Growth and physiological responses of grassland soil bacteria (Bacillus and Pseudomonas) to drought treatment in experimental microcosms

Bacterial isolates from BCCIL (twelve from each of genera Bacillus and Pseudomonas) were raised in liquid media and used to inoculate sterilised soil in kilner jars (approx 100 ml soil in 500 ml jars). Each isolate of each bacterial taxon was inoculated into five replicate microcosms per moisture treatment (drought and control) and microcosms were assigned random positions within four experimental blocks. Microcosms in the control treatment were topped up weekly to a volumetric water content of 40%. Those in the drought treatment were subjected to a series of four simulated droughts lasting six weeks, where watering was not carried out. Soil moisture content determination and watering was carried out by assessing the mass of each kilner jar and the dried soil it initially contained, allowing controlled watering to the target soil water content. Substrate utilisation profiling was carried out by taking soil samples from the microcosms at the end of the final drought cycle, which were amended with different substrates (water, glucose, cellulose, artificial root exudate)in MicroResp(TM) colourimetric microplates, before CO2 efflux detection on a FLUOstar Omega spectrophotometer (BMG Labtech, Ortenberg, Germany). q-PCR was carried out by taking soil samples from the microcosms in the experimental evolution. RNA was extracted from the soil samples and amplified using qPCR primers specific to the isolates which had been used to inoculate the microcosms.