Effects of elevated atmospheric carbon dioxide on scrub-oak root carbon pools and soil microbial processes

The levels of atmospheric CO2 are rising and this affects the growth of plants and the ecosystems in which they reside. Plants take up additional C from the atmosphere and have potential to sequester C in the soil. I investigated the sequestration of C belowground and the microbial processes that control C retention in the soil. This study was conducted in a Florida scrub-oak ecosystem, where CO2 levels have been elevated to twice ambient since 1996 in open top chambers. There were eight replicates of ambient CO2 chambers and eight replicates of twice-ambient CO 2 levels. The chambers were blocked according to the vegetation present at the beginning of the study and the site was burned prior to construction of the chambers. Soil cores were taken to investigate the effects of elevated CO2 on soil biomass pools, microbial response and nutrient limitations. Elevated CO2 did not affect total biomass of roots as of May 2002. There was less biomass of the smallest roots (<0.25 mm) in elevated CO 2 in the top 10 cm. The C and N contents of root and organic matter pools reflected the trends in biomass. N concentration was lower for <0.25 mm and 1-2 mm roots in the upper portion of the soil. Dissolved organic C and soil pH were unaffected in elevated CO2. An oxygen biosensor system was used to examine microbial function in the scrub-oak soils. Microbial response was affected by CO2 treatments. The soil microbial communities had greater N limitation in elevated CO2 than ambient CO2 , while the litter community was unaffected. The rhizosphere community had greater P limitation in elevated CO2 than ambient CO2 . Substrates for the microbes derived from roots and litter grown in elevated CO2 seemed to have more energy available to microbes, but this was dependent upon N conditions. Overall, there was greater nutrient limitation of microbial activity in elevated CO2 than ambient CO 2, but the scrub-oak ecosystem was nutrient limited regardless of CO 2 conditions preventing full use of the potential C available for energy.