46664-G2
Stable Isotopic Constraints on Carbon Exchange During Heterotrophic Decomposition of Soil Organic Matter
Progress in year 1 follows three objectives of this grant: (1) Elucidate processes accounting for differences in rate of turnover of C3, C4-derived soil organic carbon. This objective was essentially addressed during work just prior to starting PRF funding, and has been published in two papers now published in the interim: 2007 in Global Change Biology, and 2008, Tellus B, giving me the opportunity to move on to additional research questions. Building on these observations, during May 2008, I set up biomass decomposition experiments along a 4 km altitudinal transect in the Peruvian Andes. Replicate samples of biomass from above and belowground C3 and C4 plants (both local and exotic, herbaceous and woody) were planted under native conditions at sites along the transect. These samples will be 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. I also collected soil respired-CO2 samples from each of the sites, and have measured the carbon isotopic composition of these gases. 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, and will contribute to an understanding of belowground carbon cycle processes, where the major focus of the group is currently on above-ground processes. (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 (Jennifer Chelladurai and Katherine Powell), I established five monitoring sites in a relatively undisturbed wetland, where 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. Monitoring of these data is ongoing, and will continue for 1 complete annual cycle. We have collected and analyzed soil organic carbon from 25 replicates of two surface pools (0-5 and 0-30 cm), and have collected 1m-deep cores from each of the monitoring sites. These data, with 1 complete year of monitoring data will be used in year 2 in a modeling study. (3) Elucidate environmental controls on decomposition rates of organic substrates. I 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). One set of experiments monitors the isotopic composition of CO2 flux from replicate samples of each litter in contact with a pure quartz sand vs. the same sand with an additional 10% montmorillonite clay. With current data, there is relatively little trend, and d13C of respired CO2 reflects substrate. Samples will be harvested after 1 year (and longer periods) to examine the role of protection of 13C-enriched compounds by clays. In a second set of experiments, Jennifer Chelladurai has built a series of soil columns with gas sampling ports from which we can monitor 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), with some interesting deviations of the two-point mixing model evident in Keeling Plots. Jennifer will present an early version of this work at the American Geophysical Union meeting in December 2008.
