The nitrogen and sulfur status and isotopes of soils within the vicinity of a coal-fired power station in South Africa

[摘要] Amplified loads of sulfate and nitrate have caused increased stress on soil systems in many areasof the world, as both are dominant components of acid rain. This is a critical environmental stress dueto the damage caused to soil, water quality and ecosystem functioning. Issues concerning the risingemissions of these elements from local industries have begun to attract increasing attention in SouthAfrica, as the rates of deposition in the Mpumalanga Highveld region alone is comparable to thoseexperienced in First World countries. This study sought to investigate the use of natural stable isotopesof sulfur and nitrogen to identify the process transformations that these species undergo inenvironmental cycles. Total δ34S, δ15N and δ13C isotope signature of soils in the Mpumalanga regionwere combined with total elemental concentrations to determine the effect of deposition on the soilsystem. Soil samples from two soil depths (0 – 10 cm and 20 – 40 cm) were taken along a distancegradient from an identified pollution source, the Majuba power station. Long-term air quality datafrom the study area were also obtained from Eskom’s air quality monitoring stations, as well as sulfurand nitrogen deposition data from selected literature.Elemental concentrations decreased with soil depth as expected, while sites locatedapproximately 25 km downwind of the power station were seen to contain higher concentrations ofboth soil sulfur and nitrogen. The mean per site soil sulfur concentration across all depths ranged from0.009 % to 0.048 %, while the mean per site nitrogen concentration across all depths rangedfrom 0.056 % to 0.346 %. The mean soil carbon concentration in the top-soils ranged from 0.97 % to7.93 %, and decreased in the sub-soils to 0.490 % to 3.270 %.The mean δ34S value for the top-soilswas found to be 8.28 ‰ and increased to 10.78 ‰ in the sub-soils. Soil δ15N also increased with soildepth from 6.55 ‰ to 8.28 ‰. Soil δ13C values were seen to increase from -12.83 ‰ in the top-soils to-11.90 ‰ in the sub-soils. Lighter δ34S values at the surface may be due to anthropogenic deposition.The positive δ34S shift was attributed to a two-source mixing model (atmospheric deposition andbedrock) and isotopic fractionation processes that occur within the soil profile. The δ15N values of thetop-soil were higher than what is expected if all nitrogen was derived from atmospheric nitrogen gasfixation. The increase in δ15N with depth suggested that isotope fractionation occurred during nitrogenexport due to the faster reaction rate of 14N compared to 15N. The soil δ13C values indicated a typicalC4 grassland system. New carbon at the top-soil depths was enriched in 13C due to the slower decay of13C-depleted lignin; whereas in the sub-soils microbial recycling of carbon dominates and explainedthe higher 13C content of the older carbon. The conceptual framework presented for this projectinvolves simultaneous processes of deposition and export in the soil system. This was particularly truefor sulfur, where sites with lower isotope values had lower soil sulfur concentrations and vice versa.This indicates that high levels of deposition correspond to high net export. The sulfur and nitrogenisotopic signatures could not be used to as a direct means of source identification; however, theeffectiveness of isotopes in elucidating transfer of these nutrients in the soil system was illustrated.

[发布机构] University of the Witwatersrand