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A semi-arid mangrove estuary system in the northeast Brazilian coast (Ceara state) was selected for this study to (i) evaluate the impact of shrimp farm nutrient-rich wastewater effluents on the soil geochemistry and organic carbon (OC) storage and (ii) estimate the total amount of OC stored in mangrove soils (0-40 cm). Wastewater-affected mangrove forests were referred to as WAM and undisturbed areas as Non-WAM. Redox conditions and DC content were statistically correlated (P < 0.05) with seasonality and type of land use (WAM vs. Non-WAM). Eh values were from anoxic to oxic conditions in the wet season (from -5 to 68 mV in WAM and from <40 to >400 mV in Non-WAM soils) and significantly higher (from 66 to 411 mV) in the dry season (P < 0.01). OC contents (0-40 cm soil depth) were significantly higher (P < 0.01) in the wet season than the dry season, and higher in Non-WAM soils than in WAM soils (values of 8.1 and 6.7 kg m(-2) in the wet and dry seasons, respectively, for Non-WAM, and values of 3.8 and 2.9 kg m(-2) in the wet and dry seasons, respectively, for WAM soils; P < 0.01). Iron partitioning was significantly dependent (P < 0.05) on type of land use, with a smaller degree of pyritization and lower Fe-pyrite presence in WAM soils compared to Non-WAM soils. Basal respiration of soil sediments was significantly influenced (P < 0.01) by type of land use with highest CO2 flux rates measured in the WAM soils (mean values of 0.20 mg CO2 h(-1)-g(-1) C vs. 0.04 mg CO2 h(-1)-g(-1), C). The OC storage reduction in WAM soils was potentially caused (i) by an increase in microbial activity induced by loading of nutrient-rich effluents and (ii) by an increase of strong electron acceptors [e.g., NO3-] that promote a decrease in pyrite concentration and hence a reduction in soil OC burial. The current estimated OC stored in mangrove soils (0-40 cm) in the state of Ceara is approximately 1 million t. (C) 2013 Elsevier B.V. All rights reserved.