Contribution of foliar leaching and dry deposition to sulfate in net throughfall below deciduous trees

Experiments were conducted at Walker Branch Watershed, TN, in 1986 with radioactive 35S (87 day half-life) to quantify the contribution of foliar leaching and dry deposition to sulfate (SO 2− 4) in net throughfall (NTF). Two red maple ( Acer rubrum) and two yellow poplar ( Liriodendron tulipifera) trees (12–15 m tall) were radiolabeled by stem well injection. Total S and 35S were measured in leaves; 35S and SO 2− 4 were measured in throughfall (THF). The contribution of foliar leaching to SO 2− 4 in NTF, THF minus incident precipitation, was estimated by isotope dilution of 35S in NTF arising from nonradioactive S in dry deposition. The per cent contribution of foliar leaching to SO 2− 4 in NTF was greatest during the week following isotope labeling and during the period of autumn leaf fall. During the growing season, foliar leaching accounted for < 20% and dry deposition accounted for > 80% of the SO 2− 4 in NTF beneath the study trees. Dry deposition of S to these tree species can be reasonably approximated during summer from the measurement of SO 2− 4 flux in NTF.