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 **3**5S (87 day half-life) to quantify the contribution of foliar leaching and dry deposition to sulfate (SO//4**2** minus ) 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 **3**5 S were measured in leaves; **3**5S and SO//4**2** minus were measured in throughfall (THF). The contribution of foliar leaching to SO//4**2** minus in NTF, THF minus incident precipitation, was estimated by isotope dilution of **3**5S in NTF arising from nonradioactive S in dry deposition. The percent contribution of foliar leaching to SO//4**2** minus in NTF was greatest during the week following isotope labelling and during the period of autumn leaf fall. During the growing season, foliar leaching accounted for less than 20% and dry deposition accounted for greater than 80% of the SO//4 **2** minus 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//4**2** minus flux in NTF.