The contribution of root-derived sulphur to sulphate in throughfall in a Douglas fir forest

The relative contribution of foliar leaching to sulphate in throughfall is determined using radiotracer sulphate. The experiment took place in a Douglas fir forest area at Speuld, The Netherlands, from July 1993 until September 1994. The radiotracer was applied at 3–4 week intervals to the forest soil with a 4-month stop during winter. Isotopic near-equilibrium is obtained between total sulphur in the needles and water-extractable sulphur from the needles. The contribution of root-derived sulphur to sulphate in throughfall is calculated from the 35S-content in throughfall and the water-extractable sulphur pool of the needles. The relative contribution of foliar leaching of root-derived sulphur is less than 3% on average. The influence of doubling the annual input of ammonium nitrate and ammonium sulphate on the forest floor on sulphur leaching is negligible.