Tannins and nutrient dynamics in forest soils: Plant-litter-soil interactions.

摘要

Tannins affect ecosystem processes such as organic matter degradation, nitrogen cycling, soil formation and successional dynamics. In woody species, foliar tannin levels commonly range from 10 to 25% dry weight. Through litter leaching and deposition forest soils receive appreciable tannin inputs. Tannins in litter and soil may affect nutrient cycling by hindering decomposition, complexing proteins, inhibiting enzyme activities and inducing toxicity to microbial populations. Research presented in this thesis addressed the following questions: (1) What is known about the influence of tannins on nutrient dynamics? (2) How does tannin structure influence tannin reactivity? (3) How do soil conditions influence plant tannin production? and (4) How do tannins with different structures affect soil C and N mineralization? Standard assays used to quantify tannins were found to be affected by tannin structural characteristics. Particularly important was the distinction between condensed and hydrolyzable tannins, the hydroxylation pattern of the condensed tannin B-ring and the stereochemistry at the C2–C3 position. In greenhouse studies using soil and sand culture, fertility and pH effects on foliar and fine root growth and chemical composition were found. While lower nutrient levels were clearly related to higher concentrations of total phenols and tannins, acidity effects were less consistent. Results derived from laboratory C and N mineralization studies in the presence of different purified tannins support a hypothesis that ascribes reduced N availability in the presence of tannins to sequestration of organic N, as well as to immobilization associated with the addition of a labile C source. Differences in reactivity among tannins have important implications both for tannin quantification and their effects on ecological processes. The chemical structure of individual tannins can most likely explain differences in the type and magnitude of effects observed among tannin sources. Because plant species, as well as foliar and root materials, do not respond uniformly to changing soil fertility and pH it is difficult to generalize about how site conditions influence phytochemistry. Studies on tannins should recognize that tannins with varying structures may have different chemical reactivities and thus are expected to have distinct fates and effects on biogeochemical processes and ecosystem function.