Parent material modulated effects of soil degradation on fertility and organic carbon of tropical cropland soils in Eastern Africa

Deforestation for cropland expansion in the sloping landscapes along the East African Rift system causes severe soil erosion and thus the loss of fertile, organic rich topsoil. However, the varying effect of land degradation in the region on soils developed from different parent material - which may influence soil fertility and carbon stabilization - are still largely unknown. To examine these factors, we compared soil organic carbon (SOC) and soil fertility indicators in undisturbed forest topsoils with cropland hillslope topsoils along a chronosequence after deforestation (2–7, 10–20, 20–40, > 60 years of cropping, land abandonment) on mafic (South Kivu, Democratic Republic of Congo) and felsic parent material (western Uganda). From previous studies, we expected higher soil fertility and SOC contents and therefore slower degradation on mafic soils due to the higher amounts of clay and pedogenic metal phases which stabilize SOM and thus further maintain soil fertility.
However, we found similar SOC contents on both parent materials and a consistent decrease with time after deforestation. SOC values were significantly lower in soils that were cleared more than 60 years ago, compared to cropland which was cleared 2–7 years ago and nearby undisturbed forest topsoils (0–10 cm soil depth). While the effective cation exchange capacity (ECEC) positively correlated with SOC in soils on felsic parent material, this was not observed in soils with mafic parent material, where it correlated with mineralogical proxies (total reserves in bases). In both regions, SOC did not correlate with clay content. Mid-Holocene carbonate volcanism appears to have offset soil degradation in the felsic region, contributing to higher pH and ECEC and impeding land abandonment due to the maintenance of acceptable soil fertility levels. Surprisingly, abandoned cropland sites in the mafic region still had an average SOC content of 14–29 g kg^-1 in topsoils, likely due to strong fixation of SOC with reactive metal phases; however, they were characterized by extremely low pH values and high Al³⁺ mobility, combined with low available nutrient status.
Our results emphasize that soil fertility and carbon stabilization are reliant on the mineral composition of the underlying parent material, even in deeply weathered soils of the humid tropics. Soil organic matter in degraded tropical cropland soils does not appear to be a reliable indicator of soil fertility.