Soils with seasonal climates have highest potential to stabilize carbon by minerals in sub-Saharan Africa

Abstract

With climate and land use changes, it is becoming increasingly important to understand not only how much carbon is and will be stored in soils, but also how long this C will remain in soils. Estimates of C age can provide useful information about the timescales on which C will respond to such changes. It is generally accepted that the interaction of climate and soil mineralogy have a strong influence on C age. However, our current understanding is primarily based on findings from temperate regions and from small-scale studies. Large knowledge gaps persist in (sub-)tropical regions where soil processes are often understudied. Here, we use a systematic continental-scale approach to better understand the processes controlling C age on a larger scale in these understudied soils. In total, 510 samples were analyzed for radiocarbon (Δ14C), consisting of topsoils (0–20 cm) and subsoils (20–50 cm) collected from 30 sites across 14 countries. The sampled soils are part of a comprehensive soil survey (AfSIS) for sub-Saharan Africa, for which soil mineralogy (based on X-ray powder diffraction) and soil chemistry were determined. Soils with the youngest C ages are generally highly weathered, and are characterized by humid climates, high gross primary productivity (GPP), and a dominance of 1:1 clay minerals. In contrast, older C ages are either found in arid regions characterized by low C inputs and low mineral stabilization, or in seasonal climates, where GPP is high but a portion of the C is stabilized by 2:1 clay minerals and poorly crystalline minerals. Cultivation and erosion appear to play a secondary role at this large scale, but widen the range of C ages. Our data suggest that soils from seasonal climate zones have the most favorable climatic and pedogenic conditions to stabilize and store C. Yet, they are also the most vulnerable climate zones according to future projections for sub-Saharan Africa. Understanding how the stabilizing minerals will react to climate change is key to understanding short and long-term changes in C storage and stabilization.