Tidal elevation is the key factor modulating burial rates and composition of organic matter in a coastal wetland with multiple habitats

This study examines long-term burial rates of organic carbon (OC), organic nitrogen (ON), and total sulphur (TS) in a tidal-dominated coastal wetland with a high spatial heterogeneity and habitat diversity, and long history of human impacts, Cadiz Bay (SW Spain). Using replicate sediment cores, we quantified fluxes of these elements over a transect, extending from the lower saltmarsh (Spartina maritima, similar to 0.3 m mean sea level, MSL) to the lower intertidal region (Zostera noltei, similar to - 0.7 m MSL). Potential organic matter (OM) sources to the sediment were examined using an extensive dataset on carbon and nitrogen stable isotopes, and C:N molar ratios of primary producers in the region. OC burial rates decreased from the sites below MSL (similar to 80 gC.m(-2).y(-1)) to the lower saltmarsh (similar to 50 gC.m(-2).y(-1)), whereas ON burial rates showed an opposite pattern (similar to 3 gN.m(-2).y(-1) and similar to 4 gN.m(-2).y(-1) observed below and above MSL, respectively). TS burial rates (0.5-46 gS.m(-2).y(-1)) did not show any trend along the sea-land gradient. Hence, (tidal) elevation appeared to be an important determinant of sediment biogeochemical properties, and predictor of OM burial rates. The Bayesian mixing model suggested a well-mixed combination of subtidal and terrestrial/high-marsh OM sources to the surface sediments, with no clear indication of an increased contribution from the particular vegetation species inhabiting the sediments. The indication that there is substantial transport, remineralization and cycling of OM between habitats, suggests diversity may play an important role in maintaining this function, reinforcing the idea that a holistic, catchment-scale view is appropriate for understanding and preserving the long-term burial of OM in coastal wetlands. (C) 2020 Elsevier B.V. All rights reserved.