Understanding the availability of metals in agricultural soils and the impact of manure application

Metal concentrations and distributions in a Belgian agricultural soil and the impact of manure application were studied. Most metal contaminants, except Pb and Co, reached their highest levels in winter (between 1.7 and 5.3 times higher than in other seasons) due to manure application. The vertical metal profiles in the soil also varied seasonally because of agricultural activities and plant growth. Meanwhile, sequential extraction on the soil solid phase pointed out that on an annual basis, Cd, Co, and Mn were primarily present in the exchangeable fraction at 42, 34 and 41 % respectively (also Ni was quite present in the exchangeable fraction at 20 %), that Pb was in the reducible fraction at 52 %, while Cr and Fe were more enriched in the residual fraction at 66 and 64 %, respectively. In addition, metal interactions between soil porewater and solid phase were investigated by using the passive sampling technique of Diffusive Gradients in Thin-films (DGT) and the one-dimensional DGT-induced flux in sediments (1D-DIFS) model. These results suggest a high release rate of Cd, Co, Mn and Ni from the soil solid phase to the pore water, and a low release rate of Cr, Cu, Fe and Pb. Therefore, the first group of metals poses a much greater risk for plant uptake and intoxication than the second group of metals.