Impact of soil salinity on microbial growth responses to drying and rewetting
Saline soils are widespread in arid and semi-arid areas of the world, where droughts are a common occurrence. During drying of soils, ions dissolved in the soil solution become more concentrated. The combination of low matric potential due to drying and low osmotic potential caused by high solute concentrations could interact and exacerbate the negative effect of the factors salinity and drying on the soil microbial community. Most studies on the response to drying and rewetting of soil have been performed in non-saline environments. However, little is known about the combined effect of salinity and drying on soil microorganisms and the response of dried saline soils to rewetting events. We will present experiments consisting of two parts. To study the interaction of salinity and drying, in the first part of the experiment a soil was adjusted to a range of salinity levels and then dried. During drying we continuously measured bacterial growth rates. This was done to determine the direct moisture dependence of bacterial growth and to test if salinity modified it. In the second part of the experiment we rewet dry soil of different salinities to monitor the dynamics in respiration and bacterial growth following rewetting.
Salt exposure exerts a strong inhibitory effect on bacterial growth rate. Starting from about 50% water holding capacity bacterial growth rates decrease in a sigmoidal fashion with decreasing soil moisture to zero activity in air-dried conditions. We expect that in a saline soil bacterial growth rates are decreasing more quickly with drying, due to the combination of low matric and osmotic potentials. In the second part of the experiment, the rewetting from dry conditions was studied. Without salt, rewetting air-dried soil back to optimal moisture resulted in an immediate recovery of bacterial growth, and a gradual increase of rates back to the level observed in a continuously moist soil within a few hours. Prior experiments have shown that treatments that compromise the status or size of the microbial community, such as long duration of drought, long-term storage prior to rewetting, and exposure to toxins (e.g. heavy metals) can result in a lag-period of zero growth before rates start to increase in response to favourable conditions (re-moistening, or substrate additions). Salinity is predicted to result in a similar result. Thereby we will attempt to close the knowledge gap on how the rewetting response of soils after drying is modified by salinity.