How Tree Encroachment and Soil Properties Affect Soil Aggregate Stability in an Eroded Grassland in South Africa

Abstract

Gully erosion and woody plant encroachment are frequently observed in grasslands worldwide. Soil aggregate stability is one of the drivers of gully erosion and needs to be studied, especially in the context of both tree encroachment and gully erosion. This study compared the effects of tree encroachment [paperbark thorn; Vachellia sieberiana (DC.) Kyal. & Boatwr. previously called Acacia sieberiana DC.] and soil surface horizon properties on the aggregate stability (as measured by mean weight diameter, MWD) in a degraded semihumid grassland in the KwaZulu‐Natal province of South Africa. Soil properties from under trees (UT) and open areas (OA) of both an Entisol within deep gullies (locally called dongas) and Alfisol from grasslands outside the gullies (grassland). The MWD values were higher in the grassland (3.06 ± 0.21 mm) than in the dongas (1.84 ± 1.03 mm). In the grassland, the MWD was significantly higher in the OA than UT, associated with more fine roots in the OA. In the donga, soil properties UT were not significantly different than those in OA. The soil aggregate stability showed strong local variability that depends on the age of the donga and the nature of the material that composes the pediments. Illite was associated with higher MWD values. The presence of interstratified illite–vermiculite located in active donga increases the shrink–swell potential of the aggregates, exacerbating their dispersion reflected in the low MWD values measured, especially when C and root contents were also low. These results suggest that the local soil clay mineralogy controls the orientation and extension of gullies rather than soil chemistry, topography, or the size of the drainage area.