Doctoral thesis, 2001
Catchment hydrochemical processes controlling acidity and nitrogen in forest stream waterFölster, Jens
AbstractAtmospheric deposition of air pollutants has been a severe threat to terrestrial and forest ecosystems for several decades. In Sweden sulphur deposition has caused acidification of soils and runoff, while nitrogen deposition only had a minor or local impact on runoff quality so far. During the last three decades, emission control has caused a decline in sulphur deposition, whereas nitrogen deposition on the other hand, has continued to increase to a rate several times above the natural background level. Long term changes in runoff acidity and nitrogen chemistry after these changes in deposition are of great concern. Monitoring of small, well-defined catchments including hydrochemistry of precipitation,soil and runoff, is a valuable tool for addressing this concern. When interpreting runoff data from such sites,the near-stream zone has been identified to be of crucial importance. The main objective for this thesis was to explain how catchment processes were related to short-term variation and long-term trends in the hydrochemistry of forest stream water. The field work was conducted on the strongly acidified and nitrogen limited Kindla catchment, with a special emphasis on the relationship between the near-stream zone and both stream acidity and nitrogen leaching. Furthermore, time series of hydrochemistry in forest stream water from 13 catchments were analysed for changes in acidity and nitrogen leaching. In three of these sites, soil water from E-and B-horizons was also analysed with regards to these questions. The investigations revealed that the near-stream zone was a net source of acidity in runoff at Kindla due to leaching of organic acids, although this contribution was overshadowed by sulphate from upland soils and deposition. The near-stream zone was also the main source for both organic nitrogen and nitrate to the stream, but the leaching rate was low, especially for inorganic nitrogen.In the 13 reference streams, sulphate concentrations declined in all cases. In the five most acidified streams, a recovery from acidification was detected,although at a low rate. Sulphate mobilisation and decreased leaching of base cations probably delayed the recovery. Similar trends of declining sulphate concentrations and a tendency for decreasing acidity were also found in soil water in two of the southern sites. The possibilities of a continued acidification of the soil with a concurrent recovery of runoff was discussed and indicated at one site as a decrease in the Ca/H 2 ratio. Leaching of inorganic nitrogen did not show any general trends in the reference streams, but a significant decrease of TOC/TON in most streams indicated a change in the quality of the organic matter in runoff.
Keywordsacidification; near-stream; nitrogen leaching; recovery; soil water; stream; Sweden; trends
Published inActa Universitatis Agriculturae Sueciae. Silvestria
2001, number: 190
Publisher: Department of Environmental Assessment, Swedish University of Agricultural Sciences