Effects of precipitation regime on soil bacterial and fungal activity upon rewetting of a plant-soil system using 18O-SIP: depth matters

EASPEBIOmEDOCT INRAClimate change is predicted to affect not only the amount but also the temporal distribution of rain. Changes in frequency and amplitude of rain events, likely shape the activity of plants and soil microbes. Fluctuating water conditions will differ with soil depth between precipitation patterns, affecting plant growth and may result in differential microbial response upon rewetting. Our objective was to investigate, in plant-soil systems, the response of the metabolically active microbial communities to a rewetting event and to which extent this was modulated by 1) soil depth and 2) precipitation legacy. Wheat planted in soil mesocosms were subjected to frequent or infrequent watering for 12 weeks (equal total water input). 18O-labelled water was applied at final rewetting to 3 different soil depths. Extracted DNA was separated by density using 18O stable isotope probing (SIP) to differentiate the active and inactive microbial fractions. Bacterial 16S and fungal 18S rRNA from both fractions were amplified and sequenced using the Illumina MiSeq platform. Frequent watering stimulated above- and belowground plant biomass, and shifted root distribution towards the top soil layers. Soil depth, more than precipitation pattern, was the most influential in shaping the response of actively growing bacterial and fungal communities to rewetting. Despite a microbial diversity gradient with depth, activity upon rewetting involved consistent microbial groups