Nitrates in a catchment cleared of alien woody legumes in relation to ground water quality in the Atlantis aquifer (South Africa)

[摘要] ENGLISH ABSTRACT: The sandy soils of the Riverlands Nature Reserve, near Malmesbury, and theneighbouring farm were studied to determine the effects of long term invasion of thelegume Acacia saligna on the soil nutrient content of a soil previously vegetated withfynbos. The effect of the removal of this alien legume on general soil properties andgroundwater quality were also studied. The changes in nitrates and nitrites (NOX) due tothe invasion and removal of the alien legume were investigated in more detail thanchanges in other soil nutrients. In addition to that emphasis was placed on the effect ofvegetation clearing on groundwater quality, specifically relating to potentialcontamination with nitrates.This study was initiated after Conrad et al., (1999) found increased NOX concentrationsin ground water while studying the effects of pig farming on ground water nitrogen (N)near a site cleared of Acacia saligna by Working for Water (WFW). Since many sites arescheduled for removal of this alien vegetation it was deemed necessary to study theeffects that clearing alone had on groundwater quality. It was suspected that there wouldbe an increase in soil and groundwater NOX with vegetation removal due to the inputsfrom the legume alien invader.Soil sampling was done continuously throughout the rainy season of 2007 (From May toDecember) on three adjacent sites separated by some 50 m of distance, consisting of anatural fynbos site and two Acacia saligna sites. The sites were selected approximatelyon the same contour line to prevent interaction through lateral water flow. One of theAcacia sites was cleared by the Working for Water programme in the usual mannerleaving slash on the ground. Soil samples were collected at regular intervals throughoutthe season from all three sites (fynbos, Acacia and cleared site) using a Jarrett soil auger.They were airdried (to achieve full oxidation of mineral N) and sieved though a 2mmsieve. Soils were analysed by atomic absorption spectroscopy for basic cations and by ionchromatography for anions, including nitrates and nitrites. Total carbon and nitrogen wasdetermined by combustion, pH (1M KCl and H2O) and EC (1:5 H2O) were also measured. The present study was part of a wider investigation into the quality ofgroundwater, modelling flow and contaminant transport (Jovanovic et al., 2008).The largest changes in soil properties were observed in the top (0-5 cm) layer. The fynbossite had a lower nutrient status by comparison to the Acacia site and the cleared site. Thesum of cations from the soils of the fynbos site at a soil depth of 0-5cm was 100mg/kgand soils from the cleared and Acacia sites were about 190mg/kg. The Acacia site had ahigher NOX status and experienced a larger NOX fluctuation during the rainy season thanthe fynbos site; minimum NOX values were similar (below 10mg/kg) but the Acacia sitehad a maximum NOX value of near 60mg/kg and the fynbos site just below 30mg/kg.There was little difference in general soil characteristics (exchangeable cations, pH andEC, total soil C and N) during the first season after clearing, between the Acacia and thecleared sites.The effect of soluble nitrogen changes due to alien legume invasion and removal ongroundwater quality, relating to NOX, during the first season after clearing, wasdetermined. It was found that the Acacia site had higher NOX concentrations than thefynbos site. At 0-5cm the fynbos site NOX was less than 30mg/kg and the Acacia site wasbetween 30 and 110mg/kg for most of the season, with values lower than 30mg/kg for thelast four sampling dates only. N concentrations on the cleared site behaved in a similarmanner to the uncleared Acacia site, but generally N values were lower on the clearedsite, there were only two sampling dates where the cleared site had higher NOX valuesthan the Acacia site at 0-5cm. The average groundwater N in NOX under the cleared sitewas 4.34 mg/l, and under the Acacia site 3.78mg/l, these values are both below the leveldetermined for water contamination with nitrates. However, the increase in ground waternitrate levels after A. saligna clearing was significant.It was concluded that there is a change in the nutrient status of soil with Acacia invasionand again with removal. NOX migrates to the groundwater to a larger degree once vegetation has been removed, although during this study the nitrate pollution threshold of10 mg/l nitrate N was not reached.

[发布机构] Stellenbosch University