The key scientific significance of this work is the integration of microbial community and chemical interactions during the long-term degradation and stabilization of plant litter because there has not yet been a working synthesis of this knowledge expressed in a comprehensive quantitative framework. Plant litter decomposition is the main control on soil fertility and carbon (C) sequestration. There is growing interest in understanding the conditions under which soils gain or lose C, in part, because soils contain more C than the atmosphere and could play either a moderating or exacerbating role in global warming. An integrated field, laboratory, and modeling study of the biochemical mechanisms driving interactions between soil C sequestration, plant litter chemistry, and microbial community composition and activity during decomposition is needed to define these relationships. Moorhead and Sinsabaugh (2006) developed a new model that incorporates microbial dynamics into the decomposition process, the Guild Decomposition Model (GDM), which has generated a series of specific hypotheses about the mechanisms controlling the interactions between plant litter chemistry, C stabilization and microbial community composition and activity during decomposition. The proposed research will evaluate these hypotheses experimentally by monitoring changes in plant litter chemistry and microbial community composition and function during decomposition, with and without added N to mimic N deposition. Stable C isotope (13C) labeling will further determine the community composition of active microorganisms capable of metabolizing specific chemical components of plant litter. This research will provide mechanistic insight into the impacts on plant litter decomposition and soil C sequestration from N deposition, elevated CO2, plant community composition shifts, and climate change.
An integrated education and outreach program will complement this research, assisted by the University of Toledo's Center for Creative Instruction (CCI). For this project, the CCI will develop an online interactive Model Of Leaf Decomposition (iMold) to provide educational outreach about decomposition for grades 5-12. Visitors will be able to visualize the progress of the decomposition of litter as a whole, or different individual litter chemical constituents, along a time line to see how time, litter type, and environment relate to how quickly or slowly something decays. Teachers and students will be able to network between researchers and classrooms at other schools, enabling them to share data, ask questions, and collaborate on experiments. A communication center will be created so that the researchers and classrooms can share ideas and results, as well as ask questions in a blog-like environment. Additionally, this project will provide training for several undergraduate and graduate students. University of Toledo and Kent State University serve significant populations of underrepresented groups in science, and they will be recruited to this project.
At the broadest scale, there are two processes that comprise the heart of ecosystems: plants photosynthesize, removing carbon dioxide (CO2) from the atmosphere; and soil microorganisms (bacteria and fungi) live by metabolizing dead plant material, returning CO2 to the atmosphere. We understand the plant part of this cycle well; the microbial decomposition part remains challenging. We understand the general patterns—why decomposition slows down over time, how the chemical composition of "litter" changes over time, etc. However, we do not understand how these chemical changes are associated with the organisms that make them happen--a single dead leaf may have thousands of species of bacteria and fungi living on it, they may be active at different times in the decomposition process, etc. The goal of this project was to connect the microorganisms that "eat" litter and recycle the chemicals to the patterns of decomposition and the chemical changes that occur. We used modern DNA-based methods to assess not only which organisms are present but which organisms are actually growing during each stage of decomposition. We established an experiment where we decomposed maple and oak litters under carefully controlled conditions. We measured the rate of decomposition, the chemical changes, and the activities of the critical enzymes that break up the plant tissues. To assess the total community of organisms present, we extracted DNA from the entire aggregated microbial community and sequenced it to identify all the groups of bacteria and fungi present. To assess which of those organisms were growing during each decomposition stage (and were therefore presumably the active decomposers at that time) we added a compound (Bromodeoyuridine, BrDU) that growing cells take up and assimilate into their newly formed DNA as a substitute for one of the normal DNA bases, thymidine. The BrDU-labeled DNA can be separated from the unlabeled DNA to tell which organisms were growing at that specific time. By combining the data on the organisms growing with the changes in the litter chemistry we are finally putting together the "actors" in decomposition with their actions, and so to establish the roles each plays in converting a freshly fallen leaf back into CO2 and the mineral nutrients that will be used by plants to maintain the cycle.
