Threshold Increases in Soil Lead and Mercury from Tropospheric Deposition Across an Elevational Gradient

Atmospheric deposition is the primary mechanism by which remote ecosystems are contaminated, but few data sets show how fluxes change and control soil metal burdens at the landform scale. We present mercury (Hg), lead (Pb-210 and total Pb), and cosmogenic beryllium-7 (Be-7) measurements in organic (O) soil horizons at high-resolution elevation intervals of similar to 60 m from 540 to 1160 m on Camels Hump in northern Vermont, USA. Across this gradient, average O horizon Hg ranges from 0.99 mg m(-2) in the low elevation deciduous forest zone to 7.6 mg m(-2) in the higher elevation coniferous forest at 1030 m. We measure two pronounced threshold increases in soil metal burdens above 801 and 934 m, corresponding to the two most common altitudes of cloud base, which coincide with changes in vegetation species. Lead-210, a unique tracer of tropospheric deposition, also increased from 3200 Bq m(-2) to 11 500 Bq m(-2) in O horizons, exhibiting threshold responses at the same elevations as Hg and total Pb. Concentrations of Pb-210 and Hg in foliage double from 760 to 900 m elevation, indicating enhanced deposition across the transition from deciduous to coniferous forest. In contrast, Be-7 is constant across the entire elevational gradient because of its upper atmospheric source. This indicates that the effects of orographic precipitation have a smaller control on soil contaminant burdens than the coupled cloudwater deposition-vegetation scavenging effect in the presence of upwind sources. By measuring soil contaminants and unique tracers of atmospheric deposition, we show that tropospheric fluxes of Hg and Pb are higher by a factor of 2 in high-elevation coniferous forests than in adjacent lowlands. Total O horizon Hg and Pb burdens increase by over 4-fold with elevation because of the compounding effects of enhanced deposition and longer metal residence times at higher elevations (>50 years).