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 CO 2 mitigation disregard the co-occurring inorganic carbon cycle, whose antagonist effect on CO 2 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 CaCO 3 soil content. Seagrass soils in SE Brazil contain an average of 67.6 6 14.7 Mg C org ha 21 in the top 1 m, compared to an average of 175.0 6 20.4 Mg C org ha 21 for their counterparts in Florida Bay. C inorg as CaCO 3 in SE Brazil averaged 141.5 6 60.0 Mg C inorg ha 21 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 6 26.7 Mg C inorg ha 21 . When the CO 2 evasion related to CaCO 3 production is considered, seagrass ecosystems with high CaCO 3 content may have CO 2 sequestered via C org accumulation negated by CO 2 produced by calcification. These findings prompt the reconsideration of carbon inventory methods and encourage regionally-and community-specific assessments of CO 2 sequestration abilities of seagrass ecosystems.

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