Jason L. Howard

Seagrass ecosystems have been identified as long‐term carbon sinks whose conservation could serve as a tool to mitigate carbon emissions. Seagrasses alter landscapes in a way that stimulates carbon biosequestration, but discussions of their role in atmospheric CO₂ mitigation disregard the co‐occurring inorganic carbon cycle, whose antagonist effect on CO₂ sequestration can buffer and potentially outweigh the effects of C_org production on net carbon exchange with the atmosphere. This study examines the extent of both organic carbon (C_org) and inorganic carbon (C_inorg) stocks as proxies for long‐term production and calcification in the poorly studied seagrass meadows of southeastern (SE) Brazil and compares values to Florida Bay (U.S.A.), a well‐studied system known for both high autotrophy and calcification, representing extremes of CaCO₃ soil content. Seagrass soils in SE Brazil contain an average of 67.6 ± 14.7 Mg C_org ha⁻¹ in the top 1 m, compared to an average of 175.0 ± 20.4 Mg C_org ha⁻¹ for their counterparts in Florida Bay. C_inorg as CaCO₃ in SE Brazil averaged 141.5 ± 60.0 Mg C_inorg ha⁻¹ in the top meter of soil while the warmer, calcification‐promoting waters of Florida Bay had higher soil C_inorg areal stock, averaging 754.6 ± 26.7 Mg C_inorg ha⁻¹. When the CO₂ evasion related to CaCO₃ production is considered, seagrass ecosystems with high CaCO₃ content may have CO₂ sequestered via C_org accumulation negated by CO₂ produced by calcification. These findings prompt the reconsideration of carbon inventory methods and encourage regionally‐ and community‐specific assessments of CO₂ sequestration abilities of seagrass ecosystems.