Abstract
Fluctuations in soil moisture create drying-rewetting events affecting the activity of microorganisms. Microbial responses to drying-rewetting are mostly studied in soils that are air-dried before rewetting. Upon rewetting, two patterns of bacterial growth have been observed. In the Type 1 pattern, bacterial growth rates increase immediately in a linear fashion. In the Type 2 pattern, bacterial growth rates increase exponentially after a lag period. However, soils are often only partially dried. Partial drying (higher remaining moisture content before rewetting) may be considered a less harsh treatment compared with air-drying. We hypothesized that a soil with a Type 2 response upon rewetting air-dried soil would transform into a Type 1 response if dried partially before rewetting. Two soils were dried to a gradient of different moisture content. Respiration and bacterial growth rates were then measured before and during 48 h after rewetting to 50% of water holding capacity (WHC). Initial moisture content determined growth and respiration in a sigmoidal fashion, with lowest activity in air-dried soil and maximum above ca. 30% WHC. Partial drying resulted in shorter lag periods, shorter recovery times and lower maximum bacterial growth rates after rewetting. The respiration after rewetting was lower when soil was partially dried and higher when soils were air-dried. The threshold moisture content where transition from a Type 2 to a Type 1 response occurred was about 14% WHC, while >30% WHC resulted in no rewetting effect. We combine our result with other recent reports to propose a framework of response patterns after drying-rewetting, where the harshness of drying determines the response pattern of bacteria upon rewetting dried soils.
Highlights
Moisture is an important determinant of microbial activity in soil (Manzoni et al, 2012a)
It is generally assumed that the size of the respiration pulse will correlate with the amount of microorganisms killed by the drying-rewetting event (Kieft et al, 1987; Blazewicz et al, 2014; Fraser et al, 2016), mobilization of carbon (C) released from soil organic matter (Xiang et al, 2008; Schimel et al, 2011) or the accumulation of osmolytes in microbial biomass (Warren, 2014; but see Boot et al, 2013) will contribute to the respiration pulse
Respiration and bacterial growth before rewetting. Both bacterial growth and respiration rates increased with soil moisture content according to a logistic model (Fig. 2)
Summary
Moisture is an important determinant of microbial activity in soil (Manzoni et al, 2012a). Fluctuations in moisture conditions create drying and rewetting events, which affect microbial growth rates and soil respiration rates (Kieft et al, 1987; Blazewicz et al, 2014), and it is well known that a pulse of carbon dioxide (CO2) often is observed after rewetting a dry soil (Jarvis et al, 2007; Sponseller, 2007; Kim et al, 2012). The increase in respiration rate induced by rewetting has been shown to be less evident when soil is partially dried before rewetting (Kim et al, 2010; Yan and Marschner, 2014) and is only detectable when soil is dried to a moisture content below a threshold level (Fischer, 2009). It is generally assumed that the size of the respiration pulse will correlate with the amount of microorganisms killed by the drying-rewetting event (Kieft et al, 1987; Blazewicz et al, 2014; Fraser et al, 2016), mobilization of carbon (C) released from soil organic matter (Xiang et al, 2008; Schimel et al, 2011) or the accumulation of osmolytes in microbial biomass (Warren, 2014; but see Boot et al, 2013) will contribute to the respiration pulse
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