报告题目一：The microbial ecology and biogeochemistry of soil desiccation and re-wetting
Professor, Associate Dean
Department of Environmental Science Policy and Management
University of California Berkeley
报告题目二：Plant-microbial interaction regulates soil C cycling
报告一：The microbial ecology and biogeochemistry of soil desiccation and re-wetting (Mary K. Firestone, University of California, Berkeley California, USA)
Abstract：The microbial mediation of biogeochemical cycles is first and foremost dependent on the amount and character of water in soil. Microbes indigenous to arid and semi-arid soils routinely experience extreme desiccation and infrequent rainfall events . Short duration carbon dioxide pulses resulting from rainfall events can comprise a major component of the C-cycle in these systems. Microbial adaptation to extreme drying and episodic rainfall events shape the composition and characteristics of indigenous microbial communities. We characterized microbial communities indigenous to three California grassland soils that experience a Mediterranean-type climate with a 4 to 5 month period without rainfall. Bacterial communities (by 16S DNA) did not change appreciably in response to dry down and wet up. However based on 16S rRNA relative abundance, the potential activities of the communities did differ between the dry periods and the moist periods. Based on 28S DNA and rRNA, no response of soil fungal communities could be detected. Different phyla of bacteria exhibited distinctly different response strategies to soil dry down and wet up. Following rRNA over fine time increments after wet up demonstrated three response strategies apparent, with some bacteria resuscitating and initiating activity within 15 minutes of wet up. Stable isotope probing with 18O – H2O allowed us to follow death and growth resulting from dry down and wet up. Significant growth was generally detectable only after 24 hours with patterns of growth also exhibiting phylogenetic coherence. Microbes indigenous to semi-arid systems appear to be highly adapted to tolerate extreme desiccation and rapid wet ups.
报告二：Plant-microbial interaction regulates soil C cycling (Shengjing Shi, University of California, Berkeley California, USA)
Abstract: Plants play an important role in transferring atmosphere CO2 into belowground soil C pools, while soil microbes are primary mediators of C transformation and mineralization. The molecular mechanisms underlying plant-microbial interactions are poorly understood as are the possible modulations by changing climate. We first examined the effects of live Avena fatua roots (a common annual grass) on decomposition of 13C-labeled root litter in a California grassland soil over two simulated growing-seasons. The presence of live roots consistently suppressed rates of litter decomposition; however this effect disappeared with plant senescence. Presence of live roots significantly altered the abundance, composition and functional potential of microbial communities (assessed by qPCR, MiSeq 16S and ITS sequencing, and GeoChip 4, respectively). Two possible mechanisms (preferential substrate utilization and drying stress) were identified for explaining the observed negative priming on soil organic C decomposition by live roots. In another study, we investigated the influence of elevated CO2 (eCO2) on microbial communities associated with Avena fatua roots across different plant growth stages over 12 weeks by Illumina 16S sequencing. Significantly more C was allocated to belowground by plants grown under eCO2 than ambient CO2; however, eCO2 had little effect on microbial communities in rhizosphere or bulk soil. Plants demonstrated an important role in shaping rhizosphere microbial communities and driving their succession. Network analyses revealed greater complexity of microbial interactions in rhizosphere microbial communities compared to those in bulk soil. These results demonstrate the important role of plant roots in the assembly of the rhizosphere microbiome.