Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 (<span class="inline-formula"><i>δ</i><sup>13</sup></span>C) 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 <span class="inline-formula"><i>δ</i><sup>13</sup></span>C 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<span class="inline-formula"><sup>−1</sup></span>) than in old sites (78–92 Mg C ha<span class="inline-formula"><sup>−1</sup></span>). 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 CO<span class="inline-formula"><sub>2</sub></span> 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.

Highlights

  • Situated at the interface between terrestrial and marine environments, mangrove forests are unique wetland ecosystems occupyingtropical intertidal zones (Burkett & Kusler, 2000; Duke et al, 2007; Polidoro et al, 2010; Tue et al, 2012)

  • This suggests that the sources of sediment organic carbon (SOC) are predominantly of allochthonous origin for younger sites, while for older sites there is a slight dominance of autochthonous SOC origin

  • We expect that the higher volume of tidally supplied sediment input in young sites largely consists of mineral suspended sediments that are relatively low in particulate organic carbon (POC) content (Duarte et al 2004; Saintilan et al 2013; Alongi, 2014)

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Summary

Introduction

Situated at the interface between terrestrial and marine environments, mangrove forests are unique wetland ecosystems occupying (sub-)tropical intertidal zones (Burkett & Kusler, 2000; Duke et al, 2007; Polidoro et al, 2010; Tue et al, 2012) They provide a myriad of ecosystem services, such as their ability to contribute to global climate regulation by effectively sequestering carbon (Donato et al, 2011; Mcleod et al, 2011; Taillardat et al, 2018). Compared to other ecosystem types, such as rain forests, peat swamps, salt marshes and seagrasses, mangroves store much higher carbon stocks which approximately range from 140 - 1023 Mg C ha-1 (Donato et al, 2011; Alongi, 2014; Schile et al., 2017) This demonstrates the importance of the carbon capture and storage capacity of mangrove ecosystems. Recent estimations by Atwood et al (2017) equate to 2.6 billion Mg of C stored in mangrove sediments down to a 1 m depth worldwide

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