Positive vegetation response to nitrogen enrichment offsets salt marsh cover decline without altering dissolved carbon concentration in sediment pore water

Fertilization in salt marshes is expected to enhance primary production due to the addition of nitrogen (N), a commonly limiting nutrient in these environments. As a consequence, the concentration of dissolved carbon (C) in sediment pore water may increase due to enhanced organic matter (OM) degradation. We tested this hypothesis through a field experiment involving N addition in the salt marsh of the Mar Chiquita coastal lagoon (Buenos Aires Province, Argentina). Spartina densiflora showed a positive response to nitrogen enrichment, counteracting an overall decline in its cover observed from 2021 toward 2024. Dissolved C concentrations in pore water did not show a direct response to fertilization; however, both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) increased by 2024. This suggests the emergence of new potential sources of OM in the salt marsh, both allochthonous and autochthonous. Additionally, low rainfall during the study period likely reduced the dilution of DOC and DIC, thereby contributing to their accumulation. Since dissolved C is a mobile form of C, increases in its concentration in sediment pore water could have implications for C loss from salt marshes to adjacent environments. Our results indicate that the effect of N addition on primary production was decoupled from changes in dissolved C concentrations in sediment pore water.