The high organic carbon accumulation in estuarine wetlands necessarily does not represent a high CO2 sequestration capacity

Nutrient enrichment by high aquaculture effluent input exacerbates imbalances between methane production and oxidation in mangrove sediments

Frequent aquaculture activities introduce substantial nutrients into mangrove ecosystems; however, the impact of this nutrient enrichment on methane (CH4) emissions and the associated microbial communities remains largely unexplored. In this study, we used the static chamber method, combined with 16S rRNA-based, metagenomic sequencing and binning techniques, to investigate the emission patterns of greenhouse gases (GHGs), with a particular focus on CH4, in mangroves subjected to different levels of effluents. The results showed that the effluent input decreased the mineral protection of sediment carbon (C) pools and increased C loss by more than double. In particular, high effluent input increased CH4 emissions by 243.3 %. Random forest analysis revealed that changes in methanogens were an important factor in explaining the variation of CH4 emissions. Amplicon data showed that the proportion of methylotrophic methanogens increased after effluent input, and metagenomic binning further attributed this change to the adaptability of methylotrophic methanogens to the substances transporting by the effluent. The enhanced hypoxia in sediments resulting from effluent input promoted the transition of methanotrophic communities from aerobic to anaerobic types and made anaerobic oxidation of CH4 more reliant on sulfur reduction rather than nitrate reduction. The PLS model further revealed that the nutrients brought by effluent input stimulated an increase in DOC content which induced an imbalance between CH4 production and oxidation in sediments by facilitating methanogens but inhibiting methanotrophs, ultimately resulting in an increase in CH4 fluxes. These findings underscore the significance of mangroves receiving effluent input as critical consequent reactors, highlighting the necessity to consider effects of high nutrient enrichment by aquaculture effluent input on GHG emissions and blue C potential in mangroves.

Spatial-temporal impacts of invasive Spartina anglica on the rates and pathways of organic carbon mineralization and resulting C-Fe-S cycles in the intertidal wetland of the Han River Estuary, Yellow Sea

To elucidate the spatial-temporal impact of invasive saltmarsh plant Spartina anglica on the biogeochemical processes in coastal wetlands, we investigated the rates and partitioning of organic carbon (Corg) mineralization in three representative benthic habitats: (1) vegetated sediments inhabited by invasive S. anglica (SA); vegetated sediments by indigenous Suaeda japonica; and (3) unvegetated mud flats. Microbial metabolic rates were greatly stimulated at the SA site during the active growing seasons of Spartina, indicating that a substantial amount of organic substrates was supplied from the high below-ground biomass of Spartina. At the SA site, sulfate reduction dominated the Corg mineralization pathways during the plant growing season, whereas iron reduction dominated during the non-growing season. Overall, due to its greater biomass and longer growing season than native Suaeda, the expansion of invasive Spartina is likely to greatly alter the Corg-Fe-S cycles and carbon storage capacity in the coastal wetlands.

Linking carbon storage with land use dynamics in a coastal Ramsar wetland

Coastal wetland ecosystems make an important contribution to the global carbon pool, yet their extent is declining due to aquaculture-related land use changes. We conducted an extensive investigation into the carbon stock and area coverage of macrophytes in a tropical coastal Ramsar wetland, Kolleru in Andhra Pradesh, India. A total of 72 quadrats of size 1 × 1 m2 were laid in the wetland, 19 species of macrophytes were collected and analyzed for carbon content using a CNHS analyzer. To assess changes in the wetland macrophytes, Normalized Difference Vegetation Index (NDVI) was estimated using Landsat time series data from 1975 to 2023. The importance value index (IVI) of macrophytes scored highest for the Ipomoea aquatica (41.4) and the lowest for Ottelia alismoides (1.9). Non-metric multidimensional scaling (NMDS) significantly (r = 0.1905, p = 0.0361) revealed a clear separation of macrophytes in ordination space. ANOVA indicated highly significant (p < 0.0001) variations in the carbon content of aboveground and belowground components of macrophytes. Among the different macrophytes, the highest carbon content was found in Phragmites karka (0.6 g. g-1) and the lowest was recorded in Utricularia stellaris (0.2 g. g-1). On an average, emergents in the Kolleru wetland sequester 1525 ± 181 g C m-2 yr-1, rooted floating species sequester 858 ± 101 g C m-2 yr-1, submerged macrophytes sequester 480 ± 60 g C m-2 yr-1, and free-floating macrophytes sequester 221 ± 90 g C m-2 yr-1. Land cover mapping revealed a decrease in spread of aquatic vegetation from 225.2 km2 in 1975 to 100.6 km2 in 2023. Although macrophytes are vital carbon sinks, the wetland conversion into fishponds has resulted in a loss of 55.3 % of carbon storage. Therefore, immediate restoration of macrophyte cover is vital for the proper functioning of carbon sequestration and mitigation of climate change impacts.

Preliminary manifestation of the Yangtze River Protection Strategy in improving the carbon sink function of estuary wetlands

In 2016, the Yangtze River Protection Strategy was proposed and a series of measures were applied to restore the health and function of the Yangtze River ecosystem. However, the impact of these measures on the carbon (C) sink capacity of the Yangtze River estuary wetlands has not been exhaustively studied. In this work, the effects of these measures on the C sink capacity of Yangtze River estuary wetlands were examined through the long-term monitoring of C fluxes, soil respiration, plant growth and water quality. The C flux of the Yangtze River estuary wetlands has become increasingly negative after the implementation of these measures, mainly owing to reduction in soil CO2 emission. The decrease in the chemical fertilizer release and returning farmland to wetland had led to the improvement of water quality in the estuary area, which further reduced soil heterotrophic microbial activity, and ultimately decreasing soil CO2 emissions of estuary wetlands.