Litter Decay and Leaching from the Forest Floor in Pinus Contorta (Lodgepole Pine) Ecosystems

Abstract

SUMMARY (1) The dynamics of organic matter and nutrient elements in the forest floor of Pinus contorta spp. latifolia (lodgepole pine) forests in Wyoming, U.S.A., were investigated by combining three related measurements: leaching of the forest floor; long-term decay of leaf litter; and steady-state residence times in the forest floor. (2) Elemental fluxes (g m-2 yr-1) resulting from leaching by spring snowmelt were: Ca = 135, Mg = 0.30, K = 0*65, N = 0-46, P = 0.022 and C = 8.3. Over 95% of the N flux was in organic forms. (3) Most of the organic compounds were acidic, and anions of these organic acids were the principal mobile species accompanying cation flux; smaller, but important, contributions were S04 and HCO3. The chemical nature of the dissolved organic compounds changed as snowmelt proceeded, with polyphenolics and carbohydrates (leached from fresh litter) being important in the early stages, and polymeric acid compounds (fulvic and humic acids) dominating in the later stages. (4) About 35% of the original mass of leaf litter remained after 84-96 months. Potassium and magnesium were lost rapidly from leaf litter, whereas there was temporary immobilization of Ca. Nitrogen accumulated in decaying leaves for 72 months, reaching 180% of the original content before net mineralization and release occurred. Relatively small changes in P content of leaf litter were noted during the decay process. The loss of soluble organic constituents (polyphenols, carbohydrates, lipids) was more rapid than for solid residues (holocellulose, lignin). (5) The combination of approaches enabled several features of organic matter and nutrient dynamics in the forest floor, which would not be apparent using any of the methods in isolation, to be deduced: (i) about 30% of annual C release from the forest floor resulted from leaching of organic compounds; (ii) over 20% of the leaching flux for Ca and Mg was attributed to release from 01 litter, compared with less than 10% for K; (iii) high amounts of organic-N leaching from forest-floor layers, combined with rapid accumulation in 0- to 8-year-old litter, indicated rapid translocation of N from subsurface layers to 01 litter via heterotrophs; and (iv) leaching of polyphenols from the forest floor was 50% lower than the apparent loss from 01 litter, suggesting chemical transformation and changes in solubility of these compounds during litter decay. An association of this phenomenon with protein accumulation observed in the 02 horizon is postulated.