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

Inputs of surface waters high in chloride and sulphate increased the availa bility of nutrients in fen peatlands. This `internal eutrophication' was de monstrated with test plants (`phytometers') and through water and soil anal ysis. 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 Na Cl and CaCl2 treatments. The plants in these treatments had taken up much m ore P. (iii) Experiment 2 showed an increase in soil BAP after treatment with chlo ride and sulphate in pots planted with Anthoxanthum odoratum. The chloride treatment had no effect on plant biomass, whereas the sulphate treatment re sulted 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, wh ile N content remained the same. (iv) In experiment 3, treatments with chloride and sulphate led to signific antly increased biomass and P uptake of Anthoxanthum odoratum. Again, no ef fects 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 losse s due to agricultural drainage in the region. The results of this study sho w that phosphate availability to the vegetation may rise drastically, with detrimental effects on the species density and the occurrence of rare speci es in the vegetation. Hence, supply of this water should be avoided.