Nitrogen dynamics in land cleared of alien vegetation (<i>Acacia saligna</i>) and impacts on groundwater at Riverlands Nature Reserve (Western Cape, South Africa)

Abstract

Woody invading alien plants, many of which are nitrogen-fixing legumes (Fabaceae family), are currently cleared in South African catchments to reduce water loss and preserve streamflow, and for the restoration of the ecosystem. This study tested the hypothesis that clearing invasive alien vegetation may disturb the vegetation-micro-organism-soil N cycling system by producing a large once-off input of fresh tree litterfall rich in N and by eliminating a large N sink. Three experimental plots were established at the Riverlands Nature Reserve (Western Cape, South Africa): a site invaded by Acacia saligna to be used as control; a site cleared of Acacia saligna; and a site with natural vegetation to be used as background. Nitrogen concentrations in soil and groundwater, volumetric soil water contents, root density and weather conditions were measured during 2007. Oxidised forms of nitrogen, in particular NO 3 -, were dominant in the system. Recharge and leachate were simulated with the HYDRUS-2D model and used as inputs into Visual MODFLOW to predict the spatial distribution of nitrate plus nitrite (NOx) in groundwater. NOx levels in soil and groundwater were higher in alien-invaded areas compared to fynbos-covered land. A quick release of NOx into groundwater was observed due to high residual N reserves in the rooting zone, decreased evapotranspiration and increased recharge in the treatment cleared of alien vegetation. In the long run, high NOx concentrations in groundwater underlying cleared land will last only until all the excess nitrogen has been leached from the soil. A decrease in NOx concentration in groundwater can be expected thereafter. Clearing land of alien invasive legumes may therefore have a beneficial effect by reducing groundwater contamination from NOx and reducing water losses in catchments.

Selection of Tree Species and Soil Management for Simultaneous Fuelwood Production and Soil Rehabilitation in the Ethiopian Central Highlands

“…In our results, it is surprising that Acacia species did not increase nitrogen concentrations in the topsoil, which can suggest strong leaching from top soil to deeper layers ( Figures S6 and S7). This might affect groundwater as strong NOx contamination has been found in catchments afforested with Acacia saligna (Jovanovic et al, 2009), so large restoration programs with these species should take into account long-term negative effects.…”

Acacia saligna's soil legacy effects persist up to 10 years after clearing: Implications for ecological restoration

“…Moreover, we found a low N 6 years after clearing compared to the reference invaded and non-invaded areas. The differences are most likely a result of high N losses in the cleared area (Stock & Lewis 1986;Witkowski 1991;Malcolm et al 2008;Jovanovic et al 2009;Staska et al 2014). It is worth noting that low N in the cleared area compared to the invaded and non-invaded areas were not standard across all study sites.…”

“…The differences are most likely a result of high N losses in the cleared area (Stock & Lewis 1986;Witkowski 1991;Malcolm et al 2008;Jovanovic et al 2009;Staska et al 2014). The differences are most likely a result of high N losses in the cleared area (Stock & Lewis 1986;Witkowski 1991;Malcolm et al 2008;Jovanovic et al 2009;Staska et al 2014).…”

Invasion of Abandoned Agricultural Fields by Acacia Mearnsii Affect Soil Properties in Eastern Cape, South Africa

“…Soils were collected using a soil auger at a depth of 10 cm after the removal of the overlaying debris. Collection of soils at the above mentioned depth was based on results by Jovanovic et al (2009) who reported that soil nutrients were represented at a depth of 0 -5 cm underneath Acacia sites. After soil collection, soil nutrient analyses, pH, soil resistivity, gravimetric soil moisture and soil repellency were assessed under laboratory condition at Rhodes University in Grahamstown.…”