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Research progress follows three objectives of this grant:

(1) Elucidate processes accounting for differences in rate of turnover of C3, C4-derived soil organic carbon. Building on observations outlined in my PRF research proposal, and in publications in 2007 and 2008, we have now completed studies of biomass decomposition experiments along a 4 km altitudinal transect in the Peruvian Andes. These sites provide a unique opportunity to link with a network of carbon cycle monitoring sites by a large research group based in the US and UK whose main focus to date has been on above-ground processes. Replicate samples of biomass from above and belowground C3 and C4 plants (both local and exotic, herbaceous and woody) were planted under natural field conditions at sites along the transect and retrieved in May 2009 for a direct comparison of decomposition rates, and changes in the isotopic composition of a variety of litters to examine variation with photosynthetic pathway, litter chemistry, particle size, etc. Soil respired-CO2 from transplanted soils was also collected from native soils at each of the sites, and carbon isotopic composition measured. This data set is now complete. The data demonstrate significant trends in decomposition rate and change in isotopic composition with altitude (temperature) and this component of the project is at the data analysis and early publication stage.

(2) Closing the carbon isotope budget for the soil carbon cycle. This objective is addressed by a monitoring study of the carbon budget along a classical soil moisture gradient from xeric pine-shrub uplands to cypress swamps in Florida wetlands. With two graduate students assistants, and a semester-long course project for a “Critical Zone Biogeochemistry” class (with 14 students over 2 semesters), I established five monitoring sites in a relatively undisturbed wetland in the USF Eco Area. We have installed soil moisture meters, tensiometers, and soil water sampling ports (with an automated vacuum system) to collect soil water and dissolved organic carbon from five levels in each profile. We have also installed two soil respiration flux monitoring systems, and time series of data are continuously logged and downloaded on a bi-monthly basis. These data are complemented by a weather station measuring temperature, rainfall, solar radiation, relative humidity. Although monitoring of these data is ongoing, I have used the preliminary data from this work to seed an NSF proposal (in review) for a more detailed partitioning of the components of carbon production and storage. With two colleagues in hydrology and ecohydrology as collaborators on this project, we plan to install litter traps and rhizotrons to measure carbon input (NPP), in addition to more thorough monitoring of the isotopic composition of respired CO2 so as to close the carbon isotope budget. At present, we have established a significant relationship between soil moisture and carbon inventory along this gradient, and I am currently applying the Neff soil carbon model with the data in hand. Graduate research assistant Katherine Powell has completed a MS project which establishes a relationship between changes in water level in a group of representative wetlands and their capacity to store soil carbon over longer time scales. This work complements the more detailed monitoring study from the Eco Area site, in order to extend relationships to historical water level data, and provide a context for water management decisions. Katherine's MS study is complete, and a publication is in preparation for submission to Wetlands.

(3) Elucidate environmental controls on decomposition rates of organic substrates. With graduate research work by Jennifer Chelladurai, we have set up several laboratory projects that address the role of litter quality, interaction with minerals, and soil depth on decomposition of two end-member litters (pure C3 and C4 grass). Jennifer's set of experiments built a series of soil columns with gas sampling ports from which she monitored the profiles of 13CO2 in synthetic profiles containing each type of litter. These results validate a model of production-diffusion-equilibration of soil CO2 with soil water (Cerling, 1984) using the two-point mixing model analyzed through “Keeling Plots.” However, CO2 in the subsurface horizons where the experiment was designed to accumulate soil carbonate “Bk horizons,” show a significant deviation from the predicted Keeling Plot relationship, and thus an additional sink. The data are consistent with 13C fractionation during carbonate precipitation, leaving the remaining reservoir of soil CO2 13C-depleted. Jennifer presented an her Keeling Plot analysis of this data at the American Geophysical Union meeting in December 2008, and a modeling study and validation with Jennifer's Keeling plot data are in review with Earth and Planetary Science Letters. A second component of this project has incubated these litter samples in contact with pure quartz sand, and sand mixed with montmorillonitic clay. After over 1 year of data collection, carbon isotopic composition of respired CO2 reflects substrate, but shows a trend towards increasing 13C. Samples of the remaining pool of soil carbon have been harvested after 1 year (and longer periods) to examine the role of protection of 13C-enriched compounds by clays. These data are still being collected, as this component of the project requires long-term incubation experiments.