Wednesday, November 4, 2009: 12:55 PM
Convention Center, Room 305, Third Floor
Soils experience immense fluctuations in environmental variability that strongly regulates belowground processes. For example, when rainfall wets a previously dry soil, microbial respiration dramatically increases and leads to large pulses of CO2. These pulses often last for days and when scaled over longer periods may account for a significant portion of an ecosystem’s CO2 flux. However, it remains unclear what portion of the soil microbial community is responding to soil moisture variability. The goal of this study was to identify the microorganisms responsible for CO2 pulses following rainfall, and to determine what fraction of the microbial community “wakes up” in response to soil rewetting. To this end, we simulated rainfall events in four ecosystem soils (i.e., agricultural field, grassland, deciduous forest, and coniferous forest) with “heavy” water (H218O), and over 72 hours tracked CO2 production and changes in microbial metabolic activity using 18O DNA stable-isotope probing. Within 72 hours following rewetting, soil CO2 concentrations in all soils had elevated at least 1300%, and metabolically active bacteria had incorporated 18O and produced isotopically labeled DNA. This labeled DNA was isolated from unlabeled DNA using density gradient centrifugation and then amplified using quantitative PCR. We characterized the identity and composition of metabolically active bacteria using data generated via pyrosequencing of 16S rRNA genes. Our research directly links soil moisture variability to the bacteria responsible for pulses of CO2, and identifies specific trends in microbial life history traits that may predispose bacteria to rapidly respond to rainfall events.