Soil Nutrient Depletion and Tree Functional Composition Shift Following Repeated Clearing in Secondary Forests of the Congo Basin

The Congo Basin's rapidly growing population still largely depends on shifting cultivation for both energy and food security. This nexus of population growth and ecological impact will continue to exacerbate landscape degradation in the coming decades. To quantify the effects of land-use intensity on soil nutrient stocks and the functional composition of young regrowth forest in the Congo Basin, we used fallows of different ages that had been subjected to a varying number of clearing cycles. We show that repeated clearing substantially affected soil cation stocks, reducing total K, Mg and Ca in the upper 20 cm of soil by roughly 20% per clearing cycle. Additionally, we show that plant-available nitrogen (ammonium and nitrate) and phosphorus decline in the topsoil with increasing land-use intensity. Furthermore, the tree functional composition of young fallows changed after repeated clearing cycles: we observed a decrease in abundance of pioneer species and an increase in nitrogen fixing species early in succession. Variation in soil total nutrient stocks was decoupled from changes in vegetation, and soil plant-available nutrients only marginally explained tree functional composition changes. We conclude that land-use intensity substantially affects both soil total and plant-available nutrients in a shifting cultivation system, as well as the functional composition of the regenerating vegetation. However, compositional changes of the tree community are only partly driven by land-use intensity effects on soil plant-available nutrients.

Automated summary

The study reveals that in the Congo Basin, land-use intensity significantly affects soil ion stocks and the functional composition of young regrowth forest, with repeated clearing leading to a substantial reduction in potassium, magnesium, and calcium in the soil. Plant-available nitrogen and phosphorus in the topsoil decrease with increasing land-use intensity.