Decreased use of leaded gasoline has caused a rapid reduction in atmospheric Pb deposition to terrestrial and aquatic ecosystems of the northeastern United States. In forest soils, the response to decreased Pb deposition has been more rapid than was anticipated based on previous studies of Pb cycling in forested ecosystems. To better understand the observed ecosystem response, we estimated both short- (25 y) and long-term (190 y) time series of the regional average atmospheric Pb concentrations in precipitation, cloudwater, and air. We combined these time series with modeled wet and dry deposition velocities to calculate the time-dependent Pb flux at different elevations in montane forests of the northeastern United States. The total integrated flux of anthropogenic Pb to northeastern forests since industrialization is estimated to range from similar to 17 kg ha(-1) at low elevations to >30 kg ha(-1) at elevations above 1200 m. We used the Pb deposition time series and measurements of organic soil horizon Pb content made in 1966, 1980, and 1990 to compute dynamic solutions for the response time of the atmosphere-organic soil system for Pb storage in soils of several forest types. The response time for Pb storage in organic soil increased with increasing elevation from 17 y in the northern hardwood forest zone to 77 y in the subalpine spruce-fir forest zone. These response times calculated by analysis of the dynamic system are significantly lower than previous estimates made using steady-state assumptions (150-500 y). A linear-rate model of Pb transport predicts the observed pattern of Pb accumulation and release by the forest floor of these two forest zones in response to the fluctuating atmospheric inputs. Using this model, we estimate that Pb concentrations will stabilize at similar to 18 and similar to 4 mu g/g in the organic soil horizons of the subalpine spruce-fir and northern hardwood forests, respectively, if atmospheric Pb concentrations stabilize at current values. Maximum Pb concentrations in organic soil occurred around 1980 and were similar to 200 mu g/g in the spruce-fir forest and similar to 85 mu g/g in the northern hardwood forest. The time required to reach steady-state concentrations is estimated to be similar to 400 y for the spruce-fir forest and similar to 90 y for the northern hardwood forest. The large pool of anthropogenic Pb which was deposited on northeastern forests during the 1950s to the 1980s is being redistributed in the soil profile rather than being retained in the organic horizon for several centuries as was previously thought. Analysis of Pb transit times in mineral soil horizons indicates that the large pulse of anthropogenic lead deposited after 1960 may begin to be released to upland streams beginning sometime in the middle of the next century.
