Land use changes affecting soil organic carbon storage along a mangrove swamp rice chronosequence in the Cacheu and Oio regions (northern Guinea-Bissau)

Guinea-Bissau has the largest area of mangrove swamp rice, an important cropping system that significantly contribute to the food security of the nation. Attempts to reclaim mangrove swamps for rice growing have shown the importance of a greater knowledge on the effects of land use change on soil properties and soil carbon storage. To address this problem, a study was undertaken within Cacheur and Oio regions in Northern Guinea-Bissau, along the following chronosequence: mangrove, rice and abandoned fields. Changes in C/N ratio, δ13C and δ15N values were used to study the dynamics of C3 plant-derived and marine-derived carbon (C) in order to analyze the origin of soil organic matter (SOM) and estimate the impact of marine contribution to SOC. Isotopic signatures within the mangrove swamp rice soils suggested the inwelling of marine derived C. SOC stock was estimated in 0-10, 0-20, 0-40 and 0-80cm soil layers using fixed soil depth (FD) and fixed soil mass (FM) approaches. The significantly highest values were found in mangrove soils and the lowest in the abandoned fields for both sites, while no significant differences were recorded for the topsoil (0-10cm) between mangrove and rice fields. The results of this study revealed that conversion of mangrove to rice cropping has technical potential of SOC sequestration in the upper part of the soil (0-40cm). On the other hand, the abandonment of the fields caused decreases in carbon storage along the whole soil depth. These findings may have important implications for national forest carbon monitoring systems and regional level reducing emission from deforestation and forest degradation (REDD+) strategies.