Quantifying new versus old aerosol deposition in forest canopies: throughfall mass balance with fallout radionuclide chronometry

Abstract

Net throughfall (NTF) measurements of the fallout radionuclides (FRNs) 7Be and 210Pb confirm that the atmosphere is a strong net source of particulate matter (PM) and trace metals (TMs) to a temperate forest canopy, which retains nearly 60 % of total wet and dry annual atmospheric flux of the FRNs (four trees, three species, two sites; n = 159). Estimation of dry deposition using a multiple-regression technique and predictors of precipitation depth and duration of the antecedent dry period agrees well with ecosystem mass balance, with about 25 % of both 7Be and 210Pb annual flux deposited by dry processes, and total FRN fluxes in reasonable agreement with regional soil inventories. In contrast to the FRNs, other TMs, including Pb and Hg, show large enrichments in throughfall that derive from processes of internal ecosystem recycling such as PM resuspension, leaching from tree metabolic pathways, and physicochemical weathering of non-foliar biological tissues of the tree canopy (collectively “phyllosphere”). To estimate the contributions to net throughfall from these internal pathways, which we term a change in storage (1S), a new FRN canopy mass balance is derived based on the different half-lives of 7Be and 210Pb. Estimated 1Ss for selected elements are SO4 = 7 %, 210Pb = 29 %, As = 42 %, 9Be = 45 %, Cd = 60 %, Hg = 60 %, Pb = 63 %, Fe = 79 %, Al = 79 %, and P = 91 %. The balance of throughfall (1-1S) represents new ecosystem input. Change in storage for all elements was strongly correlated with the export of particulate carbon (FPOM) and dissolved organic carbon (DOC) from the canopy, indicating that physicochemical transformation during residence within the canopy facilitates the release of metals from storage. 1S thus represents an emergent ecosystem property through which metal, carbon, and hydrologic cycles may converge to determine the fate, reactivity, and timing of metal delivery to underlying soils. The forest canopy represents a substantial reservoir of decade-aged PM and cannot be assumed to be at steady state with respect to ongoing atmospheric deposition, especially in the context of changing atmospheric composition, e.g., declining industrial emissions of Pb and Hg.