Effect of land-use changes resulting from shrimp farming on acid sulfate soils in the Can Gio coastal wetland area (Vietnam)

Acid sulfate soils in coastal wetland areas are particularly vulnerable to land-use changes. We identifid the potential impacts of land-use changes in the Can Gio coastal wetland area in Vietnam due to the reclamation of acid sulfate soils from shrimp farms. Our study applied the support of vector machine algorithm in ENVI software to observe land-use changes from 1995 to 2015, using Landsat Thematic Mapper and Operational Land Imager data. We classifid the land use of the study area into four major classes including vegetation, bare land, dedicated land and aquaculture land. Our study successfully met the overall classifiation accuracy requirement above 95% and kappa statistics above 0.95. Between 1995 and 2006, about 2,938.05 ha of bare land and 1,464.66 ha of vegetation (mangrove forest) were converted to aquaculture land. In contrast, between 2006 and 2015, 2,423.88 ha of aquaculture land converted back to bare land, mainly related to the abandonment of shrimp ponds due to crop failure and disease. The disturbance of acid sulfate soils through initial soil reclamation and subsequent fallowing is considered a key reason for hastening and extending soil acidifiation in the study area. We collected 144 topsoil samples from 17 fallowed ponds in two batches, and 142 of these were acidic: 128 samples were extremely and strongly acidic (pH < 5.5), 14 samples were moderately and slightly acid (pH between 5.5 and 6.5), and only two samples were neutral (pH over 6.5).