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Understanding the Mechanisms of Decay

其他書名	Interactive Effects of Litter Chemistry, the Microbial Community, and Nitrogen Availability
出版	University of Toledo, 2014
URL	http://books.google.com.hk/books?id=RY2JAQAACAAJ&hl=&source=gbs_api

註釋Fundamental questions remain about plant litter decomposition, which is a key control on carbon (C) turnover in terrestrial ecosystems. While it is known that decomposition involves both chemical changes in leaf litter and a succession of microorganisms that consume the various chemical constituents, the underlying biochemical mechanisms are not well understood. Hence, it is difficult to predict the magnitude and often the direction of microbial responses to environmental disturbance. This dissertation examines the changes in microbial community function and composition that occur in response to changes in litter chemistry during decomposition, as well as to environmental factors, in order to quantify the mechanisms regulating plant litter C turnover. In a laboratory incubation of sugar maple litter, decomposers preferentially used soluble substrates, and shifts in functional groups of microorganisms with different enzymatic capabilities and growth rates occurred throughout decay. This provides experimental evidence that microbial uptake of C substrates during decomposition occurs in sequence, with different decomposer groups targeting different substrates. In a complementary incubation of sugar maple and white oak litter, peaks in microbial respiration and biomass, low enzyme activities, and nutrient immobilization predominated during the first few days of decomposition across contrasting litter and soil types. This implies that rapid assimilation of soluble substrates by decomposers occurred during early decay. In contrast to these consistent features of early decomposition observed in microcosms, the same leaf litter (i.e., dogwood, sugar maple, and white oak) exhibited strikingly different decay patterns when decomposed under field and lab conditions during long-term decomposition. Nitrogen (N) accumulated in the lab microcosms, but not the field litter bags, and suppressed microbial biomass and activity in mid- and late decay. Nitrogen fertilization also influenced microbial dynamics, but not lignin monomer concentrations, in a separate incubation of three maize genotypes varying in lignin content. Exogenous N decreased oxidative enzyme activities across all maize treatments, which suggests that decomposers degrade lignin to obtain shielded N compounds and decrease production of lignin-degrading enzymes when labile N availability increases. The findings in this dissertation provide evidence that there is a predictable microbial succession tied to litter chemistry during decay, and describe how and why environmental factors alter microbial community dynamics and decomposition rates.