Adverse impacts of agricultural expansion on hydrological and nutrient dynamics in a Renosterveld landscape – can natural vegetation offer solutions

Thesis (PhD)--Stellenbosch University, 2023.

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.

AFRIKAANSE OPSOMMING: Landboustelsels lewer 'n reeks produkte aan die menslike samelewing, insluitend voedsel, brandstof, tekstiele en farmaseutiese produkte. Die wêreldwye uitbreiding van landbou-aktiwiteite het egter verskeie negatiewe uitkomste tot gevolg gehad, soos verlies aan biodiversiteit, verhoogde koolstofvrystellings, bogronderosie en waterbesoedeling. Die agteruitgang van natuurlike landskappe as gevolg van landboutransformasie het dus mettertyd 'n verlies aan ekosisteemdienste tot gevolg gehad deur toenemende habitatverlies, voedingstofbeweging, sedimentasie van riviere en plaagdodervergiftiging in nie-teikenspesies. Renosterveldplantegroei is een van die landskappe in Suid-Africa wat die meeste deur landboutransformasie geraak word. Laeland-renosterveld is 'n kleinblaar, immergroen struikveld wat op die skalieryke, vrugbare gronde van die Suidwes-Kaap van Suid-Afrika voorkom waar dit deel van die Fynbos-bioom vorm, 'n spesieryke blommeryk. Renosterveld kom tipies op fynkorrelige, kleiryke gronde voor in teenstelling met die sanderige, voedingsarm gronde waarop fynbos geleë is. Landbou-uitbreiding het tot die vernietiging van die inheemse renosterveldplantegroei gelei wat nou 'n groot mate van fragmentasie vertoon. Hierdie proefskrif dokumenteer 'n ondersoek na die impak van landbou-uitbreiding op die hidrologiese, sediment- en waterkwaliteitdinamika in die Overbergse renosterveldlandskap in die Wes-Kaap. 'n Evaluasie van die potensiaal van hierdie bedreigde plantegroei om ekosisteemdienste in terme van fitoremediëring van voedingstofinsette vanaf landbouhange te lewer, word gerapporteer. Die impak van veranderende grondgebruik op hidrologiese kenmerke van die gebied word eers op 'n landskapvlak ondersoek, gevolg deur 'n gevallestudie van die Botrivier deur die implementering van 'n volledig gedifferensieerde hidrologiese model met 'n sediment-leweringskomponent. Resultate bevestig dat hidrologie op 'n landskapvlak grootliks deur veranderinge in grondgebruik beïnvloed is, terwyl gemodelleerde gronderosie vanaf die Botrivier-opvanggebied 'n gronderosie toename van 22 t/km2/jaar onder natuurlike toestande tot 490 t/km2/jaar onder 2018 grondgebruik uitbeeld. 'n Eenjarige moniteringsprogram van die rivier is onderneem om veranderende voedingstofdinamika langs die rivier se langprofiel te evalueer deur van ioonchromatografie en stabiele isotoopanalise gebruik te maak. Die resultate van hierdie ontleding bewys dat nutriëntlading in die rivier aan landbougrondgebruike gekoppel is en dat NO[x]-N-vlakke in die rivier seisoenaal verskil en periodiek waterkwaliteitriglyne vir akwatiese ekosisteme oorskry. Laastens is 'n assessering gemaak van die potensiaal vir natuurlike plantegroeibufferstroke om voedingstofinsette vanaf landbouheuwels te versag. Dit is uitgevoer deur grondmonsters deur middel van induktief gekoppelde plasma atoomemissiespektroskopie (ICP-AES), laboratorium-gebaseerde toetsing vir bio-beskikbare fosfor, nitraat en ammonium asook isotoopverhouding massaspektrometer (IRMS) toetsing van N en C isotopiese samestelling in gronde, te ontleed. Resultate toon dat N-konsentrasies in bewerkte veld- en renosterveldgronde deur bemesting van landbougrond beïnvloed word. Daar is beduidend (p <0.05) meer P in bewerkte landerye as in renosterveldgronde, terwyl renosterveldgronde 'n beduidende (p <0.05) hoër C-inhoud as bewerkte landerye het, wat dus as 'n waardevolle koolstofsink dien. Renosterveldfragmente remedieer besoedelde landbouafloop en bied dus 'n waardevolle ekosisteemdiens in die landskap.

Doctoral