In situ measurements of solution concentrations and fluxes of trace metals in soils using DGT

Assessments of the availability of toxic or nutrient metals in soils need to consider solution speciation, mobility, and the kinetics of exchange between solution and solid phase. The newly developed technique of diffusive gradients in thin films (DGT) accumulates metals in a well-defined geometry that allows quantification of their supply from the solid phase. DGT was used to measure potential maximum fluxes of available metals in old arable and ex-woodland soils variously treated with sewage sludge with and without metal amendments. The flux increased with increasing sludge application. The results suggest that for Zn and Cd there are two separate pools of metal associated with the solid phase. In untreated soils or those with 100 m(3) ha(-1) yr(-1) sludge treatments, Zn and Cd appear to have higher resupply rate constants from the solid phase than in soils with 300 m(3) ha(-1) yr(-1) sludge treatments (assuming the ratio of labile solid-phase metal to soil solution metal does not change). However, the actual rate of resupply is higher in the treated soils due to the much higher concentration associated with the solid phase. For Ni and Cu there appears to be only one pool of metal irrespective of the extent of sludge treatment.