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

EA BIOME DOCT INRAInternational audienceClimate change is predicted to a ect not only the amount but also the temporal distribu- tion of rain. Changes in frequency and amplitude of rain events, i.e. precipitation patterns, likely shape the activity of plants and soil microbes. Fluctuating water conditions will dier with soil depth between precipitation patterns, a ecting plant growth and may result in dierential 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 1) by soil depth and 2) by precipitation legacy. Wheat planted in soil mesocosms was grown under controlled conditions, and subjected to frequent or infrequent watering for 12 weeks (equal total water input). 18O-labelled wa- ter was applied at nal rewetting to 3 diferent soil depths (0-5, 10-15, 30-35 cm). Extracted DNA was separated by density using 18O stable isotope probing (SIP) to di erentiate the active and inactive microbial fractions. Bacterial 16S and fungal 18S rRNA from both frac- tions were amplied and sequenced using the Illumina MiSeq platform. Plant performance was evaluated by leaf gas exchange rate and plant biomass. 8O-SIP clearly distinguished the active from the inactive microbial fraction, for both bacte- ria and fungi. Water regime legacy and soil depth aected the bacterial and fungal active and inactive communities diferently. Depth alected microbial communities more than precipitation pattern, and more so in the fungal community. We found no significant effects of precipitation pattern on the diversity of the active bacterial community after rewetting. In contrast, diversity of the inactive fraction di ered with depth. 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 in uential in shaping the response of actively growing bacterial and fungal communities to rewetting. Despite a microbial di- versity gradient with depth, activity upon rewetting involved consistent microbial groups