Mobility of trimethyllead and total lead in the forest floor

The forest floor represents an important compartment for the accumulation of heavy metals in forest soils. To study the mobility of Pb species and their linkage to C cycling in the forest floor, we investigated concentrations of trimethyllead (C3H9Pb+) and total Pb (Pb-total) in throughfall and forest floor percolates during 14 snow-free wk. For our study, we used a long-term field experiment with increased input of throughfall and litterfall at a Norway spruce [Picea abies (L.) H. Karst.] site. The concentrations of trimethyllead were highest in throughfall (106 pg Pb L-1) and decreased with depth of the forest floor. Strong correlations between trimethyllead concentrations in throughfall and forest floor percolates suggested that trimethyllead in forest floor percolates originates largely from throughfall. The partition coefficients of trimethyllead and Pb-total, estimated as ratios of concentrations in percolates to those in forest floors, ranged from 54 to 560 and 870 to 26,400 L kg(-1), respectively, indicating that the mobility of trimethyllead was greater than Pb-total. The Pb-total concentrations were low in throughfall and often the highest in Oa horizon percolates (15.2-57.5 pg L-1). They decreased in Oi horizon percolates after increased throughfall addition. Increased litterfell resulted in increased concentrations of Pb-total in forest floor percolates. Concurrently, dissolved organic C increased. The tight coupling of Pb-total to dissolved organic C was also indicated by correlations (r >= 0.48) that were stronger than those between Pb-total and pH (r <= -0.22). We concluded that the mobilization of Pb-total from the large pool in forest floor horizons is strongly linked to the release of dissolved organic matter. Conversely, the mobility of trimethyllead is regulated by throughfall input and not by dissolved organic C.