Nitrogen mineralization and eutrophication risks in mangroves receiving shrimp farming effluents

Abstract

Nitrogen (N) inputs originated from shrimp farming effluents were evaluated for potential changes in the net N mineralization for mangrove soils from Northeastern Brazil. Our study provides notable information and assessment for the potential enhancement of N mineralization in preserved and shrimp-impacted semi-arid mangrove soils of the Jaguaribe River estuary, which is one of the largest shrimp producers of Brazil, using an analytical and daily tidal variation experimental approach. Nitrogen-rich effluents promoted a significant (p value < 0.001) increase of the total soil N content (1998 ± 201mgkg-1 on average) compared with the preserved sites (average: 1446 ± 295mgkg-1). The effluents also increased the N mineralization in the shrimp-impacted sites (N-min: 86.6 ± 37.5mgkg-1), when compared with preserved mangroves (N-min: 56.5 ± 23.8mgkg-1). Over a daily tidal variation experiment, we found that just 30% (36.2 ± 20.6mgkg-1) of mineralized N remains stored in the soil, whereas 70% (102.9 ± 38.8mgkg-1) was solubilized in tidal waters. Therefore, the N mineralization process may trigger eutrophication by increasing N inorganic bioavailability in mangrove soils receiving N-rich effluents from shrimp ponds, which in turn might increase primary producers' activity. This approach has not been studied so far in semi-arid mangroves, where the shrimp farming activity is one of the most important economic activities.