Land use change effects on carbon yield in lowland streams of the Congo Basin

The expansion of land-use activities severely threatens primary forests in the Congo Basin. As the dominant mode of deforestation in this region, it is expected to affect soil nutrient stocks and availability and, consequently, forest productivity. To assess how shifting cultivation affects carbon species transformation and export to rivers, four catchments, of which two draining forested landscapes and two draining agricultural landscapes were selected in the Yangambi region, Democratic Republic of the Congo. The catchments were equipped with sensors to continuously quantify discharge, water temperature, oxygen concentrations and sediment loads, amongst other parameters, while periodic water sampling was conducted to quantify chemical water composition. Based on these samples, concentrations and yields of the full spectrum of carbon species (DOC, DIC, CO₂, CH₄) were calculated. We found that baseflow dissolved organic carbon (DOC) concentrations were nearly identical in both groups of streams. However, significantly higher carbon dioxide(CO₂) and methane(CH₄) concentrations were observed in streams draining agricultural landscapes compared to forested streams. This apparent paradox can be explained by much higher carbon turnover rates in agricultural streams, driven by enhanced microbial metabolism resulting from environmental changes such as increased light and temperature, greater erosion, and higher nutrient availability (N and P). Thus, agricultural streams rapidly mineralize organic carbon to CO₂ and CH₄, preventing its persistence in the dissolved organic pool.