Using atmospheric fallout to date organic horizon layers and quantify metal dynamics during decomposition

High concentrations of metals in organic matter can inhibit decomposition and limit nutrient availability in ecosystems, but the long-term fate of metals bound to forest litter is poorly understood. Controlled experiments indicate that during the first few years of litter decay, Al, Fe, Pb, and other metals that form stable complexes with organic matter are naturally enriched by several hundred percent as carbon is oxidized. The transformation of fresh litter to humus takes decades, however, such that current datasets describing the accumulation and release of metals in decomposing organic matter are timescale limited. Here we use atmospheric Pb to quantify the fate of metals in canopy-derived litter during burial and decay in coniferous forests in New England and Norway where decomposition rates are slow and physical soil mixing is minimal. We measure Pb inventories in the O horizon and mineral soil and calculate a 60–630 year timescale for the production of mobile organo-metallic colloids from the decomposition of fresh forest detritus. This production rate is slowest at our highest elevation ( 1000 m) and highest latitude sites (>63 N) where decomposition rates are expected to be low. We calculate soil layer ages by assuming a constant supply of atmospheric Pb and find that they are consistent with the distribution of geochemical tracers from weapons fallout, air pollution, and a direct Pb application at one site. By quantifying a gradient of organic matter ages with depth in the O horizon, we describe the accumulation and loss of metals in the soil profile as organic matter transforms from fresh litter to humus. While decomposition experiments predict that Al and Fe concentrations increase during the initial few years of decay, we show here that these metals continue to accumulate in humus for decades, and that enrichment occurs at a rate higher than can be explained by quantitative retention during decomposition alone.Acid extractable Al and Fe concentrations are higher in the humus layer of the O horizon than in the mineral soil immediately beneath this layer: it is therefore unlikely that physical soil mixing introduces significant Al and Fe to humus. This continuous enrichment of Al and Fe over timemay best be explained by the recent suggestion that metals aremined from deeper horizons and brought into theO horizon viamycorrhizal plants. In sharp contrast toAl andFe, we find thatMn concentrations in decomposing litter layers decrease exponentiallywith age, presumably because of leaching or rapiduptake,whichmay explain the low levels of acid extractableMn in the mineral soil. This study quantifies howmetals are enriched and lost in decomposing organic matter over a longer timescale than previous studies have been able to characterize.We also put new limits on the rate at whichmetals in litter become mobile organo-metallic complexes that can migrate to deeper soil horizons or surface waters. 2011 Elsevier Ltd. All rights reserved. 0016-7037/$ see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.01.011 ⇑ Corresponding author. Tel.: +1 757 221 2951. E-mail address: [email protected] (J.M. Kaste).