Linking Soil Moisture Variability, Metabolically Active Bacteria, and CO2 Pulses through 18o DNA Stable-Isotope Probing.

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.