Physical and chemical factors controlling carbon gas emissions and organic matter transformation in coastal wetlands

Abstract

Wetlands are important sinks for the atmospheric carbon (C) and play a major role in the global carbon cycle. However, factors impacting wetland soil C transformations and C gas production are not yet well understood. Elucidating these influences is especially important to Louisiana as wetlands are being impacted by salt water intrusion, subsidence and Mississippi river water diversion activities. This study evaluates, the effects of salinity, ions in river and sea water (K+, Ca2+) , clays and electron acceptors (NO3-, SO42-) on soil C transformations as well as C gas production from Louisiana coastal wetlands. Wetland soils were collected from forest swamp (FS), freshwater marsh (FM), and saline marsh (SM) and various characterizations were carried out. Aerobic incubations showed that addition of either K+ or Ca2+ chloride salts significantly increased CO2 production from FS soil, but had little effect on CO2 production from FM soil. Clay addition (2 and 5%) to FM soil significantly decreased CO2 production compared to unamended soil (P < 0.02). A combination of 5% clay and 5 mM Ca2+ further decreased the CO2 production in this soil. Increase in salinity decreased CO2 production from both FS and FM soils. In anaerobic incubations, addition of alternative electron acceptors, NO3- decreased CO2 production significantly whereas SO42- had little effect. Nitrate and SO42- decreased CH4 production but the NO3- almost completely inhibited CH4 production (>99%). Among the three wetland soils, FM exhibited the greatest denitrification potential (PDR), but it also tends to yield more N2O as compared to FS and SM soils. Soil organic C has significant effect on regulating PDR (P< 0.007). Among the different organic C mioties, polysaccharides positively influenced PDR (P< 0.003) while phenolics had negative effect (P < 0.03). Labile organic C as measured by aerobically mineralizable C was positively correlated with polysaccharides and carboxylic C. Further, characterization of humic acids (HA) in these wetland soils showed that FM HA had greater aromaticity whereas FS HA contained more aliphatic C. Increasing salinity tended to cause an increase in crystalline nature of aliphatics and guaicyl structural units in HA, suggesting more resistant HA formation.