Linking eutrophication to carbon dioxide and methane emissions from exposed mangrove soils along an urban gradient.

Mangroves are one of the most important but threatened blue carbon ecosystems globally. Rapid urban growth has resulted in nutrient inputs and subsequent coastal eutrophication, associated with an enrichment in organic matter (OM) from algal and sewage sources and substantial changes in greenhouse gas (GHG) emissions. However, the effects of nitrogen (N) and phosphorus (P) enrichment on mangrove soil OM composition and GHG emissions, such as methane (CH) and carbon dioxide (CO), are still poorly understood. Here, we aim to evaluate the relationships between CO and CH efflux with OM composition in exposed soils from three mangrove areas along watersheds with different urbanization levels (Rio de Janeiro State, Brazil). To assess spatial (lower vs. upper intertidal zones) and seasonal (summer vs. winter) variability, we measured soil-air CO and CH fluxes at low spring tide, analyzing elementary (C, N, and P), isotopic (δC and δN), and the molecular (n-alkanes and sterols) composition of surface soil OM. A general trend of OM composition was found with increasing urban influence, with higher δN (proxy of anthropogenic N enrichment), less negative δC, more short-chain n-alkanes, lower C:N ratio (proxies of algal biomass), and higher epicoprostanol content (proxies of sewage-derived OM). The CO efflux from exposed soils increased greatly in median (25/75 % interquartile range) from 4.6 (2.9/8.3) to 24.0 (21.5/32.7) mmol m h from more pristine to more urbanized watersheds, independent of intertidal zone and seasonality. The CO fluxes at the most eutrophicated site were among the highest reported worldwide for mangrove soils. Conversely, CH emissions were relatively low (three orders of magnitude lower than CO fluxes), with high peaks in the lower intertidal zone during the rainy summer. Thus, our findings demonstrate the influence of coastal eutrophication on global warming potentials related to enhanced heterotrophic remineralization of blue carbon within mangrove soils.