Effects of fragmentation and shifting cultivation on soil carbon and nutrients: A case study in Sitapahar forest, Bangladesh

The consequences of converting primary forests to agriculture and pasture on soil carbon (C) and nutrient stocks are important, although they are little known in tropical forests. In the Sitapahar forest of Bangladesh, we evaluated soil C and nutrient concentrations and stocks among intact, fragmented, and shifting cultivation sites. We also investigated whether there are any correlations between nutrient stocks and tree species diversity or soil C stock. We measured and identified the trees in 30 main plots (20 m x 20 m), 10 in each of the three sites. Four subplots (1 m x 1 m each) were used to measure the physical and chemical characteristics of the soil at depths of 0-10, 10-20, and 20-30 cm. Results show that during shifting cultivation as compared to intact sites, soil C stock was 50% lower whereas soil P stock was 26% greater. When compared to the other two sites, the soil N and K stocks under the shifting cultivation site were 57% and 27% greater, respectively. Soil Mg stock in the intact site was up to 155% higher compared to the other two sites. In all sites, soil C stock was positively correlated with N stock, but negatively correlated with the P stock. Tree species diversity was positively correlated with K stock, but negatively correlated with N stock in the shifting cultivation site. Sustainable management of the sites should focus on the understanding of soil-plant interactions and the protection of soil C, nutrients, and species diversity.