Distribution of zinc and lead fractions within a forest Spodosol

Soil processes play a critical role in the fate of trace metals in forest ecosystems. To explore the nature of soil-trace metal interactions, we estimated five fractions of Zn and Pb in Spodosols at the Hubbard Brook Experimental Forest in central New Hampshire: (i) soluble + exchangeable (EX); (ii) inorganically hound (IB); (iii) organically bound (ORG); (iv) amorphous oxide bound (AMOX); and (v) mineral lattice, or residual (RES). Organic matter is a critical factor in trace metal fractionation in Hubbard Brook soils; loss-on-ignition was strongly correlated to EX Zn (r = 0.91) and Pb (r = 0.85), and ORG Zn (r = 0.57) and Pb (r = 0.89). The Oa horizon accounted for only 1.9% of the soil mass above the C horizon, but contained 23% and 77%, respectively, of the EX Zn and Pb in the soil. Lead was more tightly held in the soil than Zn; the percentage of total metal in the EX fraction was greater for Zn than Pb in all horizons. Through biocycling, Zn is returned to the forest floor, helping to maintain labile pools in upper soil horizons. Nevertheless, the EX Zn pool (6.03 kg ha^-1) is insufficient to replace the pool of Zn in aboveground tissues (6.87 kg ha^-1). Lead, not a nutrient, has accumulated in the soil due to the combustion of leaded gasoline. Lead leached from the O horizon between 1926 and 1991 can account for 30% of the EX + IB + ORG Pb, or 14% of the labile Pb in the mineral soil.