| 英文摘要: | One of the largest biological reservoirs of carbon is in the wood of forest trees. Once trees die, wood-containing carbon is consumed by organisms such as microbes and insects. Such carbon can be emitted into the atmosphere as carbon dioxide or methane gas. Additionally, only some microbes can break down lignin, a critical building block of wood. When they are unable to do so, remaining lignin gets locked in soil. The tropics have vast stores of carbon in trees but little is known about where that carbon ends up. Both microbes and termites consume tropical wood, but which one does the job alters how fast and in what form carbon is released. Typically, microbes need lots of moisture to thrive on wood, but termites may require less since they can hide in their nest mounds and store water if it becomes too dry. Scientists have been trying to model global carbon cycling under environmental change, but they have yet to include the decay of wood by microbes and insects in their studies. The researchers on this study predict that these organisms have an enormous impact on global carbon cycling. The experimental field work will cut across ecosystems with varying rainfall in Queensland, Australia, from wet rainforest to dry savanna. At these different sites, the researchers will compare how fast termites versus microbes consume the wood and what gases are released during wet and dry seasons. The results will be incorporated into computer models to determine how termites and microbes affect carbon cycling at regional to global scales. Such information will be fed back to the scientific community to make better predictions about the global carbon cycle. This project is an international collaboration between scientists in the U.S., the U.K. and Australia, and is supported, in part, by the NSF Office of International Science and Engineering. Additionally, workshops will bring together project scientists with forestry stakeholders to develop potential strategies for preventing the decay of carbon in wood, and reducing natural carbon emissions. The project will also support education and training of junior scientists.
Three questions will be addressed in this project: Q1. What controls rates of coarse wood debris carbon turnover? Turnover rates will be determined by measuring termite and microbe activity, which are dependent on climate and wood construction. Wetter conditions should increase turnover, but termites should increase turnover relatively more in dry conditions due to their water conservation strategies. Dense, highly lignified wood should decay more slowly. Q2. What controls fates of carbon liberated from coarse wood debris? Carbon fates (carbon dioxide, methane, soil organic residues) will depend on the stage of wood decay and the composition of termite and microbial communities. Later stages of decay, increased abundance or activity of methanogenic archaea, (or decreased bacterial methanotropy) and changes in termite species should result in greater methane/carbon dioxide ratios. Greater organic residue formation is predicted to occur when carbon loss is via termites. Q3. How do mechanisms of wood turnover scale up to affect ecosystem-level carbon fluxes under environmental change? To date, coarse wood debris is poorly parameterized in Earth system models (e.g., coarse wood debris is assumed to decay only via physical fragmentation). Field data from Q1 and Q2 will be used to drive new predictive models of wood turnover and gas emission under environmental change. To test Q1 and determine relative wood-associated microbe and termite decay rates in response to precipitation variation, blocks of pine known to attract termites will be placed at six sites. Blocks will be enclosed in fine-mesh with or without holes to limit termite access. Blocks will be harvested at the end of wet and dry seasons for four years. To determine the influence of wood construction on decay, replicate logs of ten species/site will be placed at rainforest and savannah sites. Logs will be harvested at the end of wet and dry seasons. For logs/blocks, initial and final mass, density and chemistry will be measured. To test Q2, wood subsamples and termites in wood will be collected to determine termite community, microbial community and decay gene composition, and organic residue formation. Carbon dioxide and methane will be measured for harvested logs/blocks. As some termites live in mound nests, experimental mounds will be established and carbon dioxide and methane fluxes will be measured in mounds in a one-year experiment. To test Q3, field data will be incorporated into woodCLM, an ecosystem model derived from the Community Land Model. Results from the modified and original models will be compared. Using woodCLM, wood dynamics and gas emissions will be simulated under future environmental scenarios at the sites. The project will include training at the graduate student and postdoctoral levels, and is part of an international collaborative effort with scientists in England funded by the United Kingdom's Natural Environment Research Council (NERC). |