Internal eutrophication in peat soils through competition between chloride and sulphate with phosphate for binding sites

Inputs of surface waters high in chloride and sulphate increased the availability of nutrients in fen peatlands. This `internal eutrophication' was demonstrated with test plants (`phytometers') and through water and soil analysis. Three experiments are presented in which the level of chloride and/or sulphate was increased to 3 mmol(c) l(-1). In experiment 1 chloride levels were increased from 0.5 to 3 mmol(c) l(-1) as CaCl2 or NaCl. In experiment 2 and 3 similar increased levels for chloride and sulphate (3 mmol(c) l(-1); as NaCl and Na2SO4) were used. The following results were found: (i) No differences in soil total-N and total-P were found before and after the treatments in any of the three experiments. (ii) Experiment 1 showed a significant increase in Bio-Available P (BAP) in pots planted with Anthoxanthum odoratum as well as in bare pots for the NaCl and CaCl2 treatments. The plants in these treatments had taken up much more P. (iii) Experiment 2 showed an increase in soil BAP after treatment with chloride and sulphate in pots planted with Anthoxanthum odoratum. The chloride treatment had no effect on plant biomass, whereas the sulphate treatment resulted in a reduction in root biomass and root N and P content. The shoots showed an increase in P content in the sulphate and chloride treatments, while N content remained the same. (iv) In experiment 3, treatments with chloride and sulphate led to significantly increased biomass and P uptake of Anthoxanthum odoratum. Again, no effects on N uptake were found. These experiments provide evidence for distinctly increased availability of phosphate in peat soils when these come into contact with water with even moderately increased sulphate or chloride levels. Surface water originating from the Rhine river, which is enriched in chloride and sulphate, is often supplied to fen reserves in The Netherlands, to compensate for water losses due to agricultural drainage in the region. The results of this study show that phosphate availability to the vegetation may rise drastically, with detrimental effects on the species density and the occurrence of rare species in the vegetation. Hence, supply of this water should be avoided.