We used repeated sampling of the forest floor to determine if there was a net loss of Ca from organic horizons of Adirondack forest soils between 1930 and 2004. In 1984, we established 48 permanent plots in spruce–fir, northern hardwood, and pine stands located in areas sampled by Carl C. Heimburger in the early 1930s. Following Heimburger's protocols and analytical methods, we measured pH and dilute‐HCl‐extractable Ca in Oe and Oa horizons, and determined that there was a statistically significant decrease in Ca concentration during the 1932 to 1984 interval. In the 36 plots that we could locate in 2004, we again sampled organic horizons. During the >70‐yr interval, HCl‐extractable Ca concentrations in the Oe and Oa horizons decreased in each forest type (P < 0.05). We also measured NH4Cl‐extractable Ca and Al in the 1984 and 2004 samples and found a significant decrease in Ca concentration in the pooled Oe horizons. High‐elevation spruce–fir plots showed a Ca loss rate between 1984 and 2004 of 7.6 to 9.8 kg ha−1 yr−1 This compares well with a 4‐yr Ca cycling study conducted in an equivalent spruce–fir forest at Whiteface Mt. in the Adirondacks, which showed an annual forest floor Ca loss of 8.4 kg ha−1 yr−1 Based on uptake and anion flux data from the Whiteface Mt. study, we estimated that about 25 to 30% of the 1984 to 2004 forest floor Ca loss in the spruce–fir plots is attributable to leaching driven by atmospheric SO42− deposition.
Hematite is a semiconducting mineral with a role in natural photoelectrochemical processes, and has been studied from the viewpoint of solar energy utilization. Hematite is an anisotropic conductor, with faster conduction parallel to (001) planes. Flatband potentials and photocurrent onset potentials for natural hematite single crystals and synthetic nanocrystalline hematite films are similar, and show Nernstian behavior within error. At pH 7, the flatband potential of single crystals is -0.25AE0.1 V vs. Ag/AgCl. Photocurrent onset potential is 0.02 to 0.03 V more negative for crystal faces than for crystal edges. Photocurrent density is a factor of 5 to 10 higher for crystal edges than for crystal (001) faces, presumably because of more rapid charge separation for the edges. Falling photocurrent transients decay more slowly for edges than for faces, consistent with more rapid removal of conduction band electrons into the bulk and therefore reduced availability of such electrons for back reaction. Rising photocurrent transients occur at higher potential, and have the same rise time for both faces and edges. This suggests that the rising transients are due to slow conduction through bulk hematite. The transition from falling to rising transients occurs at a more positive potential for edges than for faces, which is also consistent with more rapid charge transport away from edge surfaces and with Fermi level pinning at edges.
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