Doctoral Degrees (Geography and Environmental Studies)
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Browsing Doctoral Degrees (Geography and Environmental Studies) by Author "De Waal, Jan"
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- ItemAdverse impacts of agricultural expansion on hydrological and nutrient dynamics in a Renosterveld landscape – can natural vegetation offer solutions(Stellenbosch : Stellenbosch University, 2023-03) De Waal, Jan; Van Niekerk, Adriaan, 1970-; Miller, J; Stellenbosch University. Faculty of Arts and Social Sciences. Dept. of Geography and Environmental Studies.ENGLISH ABSTRACT: Agricultural systems deliver a range of products to human society including, food, fuel, textiles and pharmaceuticals. However, the global expansion of agricultural activities has resulted in several negative outcomes such as biodiversity loss, increased carbon emissions, topsoil erosion and water pollution. Thus, the degradation of natural landscapes due to agricultural transformation has resulted in a loss of ecosystem services over time by increasing habitat loss, nutrient movement, sedimentation of rivers and pesticide poisoning in non-target species. One of the most impacted landscapes in terms of agricultural transformation in South Africa is renosterveld vegetation. Lowland renosterveld is a small-leaved, evergreen shrubland found on the shale-rich, fertile soils of the south-western Cape of South Africa where it forms part of the Fynbos biome, a species-rich floral kingdom. Renosterveld typically occurs on fine-grained, clay-rich soils as opposed to the sandy, nutrient-poor soils on which fynbos is located. Agricultural expansion has resulted in the destruction of the indigenous renosterveld vegetation which now exhibits a great degree of fragmentation. This dissertation documents an investigation of the impact of agricultural expansion on the hydrological, sediment and water quality dynamics in the Overberg renosterveld landscape in theWestern Cape. An evaluation is reported of whether conservation of this threatened vegetation can allow for the delivery of ecosystem services in vegetation buffers in terms of phytoremediation of nutrient inputs from agricultural slopes. The impact of changing landuse on hydrological characteristics of the area at a landscape level is examined first, followed by a case study of the Bot River by implementing a fully differentiated hydrological model with a sediment delivery component. Results confirm that hydrology on a landscape level has been greatly impacted by changes in landuse, while modelled soil erosion from the Bot River catchment depicts an increase in soil erosion from 22 t/km2/year under natural conditions to 490 t/km2/year under 2018 landuse. A one-year monitoring programme of the river was undertaken to evaluate changing dissolved nutrient dynamics down the river’s long profile through the use of ion-chromatography and stable isotope analysis. The results of this analysis indicate that nutrient loading in the river is linked to agricultural landuses and that NO[x]-N levels in the river vary seasonally and periodically exceed water quality guidelines for aquatic ecosystems. Finally, an assessment was made of the potential for natural vegetation buffer strips to mitigate nutrient inputs from agricultural hillslopes. This was performed by an analysis of soil samples via inductively coupled plasma atomic emission spectroscopy (ICP-AES), laboratory-based testing for bio-available phosphorus, nitrate and ammonium as well as isotope ratio mass spectrometer (IRMS) testing of N and C isotopic composition in soils. Results show that N concentrations in cultivated field and renosterveld soils are impacted by fertilisation of agricultural lands. There is significantly (p <0.05) more P in cultivated fields than in renosteveld soils, while renosterveld soils have a significantly (p <0.05) higher C content than cultivated fields, thus acting as a valuable carbon sink. Renosterveld fragments are shown to remediate polluted agricultural runoff, and so provide a valuable ecosystem service in the landscape.