Inorganic and black carbon hotspots constrain blue carbon mitigation services across tropical seagrass and temperate tidal marshes
Total organic carbon (TOC) sediment stocks as a CO2 mitigation service requires exclusion of allochthonous black (BC) and particulate inorganic carbon corrected for water- atmospheric equilibrium (PICeq). For the first time, we address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots. 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) and sequestration rates of 1.134 MgC ha-1 yr-1. Both ecosystems showed larger BC constraints than their pristine counterparts. However, the seagrass meadows mitigation services were largely constrained by both higher BC/TOC and PICeq/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 demonstrate a need to account for both BC and PIC within blue carbon mitigation assessments.