**3. Results and discussion**

In this study, land degradation reduced rangeland soil quality through a linear decrease in grass cover. Consequently, soil aggregate stability in the topsoil layer decreased from an average of 1.35 mm in non-degraded grassland to 0.71 mm in heavily degraded grassland, corresponding to a decline of 47% (**Figure 2**). The decline in the protective grass cover induced by degradation led to soil structural alteration, disruption of soil aggregates, increasing susceptibility of degraded soil to soil crusting and compaction. The less structural stability of the degraded soil may in turn increase soil erodibility — the inherent susceptibility of soil to detachment and transport by rainsplash and runoff [32].

**Figure 2.** Relationship between soil aggregate stability and soil surface coverage by vegetation. Data are presented as mean±SE (*n*=12) per soil surface coverage by vegetation, and bars with different letters are significantly different at the *P*<0.05 level.

Penetrometer resistance, an important mechanical property used as an indicator of soil compaction [33], increased with decreasing grass cover from an average of 11.3 kg cm–2 in nondegraded grassland to 19.5 kg cm–2 in heavily degraded grassland, corresponding to an increase of 42% (**Figure 3A**). In agreement with our study, Snyman and du Preez [26] found that rangeland degradation decreased soil compaction by 65% from 18.3 kg cm–2 in nondegraded fine sandy loam soil to 6.4 kg cm–2 in heavily degraded fine sandy loam soil in a semiarid region in Bloemfontein, South Africa. One of the profound effects of soil compaction is the reduction in pore space and macroporosity, which is associated with increased bulk density [34, 35]. Such was the case in the present study, as soil bulk density increased by 12% from an average of 1.43 g cm–3 in non-degraded grassland to 1.61 g cm–3 in heavily degraded grassland, indicating increasing compaction (**Figure 3B**). Similarly, Hiltbrunner et al. [36] observed a 20% increase in soil bulk density on degraded grassland in a Swiss subalpine grassland, and this led to changes in biomass production.

Some studies have shown that soil compaction decreases the infiltration capacity of the soil [35, 37]. At our study site, Podwojewski et al. [38] found using rainfall simulation on runoff plots that land degradation decreased the soil infiltration rate by 72% from 21.6 mm h–1 in nondegraded grassland to 6 mm h–1 in heavily degraded grassland. While in South West England, the authors [38] found that the infiltration capacity was reduced by 80% and surface runoff volumes were increased by nearly 12 times on heavily degraded grassland compared with non-degraded grassland. The decrease in the infiltration capacity of soils with increasing degradation intensity may be explained by several reasons. First, a decline in protective grass cover and associated dense sward characteristics by land degradation leads to reduced intercepted raindrops and water movement through the soil. Second, a decline in the protective cover offered by grass decreases surface roughness, leading to decreased detention storage [38]. Although not investigated here, some studies have shown that soil compaction and the reduction in pore space also decrease the hydraulic conductivity of soil [34, 39].

**3. Results and discussion**

86 Land Degradation and Desertification - a Global Crisis

*P*<0.05 level.

detachment and transport by rainsplash and runoff [32].

grassland, and this led to changes in biomass production.

In this study, land degradation reduced rangeland soil quality through a linear decrease in grass cover. Consequently, soil aggregate stability in the topsoil layer decreased from an average of 1.35 mm in non-degraded grassland to 0.71 mm in heavily degraded grassland, corresponding to a decline of 47% (**Figure 2**). The decline in the protective grass cover induced by degradation led to soil structural alteration, disruption of soil aggregates, increasing susceptibility of degraded soil to soil crusting and compaction. The less structural stability of the degraded soil may in turn increase soil erodibility — the inherent susceptibility of soil to

**Figure 2.** Relationship between soil aggregate stability and soil surface coverage by vegetation. Data are presented as mean±SE (*n*=12) per soil surface coverage by vegetation, and bars with different letters are significantly different at the

Penetrometer resistance, an important mechanical property used as an indicator of soil compaction [33], increased with decreasing grass cover from an average of 11.3 kg cm–2 in nondegraded grassland to 19.5 kg cm–2 in heavily degraded grassland, corresponding to an increase of 42% (**Figure 3A**). In agreement with our study, Snyman and du Preez [26] found that rangeland degradation decreased soil compaction by 65% from 18.3 kg cm–2 in nondegraded fine sandy loam soil to 6.4 kg cm–2 in heavily degraded fine sandy loam soil in a semiarid region in Bloemfontein, South Africa. One of the profound effects of soil compaction is the reduction in pore space and macroporosity, which is associated with increased bulk density [34, 35]. Such was the case in the present study, as soil bulk density increased by 12% from an average of 1.43 g cm–3 in non-degraded grassland to 1.61 g cm–3 in heavily degraded grassland, indicating increasing compaction (**Figure 3B**). Similarly, Hiltbrunner et al. [36] observed a 20% increase in soil bulk density on degraded grassland in a Swiss subalpine

**Figure 3.** Mean±SE values of (A) penetrative resistance, a proxy for soil compaction (*n*=15), and (B) soil bulk density (*n*=12) per soil surface coverage by vegetation, and bars with different letters are significantly different at the *P*<0.05 level.

In this study, a pattern of lower sand (49%) was observed in heavily degraded grassland, compared with 72% in moderately degraded and 73% in non-degraded grassland. The depletion in sand was so marked that the mean clay content was almost two times (34%) greater in heavily degraded grassland compared with 14% in non-degraded grassland, while the distribution of silt content was similar along the degradation gradient (**Figure 4**). Indeed, intensification of degradation can induce shifts in the distribution of texture, as indicated in the study by Dong et al. [27] in the Qinghai-Tibetan Plateau in China, which found that grassland degradation led to a shift in soil texture from loamy toward sandy loamy soils. This phenomenon was corroborated by Fullen et al. [40], whose study compared the textures of grassland and degraded sandy soils from Shropshire, UK, and concluded that degradation changed mean soil texture from a very slightly stony loamy sand to a slightly stony sandy loam. The authors also found that the degraded soil was particularly deficient in sand, especially medium and coarse sands, and the depletion in sand was so marked that the degraded bare soil had significantly greater mean percentage clay content than non-degraded grassland soil. A recent meta-analysis by Dlamini et al. [41] concluded that grassland degradation has a significantly negative effect on coarser textured soils than fine textured soils due to the lack of physical protection of organic matter and weak aggregation in sandy soils.

**Figure 4.** Relationship between sand, silt, clay and soil surface coverage by vegetation. Data are presented as mean±SE (*n*=12) per soil surface coverage by vegetation, and bars with different letters are significantly different at the *P*<0.05 level.

Land degradation results in the reduction of vegetation cover, which is unfavourable to soil protection. Degraded soils generated through the loss of vegetation cover are exposed to raindrop impact, which may lead to crust formation and a reduction in the infiltration capacity of the soil [42]. Such effects may lead to bare soil being more susceptible to surface runoff generation as drainage becomes impeded. These changes to soil hydrology have implications for runoff from degraded land, potentially modifying not only the quantity but also the quality of runoff, in terms of sediment and nutrient loads transported over and through the soil [37]. Vegetation cover by intercepting raindrops and enhancing infiltration protects the soil surface from the erosional effects of rainsplash and surface runoff, and this in turn helps preserve the water quality in surface waters of rangelands.
