Transformation of thicket to savanna reduces soil quality in the Eastern Cape, South Africa

Abstract

Xeric succulent thicket in the Eastern Cape, South Africa has been used for farming goats since the early 1900s. This habitat is characterised by a dense cover of the succulent bush Portulacaria afra and by a warm, semi-arid climate with evenly distributed annual rainfall of 250-400 mm. Heavy browsing by goats results in the loss of P. afra and transforms the thicket to an open savanna dominated by annual grasses. Eight fence-line comparisons between thicket and savanna were used to investigate differences in soil quality associated with the vegetation change. Composite soil samples were taken to a depth of 10 cm from 1 ha plots on either side of the fence-line. Associated with the change from thicket to savanna, a significant decrease (paired t-test, P < 0.05) was found in total C (respective means of 5.6 vs. 3.0%), total N (0.33 vs. 0.24%), labile C (2.8 vs. 1.5%), CO 2 flux (1.9 vs. 0.5 µmol m −2 s −1 ), soil respiration in the laboratory (144 vs. 79 ng C kg −1 s −1 ), (NH 4 )OAc-extractable Mg (55 vs. 28 mmol c kg −1 ), and laboratory infiltration rate (51 vs. 19 mm h −1 ). In the same direction there was a similarly significant increase in modulus of rupture (16 vs. 34 kPa), water-soluble Ca (2.3 vs. 3.4 mmol c kg −1 ) and pH (6.7 vs. 7.7). The soil C content of 5.6% in thicket is surprisingly high in this warm, semi-arid climate and suggests that the dense P. afra bush strongly regulates soil organic matter through microclimate, erosion control, litter quantity and perhaps chemistry. Savanna soils had a greater tendency to crust (as evident in a lower rate of laboratory infiltration and greater modulus of rupture) than thicket soils. This was attributed to their lower organic matter content, which probably reduced aggregate stability. Savannas are likely to be more prone to runoff and erosion not only because of sparser vegetation but also because of a decline in soil quality.

Soil infiltrability as a driver of plant cover and species richness in the semi-arid Karoo, South Africa

“…Figure 4 shows, nevertheless, that poorly permeable soils can also support minimal species richness. Although it is tempting to think that such soils may have been overgrazed, which can result in reduced plant cover and species richness as well as infiltrability (Mills and Fey 2004b), at least some of these soils may simply have a high pH or low nitrogen content which, as Fig. 4 shows, ensures low species richness in this dataset.…”

Restoring Soil Quality to Mitigate Soil Degradation

“…A study conducted in the sub-tropical humid grasslands in South Africa indicated that the decline in grass (vegetative) cover from 100% to 0%-5% reduced the SOC pool by 1.25 kg/m 2 and the soil organic N (SON) pool by 0.074 kg/m 2 [20], There were also attendant declines in the C:N ratio and proportion of SOC and SON in the silt + clay fraction with the decline in aerial grass cover which negatively affected ecosystem functions of the acidic sandy loam soils. Similarly, transformation of a thicket vegetation to an open savanna (dominated by grasses) due to intensive grazing decreased soil quality in the Eastern Cape region of South Africa [21]. Indeed, savanna soils have lower SOC concentration and a greater tendency to crust than thicket soils because of the decreased quantity and stability of structural aggregates.…”

Effects of different management regimes on soil erosion and surface runoff in semi-arid to sub-humid rangelands

“…The amount of rainfall not intercepted by vegetation (i.e. 'throughfall'), largely determines the amount of surface runoff (Mills and Fey, 2004). Most studies mentioned only throughfall, but 'interceptional loss' (i.e.…”

“…LeMaitre et al, 1999;van Luijk et al, 2013). For instance, plant communities in semi-arid scrublands take up water more efficiently than plants in sub-humid ecosystems(Le Maitre et al, 1999;Mills and Fey, 2004). Lower vegetation cover in…”