Inorganic and Black Carbon Hotspots Constrain Blue Carbon Mitigation Services Across Tropical Seagrass and Temperate Tidal Marshes

Abstract

Total organic carbon (TOC) sediment stocks as a CO 2 mitigation service require exclusion of allochthonous black (BC) and particulate inorganic carbon corrected for water-atmospheric equilibrium (PIC eq ). For the rst time, we address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots that represent two different blue carbon ecosystems of Malaysia and Australia. Seagrass TOC stocks were similar to the salt marshes with soil depths < 1 m (59.3 ± 11.3 and 74.9 ± 18.9 MgC ha − 1 , CI 95% respectively). Both ecosystems showed larger BC constraints than their pristine counterparts did.However, the seagrass meadows' mitigation services were largely constrained by both higher BC/TOC and PIC eq /TOC fractions (38.0% ± 6.6% and 43.4% ± 5.9%, CI 95%) and salt marshes around a third (22% ± 10.2% and 6.0% ± 3.1% CI 95%). The results provide useful data from underrepresented regions, and, reiterates the need to consider both BC and PIC for more reliable blue carbon mitigation assessments.

A re-evaluation of wetland carbon sink mitigation concepts and measurements: A diagenetic solution

“…Nevertheless, BC estimates across coastal wetland ecosystems remain globally under sampled. This is despite contributing substantial fractions to the sediments' TOC (means between 3-38%) (data from Gallagher et al 2021;Gallagher et al 2019). Estimates across freshwater wetlands, however, are scarce but may still be significant.…”

“…These can be dry bulk density (Callaway et al 2012), gravimetric losses after combustion, chemical oxidation and titration (Byers et al 1978;Heiri et al 2001), or infrared reflectance (Bellon-Maurel & McBratney 2011). Except for chemical titration, the above proxies to some degree require a global (Fourqurean et al 2012) or preferably regional (Craft et al 1991) calibration with standard methods, remembering to include regression variance in the final estimate (Gallagher et al 2021).…”

“…The carbon content can be estimated after thermal or chemical isolation within a sedimentary matrix using standard laboratory equipment . Carbon and N isotopic signatures can then be used to assist in identifying their allochthonous or autochthonous nature (Gallagher et al 2021;Leorri et al 2018). Nevertheless, BC estimates across coastal wetland ecosystems remain globally under sampled.…”

“…First, there is an implicit and untested assumption that total sedimentary organic carbon accumulation is a proxy for carbon sequestration. Second, the majority of carbon sink assessments still fail to subtract intrinsically recalcitrant allochthonous or autochthonous carbon from stocks and sequestration estimates (Gallagher et al 2021;Rillig 2018). Third, the traditional conceptual model does not account for remineralisation of its sedimentary organic carbon over climatic time scales Maher et al 2017).…”

A Re-evaluation of Wetland Carbon Sink Mitigation Concepts and Measurements: A Diagenetic Solution

“…These can be previously determined dry bulk density (Callaway et al 2012), gravimetric losses after combustion, chemical oxidation and titration (Byers et al 1978;Heiri et al 2001), or infrared reflectance (Bellon-Maurel and McBratney 2011). Except for chemical titration, the above proxies to some degree require a global (Fourqurean et al 2012) or preferably regional (Craft et al 1991) calibration with standard methods, with the inclusion of regression variance in the final estimate (Gallagher et al 2021a).…”