Biodegradable dissolved organic carbon in forest soil solution and effects of chronic nitrogen deposition

Using a flow-through bioreactor, biodegradable dissolved organic carbon (BDOC) in forest floor solution was determined for a red pine (Pinus resinosa) plantation and a naturally regenerated mixed hardwood forest (dominant species: Quercus velutina, Q. rubra, Acer rubrum, Fagus grandifolia, Prunus serotina). The forests have received chronic-N fertilization at three different rates (0, 50, 150 kg N ha(-1) year(-1)) since 1988. Both BDOC concentration and %BDOC [% of dissolved organic carbon (DOC)] were significantly higher in the summer (18-20 mg C l(-1), approximate to 30%) than spring or fall for both forest types. The BDOC depletion that was hypothesized to occur with chronic-N application and N saturation was not found in either stand. Instead, for the hardwood stand, BDOC concentration increased with N application, probably due to increased BDOC production in the forest floor with increases in available N. The strong negative relationship between litterfall mass and both BDOC concentration (r(2) = 0.36) and %BDOC (r(2) = 0.60) for the hardwood stand suggests that the primary source of BDOC in the hardwood forest floor is recent photosynthate allocated to fine roots for their growth, and organic compounds released from them (e.g., exudates and mucilage) rather than the leachate of freshly fallen autumn litter. A significant but weaker positive relationship between BDOC and ambient temperature for both stands suggests that the decomposition of forest floor organic matter is an additional source of BDOC in the forest floor. The production of non-biodegradable DOC (NBDOC) in the forest floor was not sensitive to ambient temperature or the chronic-N treatment. This suggests that an abiotic process, such as chemical equilibration between forest floor and forest floor solution, is responsible for the concentration of non-labile DOG. High %BDOC in throughfall (50-75%) for both stands suggests that throughfall is possibly an additional source of BDOC in the forest floor. (C) 2000 Elsevier Science Ltd. All rights reserved.