Uptake and recycling of lead by boreal forest plants: Quantitative estimates from a site in northern Sweden

摘要

As a consequence of deposition of atmospheric pollution, the lead concentration in the mor layer (the organic horizon) of remote boreal forest soils in Sweden is raised far above natural levels. How the mor will respond to decreased atmospheric pollution is not well known and is dependent on future deposition rates, downward migration losses and upward fluxes in the soil profile. Plants may contribute to the upward flux of lead by 'pumping' lead back to the mor surface through root uptake and subsequent litter fall. We use lead concentration and stable isotope (Pb-206, Pb-207 and Pb-208) measurements of forest vegetation to quantify plant uptake rates from the soil and direct from the atmosphere at two sites in northern Sweden; an undisturbed mature forest and a disturbed site with Scots pine (Pinus sylvestris) growing on a recently exposed mineral soil (C-horizon) containing a minimum of atmospherically derived pollution lead. Analyses of forest mosses from a herbarium collection (spanning the last similar to 100 yr) and soil matrix samples suggest that the atmospheric lead deposited on plants and soil has an average Pb-206/Pb-207 ratio of 1.15, while lead derived from local soil minerals has an average ratio of similar to 1.47. Since the biomass of trees and field layer shrubs has an average Pb-206/Pb-207 ratio of similar to 1.25, this indicates that 70% +/- 10% of the inventory of 1 +/- 0.8 mg Pb m(-2) stored in plants in the mature forest originates from pollution. Needles, bark and apical stemwood of the pine growing on the disturbed soil, show lower Pb-206/Pb-207 ratios (as low as 1.21) than the roots and basal stemwood (having ratios > 1.36), which indicate that plants are able to incorporate lead directly from the atmosphere (similar to 50% of the total tree uptake). By partitioning the total uptake of lead into uptake from the atmosphere and different soil layers using an isotopic mixing model, we estimate that similar to 0.03 +/- 0.01, 0.02 +/- 0.01 and 0.05 +/- 0.01 mg Pb m(-2) yr(-1) (mean +/- SD), is taken up from the mor layer, the mineral soil and the atmosphere, respectively, by plants in the undisturbed mature forest. These small fluxes, which are at least a magnitude lower than reported downward migration losses, suggest that plant uptake will not strongly prolong the self-cleaning rate of the mor layer. Copyright (c) 2005 Elsevier Ltd.