Microbial growth dynamics underlying the respiration pulse when rewetting dry soil
A pulse of carbon dioxide release (respiration) is often observed after rewetting a dry soil. The pulse in respiration is often assumed to be caused by increased microbial activity. Yet, the microbial growth responses that underlie this pulse remain largely unknown.
The following questions will be addressed in this presentation. 1) Do fungal or bacterial growth dynamics explain the pulse in respiration upon rewetting a dry soil? 2) How does bacterial growth respond to different duration of drought and intensities of drying before rewetting?
Soils from Sweden, the U.K. and Greenland were put in microcosms, air-dried for a range of durations between 4 days and one year, or to different moisture contents (partial drying), before rewetting. We measured soil respiration, fungal growth rates and/or bacterial growth rates at high temporal resolution during one week after rewetting.
The respiration pulse upon rewetting a dry soil is not due to high microbial growth rates. During the first hours after rewetting, bacterial and fungal growth rates were low whereas the respiration rates were very high. As such, respiration and microbial growth rates were uncoupled.
Two patterns of bacterial growth were observed upon rewetting. In “pattern 1”, bacterial growth immediately started to increase linearly from low values upon rewetting. In “pattern 2”, bacterial growth started to increase exponentially after a lag period (up to 18 h) of no growth. The “pattern 2” response showed a secondary increase in respiration beyond the high initial respiration rates, coinciding with the start of bacterial growth. Soils with “pattern 1” could be changed to “pattern 2” through exposure to longer periods of drought. Soils with “pattern 2” could be changed into “pattern 1” when subjected to less extensive drying.
Our results suggest that bacterial growth responses do not explain the initial pulse in respiration. Instead, the size of the surviving microbial biomass along with its status may explain the growth responses and the shape of the respiration pulse.