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

[摘要] Mangroves are widely recognised as key ecosystems for climate changemitigation as they capture and store significant amounts of sediment organiccarbon (SOC). Yet, there is incomplete knowledge on how sources of SOC andtheir differential preservation vary between mangrove sites in relation toenvironmental gradients. To address this, sediment depth profiles weresampled from mangrove sites ranging from river-dominated to marine-dominatedsites and including old and young mangrove sites in the Guayas delta(Ecuador). The stable carbon isotope ratios ( δ 13 C) and theelemental composition (OC %, C : N) of sediment profiles, local vegetation(i.e. autochthonous carbon) and externally supplied suspended particulatematter (i.e. allochthonous carbon) were obtained to assess variations inthe amount and sources of SOC at different locations throughout the delta.In general, across all sites, we found that increasing SOC contents and stocksare associated with decreasing δ 13 C and increasing C : N ratios,indicating that SOC stocks and sources are intrinsically related. The SOCstocks (down to 0.64 m depth profiles) are significantly lower in youngmangrove 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 allochthonousorigin (estimated on average at 79 %), whereas in the old sites there is aslight dominance of autochthonous OC (on average 59 %). Moreover, fromriver- to marine-dominated sites, a pattern was found of increasing SOCstocks and increasing autochthonous SOC contribution. These observeddifferences along the two studied gradients are hypothesised to be mainlydriven by (1) expected higher sedimentation rates in the river-dominated andlower-elevation younger sites, thereby `diluting' the SOC content anddecreasing the relative autochthonous contribution, and (2) potentialdifferences in preservation of the different SOC sources. Our finding ofhigh contributions of allochthonous SOC, especially in young mangroves,implies that this carbon is not originating from CO 2 sequestration bythe mangrove ecosystem itself but is externally supplied from otherterrestrial, marine or estuarine ecosystems. We argue that accounting forlower SOC stocks and higher contribution of allochthonous SOC in young andriver-dominated mangrove sites, as compared to old and marine-dominatedsites, is particularly relevant for designing and valuing nature-basedclimate mitigation programmes based on mangrove reforestation.

[学科分类] 大气科学