Investigation of microbial community dynamics in soil during variable hydrological forcing

Climate change is expected to alter the temporal distribution of precipitation events, leading to prolonged drought periods and an increased frequency of extreme precipitation events. Changes in precipitation pattern will directly affect soil moisture dynamics and further influence soil redox potential, biogeochemical processes and microbial community compositions. In this thesis, a series of experiments have been conducted to develop a thorough understanding of its impact. First, to investigate the interplay between soil moisture and redox potential dynamics, soil columns are manipulated to vary hydrologic and geochemical conditions and the spatio-temporal variations of their physical and biogeochemical conditions are recorded. The measurements of Eh, soil saturation and porewater chemical composition are used to develop a spatially explicit biogeochemical model that simulates reactive transport phenomena to describe quantitatively the spatio-temporal behavior of the redox potential along a vertical transect of a soil profile. The results of this experiment highlight the importance of joint spatially resolved hydrologic flow/transport and redox processes, the worth of contrasting experiments and computations for a sufficient understanding of the redox potential dynamics.Second, to investigate the impact of the redox regimes on the microbial community composition and probe the widespread hypothesis that the diversity of the redox fluctuating layer is greater than that of the zones with static redox conditions, a new 50-cm homogenized soil column is subjected to realistic intermittent artificial precipitation for 3 months. The hydrological and geochemical parameters are measured and microbial community compositions are analyzed as a function of time and depth. These results highlight the dominant role of vertical dissolved organic carbon gradients to the compositional trajectory of the microbiome and validate the previously-developed biogeochemical model with microbial and geochemical data. Finally, to investigate the impact of changing precipitation distribution on the soil microbial community composition at different depths and soil greenhouse gas emissions, three 50-cm homogenized soil columns are subjected to the sparse and intense precipitation for 3 months. Similar characterizations of soil hydrological, geochemical parameters and microbial compositions have been conducted and greenhouse gas fluxes are measured every 3-4 days. A change in the precipitation pattern increased the microbial diversity in both the oxic and anoxic zones of forest soil and the soil microbial community compositions varied between precipitation patterns in both oxic and anoxic zones. Results from this experiment highlight the importance of leaf litter decomposition in forest ecosystem as it being significantly correlated to the compositional trajectories of the forest soil microbiome and their associated biogeochemical processes. Additionally, our results reveal the dominant role of denitrification that contributes to N2O pulse emissions from soil under extended drought periods and extreme precipitation events.