No abstract

The forest floor at high elevation spruce-fir sites from southern Vermont, U.S. to the Gaspé Peninsula, Québec, Canada was sampled and analyzed in 1979 and re-sampled and analyzed in 1996 to study temporal changes in the impacts of atmospheric pollutants. We determined organic matter mass, pH, and concentrations of Al, Ca, Fe, K, Mg, Na, Cd, Cu, Hg, Pb, and Zn for the litter (L = fresh litter plus Oi horizon) and fermentation plus humic horizons (F + H) (= Oe plus Oa horizons) of the forest floor. There were no trends for Al or Fe concentrations in the 1979 or 1996 L along the transect. Several sites had significantly lower Al and Fe values in 1996 than in 1979, likely indicating less mineral soil in the 1996 samples. The 1996 concentrations of Ca in L increased along the transect from 0.22% dry weight (dw) in Vermont to 0.60% dw in Québec. Concentrations of Mg in L were relatively constant along the transect. Neither Ca nor Mg changed at sites from 1979 to 1996, indicating unchanged base status. Concentrations of Cd did not vary spatially along the transect but decreased at all sites from 1979 to 1996. Cu and Zn did not vary spatially or with time. In 1979, the concentration of Hg in L ranged between 150 and 300 µg kg −1 dw, with no spatial gradient. By 1996, Hg concentrations were 25 to 50% lower in L, with decreases generally proportional to the concentration in 1979. The concentration of Pb in 1979 L decreased significantly from 200 mg kg −1 dw in southern Vermont to 60 mg kg −1 dw in Québec. By 1996, the Pb concentration in L ranged between 32 and 66 mg kg −1 dw with no spatial trend along the transect. Decreases in Pb concentrations at sites were proportional to the absolute value in 1979. The concentrations of Cd, Hg, and Pb have declined in litter from 1979 to 1996, indicating a decline in atmospheric deposition. Higher Hg and Pb accumulation rates to the southwest are suggested for the past as indicated by (F + H) concentrations and inventories of Hg and Pb. The decline of Pb in L is consistent with the decreased use of leaded gasoline starting in the 1970s; the declines in Cd and Hg probably reflect lower emissions over the same period. Declining concentrations of Cd, Hg, and Pb in L parallel those documented in recent lake and peat sediments in the northeastern United States.

Abstract. Empirical field evidence for changing chemical processes in soils caused by atmospheric deposition of pollutants consists of: (1) Long-term water quality data including total dissolved solids, concentrations of specific metals (e.g. Ca), and conductivity; (2) Cation exchange capacity and base saturation values for soils located on precipitation pH gradients; (3) Lysimeter studies; and (4) Chemical analysis of organic soils on precipitation pH and metal gradients. For well-drained organic soils, as precipitation pH decreases, metals are differentially leached at an accelerated rate (Mn > Ca > Mg >--Zn > Cd and Na > AI). Experimental field and laboratory lysimeter studies on soil columns yield similar results, with increases in leaching rates for soil solutions with pH = 3 up to 100 x values for soil solutions with pH = 5. Nearly 100% of the Pb from precipitation is accumulating in the organic soil layer or sediments. Zn is accumulating in soils and sediments where the pH's of precipitation, soil solutions, and surface waters are generally above 5 to 5.5. At lower pH values Zn and other chemically similar elements are desorbed/leached (net) at an accelerated rate.Chemical analyses of dated sediment cores from high and low altitude lakes, with drainage basins relatively undisturbed for the last 200 + yr, reveal that increased deposition of metals on a regional scale started in the northeastern United States as early as 1880, consistent with increased fossil fuel consumption. This suggests acidified precipitation as early as 1880. Cores from historically acidified lakes (pH < ~ 5.3 to 5.5) indicate that, as acidification of surface waters occurs (caused by acidic deposition), concentrations of Zn, Mn, and Ca decrease in the sediment. Apparently the metals are leached from the detritus prior to sedimentation. This conclusion results from data from experimental acidification of sediment cores and the general observation that precipitation pH is generally >--0.5 pH units lower than lake water pH. Accelerated leaching of soil in New England dates to earlier than 1900.

No abstract

No abstract