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Abstract. In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SOi-reduction rates in the sediment, (2) depletion of SOi-in the overlying water column and (3) release of 35S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO{-in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core "SO:-injection. Rates of SOi-consumption in the overlying water were estimated by changes in SO:-concentration over time in in situ enclosures. Reduction rates ranged from 0.27-l 1.2 mm01 me2 d-i. Rates of SOf uptake from the enclosed water column varied from -0.5, -0.3 mm01 m-* d-i (November) to 0.43-1.81 rmnol mm2 d-l (July, August and April). Maximum rates of oxidation to SO:-in July 1990 estimated by combination of SOi-reduction rates and rates of in situ SOiuptake in the enclosed water column were 10.3 and 10.5 mm01 m-* d-l at an organic rich and at a sandy site respectively.Experiments with 35S2-and 35SOj-tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SOi-and (2) the presence of mainly non SOi--S derived from reduced S transported from the sediment into the overlying water. A 15S2-tracer experiment showed that about 7% of 3sS2-injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.

We conducted a field test to investigate whether ground water transfer from one site (showing complete natural reductive dechlorination of chlorinated ethenes to ethene) could induce full reductive dechlorination at another site polluted with tetrachloroethene and its partial dechlorination products trichloroethene and cis‐dichloroethene (cDCE). Addition of electron donor (lactate) at the test site established low redox conditions but did not stimulate further dechlorination past cDCE. After transferring 2 m3 of ground water from the first site to the test site, full dechlorination commenced and high levels of ethene were measured to distances up to 6 m downstream of the injection location within 7 months. Ground water samples from monitoring wells were analyzed before and after inoculation of the test site for the presence of Dehalococcoides species (16S ribosomal RNA) and vinyl chloride reductase (vCRA) genes using the polymerase chain reaction. These tests showed that Dehalococcoides species were present both before and after ground water transfer, while vCRA genes were detected at the test site only after ground water transfer. The vCRA genes were detected in ground water samples collected 6 m downstream of the injection locations 7 months after ground water transfer, suggesting that the microorganisms carrying the dehalogenase genes were effectively transported in the aquifer.

To determine the fate of atmospheric ammonium in forest soils, one calcareous and two acid forest soils were incubated with ~SN ammonium. In the calcareous soil about 65% of the applied ~5N-ammonium was recovered as nitrate after 98 days of incubation, whereas in the acid soils less than 10% of the 15N-ammonium was converted to nitrate. In all soils a large proportion of the ~5N was incorporated in organic nitrogen compounds. This incorporation limits the use of 15N tracers for the elucidation of the fate of atmospheric ammonium in soils.

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