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.

Mechanisms of lateral and linear extension of gullies (dongas) in a subhumid grassland of South Africa

“…The low structural stability of the BC horizon in both the Lixisol and Luvisol profiles is associated with the presence of interstratified illite-smectite minerals, confirming its occurrence as reported in a study of the base of active dongas in the same area (Podwojewski et al, 2014). Singer et al (1992) found that the shrinking and swelling that accompanies drying and wetting of smectite decreases aggregate stability, because the clay is more readily separated from the aggregates.…”

“…This suggests that slaking of aggregates in the surface horizon of the non‐degraded area is minimal. This structural stability is mostly maintained by the fine roots of the complete grass cover (Podwojewski et al ., 2014).…”

“…Soil samples were analysed for basic chemical properties and structural stability (Grellier, 2011; Grellier et al ., 2012; Podwojewski et al ., 2014). Soil samples for basic physicochemical analyses were air dried and sieved through a 2 mm sieve.…”

“…Location of the reference profiles and former studies: (A) area studied by Podwojewski et al . (2014) for soil structure stability; (B) geophysical survey by Grellier et al . (2013); (C) rainfall simulation site of Podwojewski et al .…”

Root Characteristics of Herbaceous Species for Topsoil Stabilization in Restoration Projects

“…Roots may influence the soil hydrological features by forming macropores within the soil and controlling the infiltration rate and movement of water; thus, the roots drive the soil moisture content (Ziegler & Giambelluca, 1998;Bernini et al, 2003;Vergani & Graf, 2015). Likewise, roots improve the soil structure and, thereafter, enhance the stability of soil aggregates: this effect is of upmost importance to limit the soil erosion and to allow the establishment of a permanent vegetation cover (Burri et al, 2009;Podwojewski et al, 2014).…”