See more from this Session: Soil Carbon and Nitrogen: Microbial Transformations and Fluxes
Wednesday, October 19, 2011: 11:15 AM
Henry Gonzalez Convention Center, Room 006A, River Level
Exotic species often alter belowground ecosystem processes creating positive feedbacks that promote invasion. Many of these feedbacks occur as exotics stimulate microbial activity in the rhizosphere that subsequently accelerates the turnover of C substrates and the release of plant-limiting nutrients, such as N. Despite these effects, interactions between root processes and microbial responses remain tenuous at best. In this study, we investigate the potential for the invasive grass, Taeniatherum caput-medusae (Taca), to enhance root exudation, shift microbial community composition, and stimulate N cycling. We tracked root exudates, rhizosphere communities, and N mineralization rates in an invaded shrub-steppe ecosystem (OR, USA). Specifically, these variables were evaluated in soils beneath Taca and two native perennial bunchgrasses, Pseudoroegneria spicata (Pssp) and Poa secunda (Pose), three-times during the growing season. We measured root exudates from intact roots in situ with liquid chromatography-mass spectrometry; communities with a quantitative PCR-based approach to estimate the abundance of major taxonomical groups of bacteria; and N mineralization through 15N isotope pool dilutions. We found that Taca enhanced the exudation of a simple organic acid and C substrate, and stimulated rhizosphere microbial biomass and specific bacteria phyla. The production of citric acid and glucose was seven-times and three-times higher, respectively, in Taca than Pssp and Pose rhizospheres during the growing season. Bacterial biomass was consistently higher in Taca than Pssp and Pose soils. Communities in the exotic’s rhizosphere had more Actinobacteria and Nitrospira and less b-Proteobacteria. These differences coincided with higher mineralization rates in invaded soils. Our results suggest that the exotic Taca may create positive N feedbacks by enhancing root exudation in the rhizosphere and stimulating microbial activity and N mineralization. Actinobacteria may stimulate C and N cycling but this phylum represented no more than 13% of the community. Therefore, feedbacks most likely stem from a general increase in microbial activity and not concomitant changes in microbial community composition.