The Role of Carbon-Based Plant Secondary Metabolites in Decomposition in Terrestrial Ecosystems

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Abstract
Litter decomposition in terrestrial habitats is affected by many factors, including temperature, moisture, and nutrient and organic composition of litter. Among organic components, lignin is the primary controlling factor of decomposition rates of surface litter during the later phase of decomposition in most habitats and during the initial phase in warm, moist habitats (i.e., those with a high actual evapotranspiration, AET). In habitats with moderate AET''s, we suggest that the decreased control by lignin over annual decomposition rates of surface litter is due, at least in part, to a significant periodic or seasonal influence of other carbon-based plant secondary metabolites over rates in the initial phase of decomposition. The influence of other secondary metabolites over decomposition rates should be a function of other correlates of AET: the phytochemical composition of the community and the persistence of various secondary metabolites in litter. As AET decreses from the highest extreme, we expect more, but perhaps short-term seasonal, influence of monomeric phenolics and tannins. In warm, dry environments with still-loewr AET''s, we expect some control by terpenes. On an annual basis, the relative influence of components other than lignin should therefore increase with decreasing AET, and the influence of lignin content alone on decomposition rates of surface litter should be reduced proportionately. Effects of these various classes of compounds should exhibit different temporal patterns in litter and soil. Complexity is added to this model by the multifarious effects of stress on the production of specific secondary metabolites, the changes that occur in secondary-metabolite composition during senescence of tissues, and the differential effects of specific secondary metabolites on decomposition.