The effect of trees and grass on soil aggregate stability in Potshini catchment, KwaZulu-Natal, South Africa.
Mthimkhulu, Sandile Siphesihle.
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Long-term environmental quality is closely linked to maintaining soil quality. Therefore, assessment of the effect of land use on soil chemical, physical and biological properties provides fundamental information about soil sustainability. The role of vegetation on soil structural and aggregate stability has received much attention over the last few decades. The loss of soil due to erosion and reduction in organic matter due to change in vegetation is usually caused by invasion of grassland by trees. The response of soils to land use depends on the inherent soil properties and environmental conditions thus the effect of land use on soil stability is site specific. The objectives of this study were to assess and compare soil aggregate stability under trees and in open grassland from open-savannah woodland and nearby deeply eroded dongas (gullies). Within the dongas, tree encroachment was expected to improve soil structural stability by increasing the organic matter content. Results that were obtained from this study are aimed at improving management of soil for smallholder and pastoral farming systems. The study site was located at Potshini, KwaZulu-Natal Province, South Africa, about 18 km south-east of Bergville. The vegetation of the area is classified as grassland biome. Acacia sieberiana var. Woodii has encroached into the valley especially onto the eroded areas. Within the study area itself the dominant parent materials are sandstone and dolerite, with colluvium covering the lower slopes. The dominant soil was classified as Hutton form. Effects of different vegetation types (grass and trees) on the soil structural stability was evaluated for their effects on soil organic carbon (SOC), mean weight diameter, bulk density, root density, clay mineralogy and some other chemical properties. For this study the site was divided into two areas i.e., the donga (D) and the grassland (G). These were then each subdivided into two parts namely, donga under a tree (DUT) with a corresponding open area (DOA), and grassland under a tree (GUT) with its corresponding open area (GOA). Three bulk samples were collected from each sampling area from 0 to 20 cm (topsoil only) using a spade. Samples for bulk density were collected from 0 - 10 cm. For soil aggregate stability determinations, samples were dried and sieved to collect soil aggregates between 2.8 and 5 mm. Some of the bulk sample was analysed for SOC, pH, exchangeable bases, nitrogen and clay mineralogy. For root biomass, soil samples were collected from 0 - 10 cm using a stainless steel cylinder of 1766 cm3 volume. The type of vegetation affected the soil physical and chemical properties of the soil in the investigated horizon (A horizon). Although the results were not significantly (p > 0.05) different, the open area showed a positive effect on soil structural stability where higher soil aggregate stability and root density were observed, as opposed to under trees in both donga and grassland. Due to the non-significant differences observed in both donga and grassland, the effect of trees and grass on the aggregate stability was considered as the positive effect. The bulk density showed an unusual trend. Bulk density was higher in the open areas where high aggregate stability was observed. From these results it was concluded that because trees have larger roots than grass these are more effective in loosening the soil particles but less effective in stabilizing the soil aggregates. In other words, the high amount of fine roots increased the soil stability while large roots improved the loosening of the soil particles. The amount of vegetation considerably affected all the physical and chemical soil properties investigated. The SOC and root density was considerably higher under grassland compared to the donga. The clay mineralogy differed between the donga and grassland. The donga soils had a higher kaolinite content and amount of interstratified smectite/mica compared to grassland. The presence of interstratified smectite/mica in the donga could cause lower aggregate stability due to shrinking-swelling cycles during wetting-drying conditions. The effects of mineralogy have been overridden by organic carbon in the grassland. From this study it was revealed that soil aggregate stability is the product of interactions between soil clay minerals, and organic fractions which are influenced by soil environment, land use and soil properties. This means that all the factors involved in aggregate formation and their stabilization are interdependent. It is suggested that factors that were driving the soil aggregate stability in the donga were different to the ones in grassland. It is suggested that the aggregate stability was driven by inorganic factors i.e., clay, clay mineralogy, bases and CEC in the donga while organic carbon and plants roots were dominating in the grassland