Effect of Vegetation Clearing on the Nitrogen Cycle and Water Resource Quality in South Africa

Clearing nitrogen-rich vegetation is known to result in increased mineralization and consequently to generate nitrate pools in excess of biota requirements. Nitrate from this pool can potentially leach into water resources. Nitrates in water pose a risk to humans and animals that depend on the resource for drinking water. The CSIR and the Universities of the Western Cape (UWC) and Stellenbosch (US) are currently conducting a 2 year (2006–2008) research project to investigate the impacts that the clearing of leguminous invasive alien vegetation, particularly A. saligna (Port Jackson willow), has on groundwater and surface water quality. Groundwater resources offer an alternative source, often of better quality than surface sources, such as ponds and streams. But it can become contaminated and nitrates are among the key groundwater contaminants. At high levels (20 mg N/L (as N) is the upper limit in South Africa), ingested nitrate causes methaemoglobinaemia, which may be fatal for infants and livestock. Most nitrate problems are due to pollution but, in semiarid to arid regions, high groundwater nitrate has been reported from areas where anthropogenic influences can be excluded. This indicates that also under natural conditions, nitrogen fixation may be followed by nitrification with subsequent leaching of nitrate into the groundwater. However, anthropogenic activities, such as deforestation, can modify the N-cycle by producing excessive mobile nitrate. Faillat and Rambaud (1991) linked the groundwater nitrogen in the rural area of Côte d’lvoire, West Africa, to deforestation.