Intense droughts lead to higher rates of CO2 from soil: Microbial growth responses to rewetting in a grass-soil system

Sammanfattning

Based on current climate data both drought intensity and frequency is expected to increase. Soil microbes react in a variety of ways to drought-imposed stresses, with only a fraction surviving until wetter conditions return. As drought ends and rain returns, these microbial communities respond with an associated pulse in soil CO2-emissions called the Birch effect. Understanding soil microbial responses to drought and rewetting (DRW) events is critical for understanding the terrestrial carbon balance. As DRW events become more common in historically wetter soils, soils not adapted to drought stress may exhibit disproportionately high carbon losses. This study investigates how drought intensity affects soil respiration, fungal- and bacterial growth, and plant condition following rewetting, using controlled drought treatments on grass-planted soils. Two drought regimes, intermediate (ID) and severe (SD) were applied for 12 days, followed by rewetting. Soil respiration and microbial growth response was monitored over the course of 19 days and compared with continually moist control. Both drought treatments exhibited typical responses to rewetting associated with low carbon use efficiency (low portion of carbon uptake allocated to growth), characterized by a decoupling of soil respiration and growth. SD induced the largest cumulative carbon efflux as well as the highest rates of bacterial growth, while ID exhibited higher rates of growth immediately after rewetting. Plant health and root integrity is suggested to play a critical role in modulating microbial dynamics and their available resources. The findings indicate that drought intensity increases soil organic matter availability through fine root death and soil structure disruption, while at the same inhibiting microbial carbon use efficiency. Through this study, microscopic processes with potential of macroscopic implications are highlighted, contributing with an important puzzle-piece to understanding soil-atmosphere dynamics and carbon cycling.