The effect of nutrient-rich effluents from shrimp farming on mangrove soil carbon storage and geochemistry under semi-arid climate conditions in northern Brazil

Abstract

A semi-arid mangrove estuary system in the northeast Brazilian coast (Ceará 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 (0–40cm). Wastewater-affected mangrove forests were referred to as WAM and undisturbed areas as Non-WAM. Redox conditions and OC 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 68mV in WAM and from <40 to >400mV in Non-WAM soils) and significantly higher (from 66 to 411mV) in the dry season (P<0.01). OC contents (0–40cm 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 6.7kgm−2 in the wet and dry seasons, respectively, for Non-WAM, and values of 3.8 and 2.9kgm−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 0.20mgCO2h−1–g−1 C vs. 0.04mgCO2h−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 (0–40cm) in the state of Ceará is approximately 1milliont.