WATER-SOLUBLE ORGANIC-MATTER IN FOREST SOILS .1. COMPLEXING PROPERTIES AND IMPLICATIONS FOR SOIL EQUILIBRIA

By applying a modified gel permeation technique, the molecular-size di stribution (MSD) and complexing properties of water-soluble organic ma tter (WSOM), isolated from the A(h) horizon under stands with either D ouglas-fir, European beech or Scots pine were established. Both with r espect to MSD and complexing properties, the dissolved organic matter was highly similar. WSOM was comprised of compounds apparently high in molecular weight (>1 kDa) and with a complexing capacity of 1.0 +/- 0 . 1 mumol mg-1 carbon as determined for Cu(II) at pH 5.5 and 0.01 M io nic strength. The effect of WSOM on the partitioning of cations betwee n soil solid phase and soil solution was evaluated in several soil bat ch experiments using loamy sand or sandy soil material. Although a lar ge part of WSOM was sorbed to the soil matrix, Al, Cu, Fe and Pb were solubilized in considerable amounts by complexation. The Mn concentrat ion in the soil solution was also significantly increased but this pro bably resulted from a redox reaction, with certain constituents of WSO M serving as electron donor. With a decrease in soil pH, cation mobili zation by WSOM was significantly lower as a result of increased sorpti on and a decrease in complexing capacity of the soluble organics. Appl ication of several low MW aliphatic and phenolic acids gave results si milar to the results obtained with WSOM.