Mangrove sediment organic carbon storage and sources in relation to forest age and position along a deltaic salinity gradient

Abstract

Abstract. Mangroves are widely recognised as key ecosystems for climate change mitigation as they capture and store significant amounts of sediment organic carbon (SOC). Yet, there is incomplete knowledge on how sources of SOC and their differential preservation vary between mangrove sites in relation to environmental gradients. To address this, sediment depth profiles were sampled from mangrove sites ranging from river-dominated to marine-dominated sites and including old and young mangrove sites in the Guayas delta (Ecuador). The stable carbon isotope ratios (δ13C) and the elemental composition (OC %, C : N) of sediment profiles, local vegetation (i.e. autochthonous carbon) and externally supplied suspended particulate matter (i.e. allochthonous carbon) were obtained to assess variations in the amount and sources of SOC at different locations throughout the delta. In general, across all sites, we found that increasing SOC contents and stocks are associated with decreasing δ13C and increasing C : N ratios, indicating that SOC stocks and sources are intrinsically related. The SOC stocks (down to 0.64 m depth profiles) are significantly lower in young mangrove sites (46–55 Mg C ha−1) than in old sites (78–92 Mg C ha−1). The SOC in the young mangrove sites is mainly of allochthonous origin (estimated on average at 79 %), whereas in the old sites there is a slight dominance of autochthonous OC (on average 59 %). Moreover, from river- to marine-dominated sites, a pattern was found of increasing SOC stocks and increasing autochthonous SOC contribution. These observed differences along the two studied gradients are hypothesised to be mainly driven by (1) expected higher sedimentation rates in the river-dominated and lower-elevation younger sites, thereby `diluting' the SOC content and decreasing the relative autochthonous contribution, and (2) potential differences in preservation of the different SOC sources. Our finding of high contributions of allochthonous SOC, especially in young mangroves, implies that this carbon is not originating from CO2 sequestration by the mangrove ecosystem itself but is externally supplied from other terrestrial, marine or estuarine ecosystems. We argue that accounting for lower SOC stocks and higher contribution of allochthonous SOC in young and river-dominated mangrove sites, as compared to old and marine-dominated sites, is particularly relevant for designing and valuing nature-based climate mitigation programmes based on mangrove reforestation.

Maximizing blue carbon stocks through saltmarsh restoration

“…Studies on natural blue carbon habitats (saltmarshes, seagrasses, and mangroves) report high spatial heterogeneity at the estuarine level (Broek et al, 2016;Ricart et al, 2020;Suello et al, 2022). Conversely, we observed relatively low spatial variability in carbon stock among natural and MR saltmarshes in the Blackwater Estuary, with the exception of Tollesbury MR. We highlight the variability seen between Tollesbury MR and the other MR sites in the estuary is mostly linked to the vegetation community composition, whereas sediment properties are a more influential driver for natural saltmarshes.…”

Carbon Removal Using Coastal Blue Carbon Ecosystems Is Uncertain and Unreliable, With Questionable Climatic Cost-Effectiveness

“…The proportion of carbon buried in CBCE sediments that is allochthonous is highly variable. For mangroves, allochthonous carbon has been estimated as 24-55% of the total for a range of mangrove sites in Vietnam (Hieu et al, 2017), 3-73% in China (Xiong et al, 2018), and 59-79% in Ecuador (Suello et al, 2022). For seagrasses, the allochthonous contribution to carbon burial can be as high as 70-90% in Australian estuaries (Ricart et al, 2020).…”

Towards environmental sustainability: further evidences from decarbonization projects in Kenya’s Blue Economy

“…According to evidences adduced from several studies, the high productivity combined with slow rates of decomposition in the soil significantly improves mangroves' capacity to capture and eventually store organic carbon, especially in the soils (Bouillon et al, 2008;Alongi, 2012;Suello et al, 2022). Estimates by Atwood et al (2017) indicate that organic carbon stowed in mangrove sediments up to a depth of 1 m, globally equates to 2.6 billion Mg of C. Furthermore, above-ground net primary productivity reported for mangroves (8.1 t DW ha−1 yr−1) match the records from highly productive tropical forests on land (11.1 t DW ha−1 yr−1) (Alongi, 2012;Cooray et al, 2021).…”