 13C NMR spectral composition of three soil depths of the freshwater marsh was similar and dominated by contribution from polysaccharide carbon (C), followed by equal contributions from aliphatic C and total aromatic C. A steady and more pronounced drop in polysaccharide C with depth, accompanied by the de-methoxylation of lignin moieties, was observed in the profiles of the saline marsh. The δ13C signature of these marsh soils suggested that much of the C in the saline profile was likely derived from the mixed debris of previous freshwater and saline marsh vegetation rather than from the current salt-tolerant Spartina species alone. It was concluded that the chemical composition of SOM was determined more by the difference between the freshwater and the saline soil environment than by the C source in the coastal wetland ecosystems.