ALBERT

Beschreibung: Mass balance studies in forested catchments in the northeastern USA show that S losses via streamwater SO42- exceed measured atmospheric S inputs. Possible sources of the excess S loss include underestimated dry deposition, mineralization of organic S in soils, desorption of soil sulphate, oxidation of recently formed sulphides and mineral weathering. Evaluating the relative contribution of these sources and processes to SO42- export is important to our understanding of S cycling as well as to policy makers in their evaluation of the efficacy of S emission controls. In order to evaluate the potential for mineral weathering contributions to SO42- export, we measured concentration and isotopic composition (δ34S and δ18O) of SO42- in stream water, and concentration and δ34S values of four S fractions in bedrock and soil parent material in catchments of varying geological composition. Geological substrates with low S concentrations were represented by catchments underlain by quartzite and granite, whereas geological substrates with high S concentrations were represented by catchments underlain by sulphidic slate, schist and metavolcanic rocks. Catchments with S-poor bedrock had stream-water SO42- concentrations 〈100 μeq L-1 and isotopic values consistent with those of atmospheric SO42- that had been cycled through the organic soil pool. Catchments with S-rich bedrock had stream-water SO42- concentrations ranging from 56 to 229 μeq L-1. Isotopic values deviated from those of SO42- in atmospheric deposition, clearly indicating a mineral weathering source in some cases, whereas in others spatial variability of mineral δ34S values precluded the isotopic detection of a weathering contribution. These results, along with evidence suggesting formation of secondary sulphate minerals in bedrock weathering rinds, indicate that mineral weathering may be an important source of S in the surface waters of some forested catchments in the northeastern USA. © 2004 John Wiley and Sons, Ltd.

Beschreibung: Background aqueous chemistry and 15Nnitrate tracer injection methods were used to calculate in-stream nitrate uptake metrics at Red Canyon Creek, a third-order stream in the Rocky Mountains in the state of Wyoming, United States. 'Net' nitrate uptake lengths, which reflect both nitrate uptake and regeneration, and 'gross' nitrate uptake lengths, which exclude re-mineralization, were quantified separately from background nitrate chemistry and 15N labelling tracer data, respectively. Gross nitrate uptake lengths, from tracer injections of 15N labelled nitrate, ranged from 502 to 3140 m. Net nitrate uptake lengths, from background nitrate chemistry downstream of a point source, ranged from 1170 to 4330 m. Diurnal changes in uptake lengths suggest the importance of nitrate utilization by autotrophs in the stream and benthic zone. The differences between net and gross nitrate uptake lengths along lower reaches of Red Canyon Creek allowed us to estimate the nitrate regeneration rate, which was 0.056-0.080 μmol m-2 s-1 during the day and 0.0062-0.0083 μmol m-2 s-1 at night. Spatial patterns of streambed pore water chemistry indicate those areas of the hyporheic zone where denitrification was likely occurring. Permanent log dams generated stronger redox gradients in the hyporheic zone than areas with transient beaver dams. By combining isotopically labelled nitrate additions, estimates of uptake from background aqueous nitrate chemistry and characterization of redox conditions in the hyporheic zone, we were able to determine the nitrate regeneration rate and the redox processes responsible for nitrogen cycling in the hyporheic zone. © 2010 John Wiley & Sons, Ltd.