Fate and distribution of sulfur-35 in yellow poplar and red maple trees

Two deciduous tree species (yellow poplar and red maple) on Walker Branch Watershed (WBW), near Oak Ridge, Tennessee, were radiolabeled with ³⁵S (87 day halflife) to study internal cycling, storage, and biogenic emission of sulfur (S). One tree of each species was girdled before radiolabeling to prevent phloem translocation to the roots, and the aboveground biomass was harvested prior to autumn leaf fall. Aboveground biomass, leaf fall, throughfall, and stemflow were sampled over a 13 to 24 week period. Sulfur-35 concentrations in tree leaves reached nearly asymptotic levels within 1 to 2 weeks after radiolabeling. Foliar leaching of ³⁵S and leaf fall represented relatively unimportant return pathways to the forest soil. The final distribution of ³⁵S in the nongirdled trees indicated little aboveground storage of S in biomass and appreciable (>60%) capacity to cycle S either to the belowground system by means of translocation or to the atmosphere by means of biogenic S emissions. Losses of volatile ³⁵S were estimated from the amount of isotope missing (∼33%) in final inventories of the girdled trees. Estimated ³⁵S emission rates from the girdled trees were ∼10^-6 to ∼10^-7 μCi cm^-2 leaf d^-1, and corresponded to an estimated gaseous S emission of approximately 0.1 to 1 μg S cm^-2 leaf d^-1. Translocation to roots was a significant sink for ³⁵S in the red maple tree (40% of the injected amount). Research on forest biogeochemical S cycles should further explore biogenic S emissions from trees as a potential process of S flux from forest ecosystems.