Radiocarbon Measurements of Root and Soil Respired CO2 in an Old-Growth Coniferous Forest for Estimates of Residence Time and Flux Partitioning

Includes bibliographical references (pages 28-31).Climate change has the potential to impact the carbon (C) cycle in unknown ways. Factors such as temperature, light, moisture, and nutrients can strongly influence whether forest ecosystems are net sources or sinks of CO???. In this study, I used radiocarbon (14C) to determine the residence time and source of soil- and root-respired CO??? using a combination of soil chamber and biomass incubation measurements in an old-growth forest at the Wind River Field Station, WA. I have two main goals for this study. First is to determine the role of recent photosynthates and stored C in fueling root respiration, and how their relative importance change between spring and summer seasons. My results show that the mean C residence time is 2-3 y old and 7-10 y old released by fine root respiration and root-free soil respiration, respectively. I found that ??14C values of CO??? produced from combusted bulk roots in the laboratory were 70 ??? higher than root-respired CO??? collected from in-situ incubation. I observed a small but significant seasonal increase in ??14CO??? of soil respiration but found no seasonal ??14CO??? difference in either root or microbial respiration. The second goal is to partition the contribution of autotrophic (RA) to heterotrophic (RH) respiration and determine how this ratio differs between the beginning and the end of a growing season. Measured ??14C values of chamber-based soil respiration fall between end member values determined by field incubations of fine roots and root-free soil organic matter. Using a mixing model, my calculations showed that 55 ?? 22% of soil respiration originated from autotrophic sources in April but this contribution dropped to 38 ?? 21% in August of 2013. The relatively high RA contribution in April is supposed to be resulting from the combined effect of increasing root respiration at a time of high productivity and/or relatively low microbial activities in the spring. Future studies making more frequent measurements throughout the year may provide additional information into C allocation patterns by these old-growth coniferous trees