Linking pollution to climate change: Increases in carbon dioxide emissions from exposed mangrove soils along a eutrophication gradient (Rio de Janeiro, Brazil)

Abstract:

Mangroves are considered one of the most productive and threatened ecosystems in the biosphere, receiving and retaining large anthropogenic inputs of nutrients, especially nitrogen (N) and phosphorus (P). These blue carbon ecosystems are particularly relevant to global biogeochemistry by the capacity of their soils to store carbon (C) as organic matter, and emit it as carbon dioxide (CO₂). Here, we aim to evaluate variations in CO₂ fluxes from exposed mangrove soils to the atmosphere along a gradient of eutrophication, derived from different levels of urbanization in the surroundings. In addition, soil-air CO₂ fluxes were measured to assess spatial (lower vs upper intertidal zone) and seasonal (summer vs winter) variability at low tide during spring tide. After these measurements, we accounted visible crab burrows and pneumatophores on the area below chambers, and sampled ⁓2 cm surface soil for elementary (C, N, P) and isotopic (δ¹³C and δ¹⁵N) analysis. The abundance of crab burrows and pneumatophores was positively related to CO₂ emissions mainly at lower intertidal sites where Avicennia sp. was dominant. Mean CO₂ emissions during the studied period ranged from 5.7±2.4 to 32.7±22.3 mmolm^-2h^-1, with highest values at the lower intertidal zone of the most eutrophic mangrove. This site still showed lower total organic carbon (TOC) content and its ratio with total nitrogen (TN), less negative δ¹³C indicating the presence of marine or estuarine particulate matter, and higher δ¹⁵N suggesting the inputs of sewage effluents. In turn, the less impacted mangrove exhibited a greater autochthonous contribution from mangrove trees in its surface soils, as indicated by higher organic carbon content and TOC:TN ratio associated to more depleted δ¹³C. These contrasting results along the eutrophication gradient support previous evidence that the remineralization of the additional organic matter (from sewage and algae) with nutrient enrichment leads to higher CO₂ outgassing from exposed mangrove soils. Further, our findings reveal that eutrophication could enhance greenhouse gas emissions and reduce organic carbon stocks, independent of intra-ecosystem spatial and seasonal variability. Therefore, mangrove pollution by untreated sewage may have climate implications, as an important component of global change.