Hydrologic history influences microbial community composition and nitrogen cycling under experimental drying/wetting treatments

Understanding how historical and contemporary environmental conditions interact to affect the relationship between community structure and function is necessary to predict the response of microbial community function to a changing environment. We focused on nitrogen transformations - denitrification and nitrification - which directly impact nitrate concentration in wetland ecosystems. Nitrate removal can occur via denitrification, while nitrate can be generated through nitrification; these two microbial transformations are strongly influenced by hydrology and redox conditions. We carried out a manipulative mesocosm experiment to determine how microbial communities adapted to different hydrologic conditions (upland vs. wetland) respond to experimental soil moisture treatments (dry, wet-dry, saturated). We assessed local soil characteristics (e.g., soil moisture, inorganic nitrogen), and microbial community composition and activity of denitrifiers and ammonia oxidizers (targeted by the nosZ and amoA genes, respectively) before and after moisture treatments. Wetland soils were characterized by higher soil fertility compared to upland soils. In response to the drying/flooding treatments, we observed a small but significant change in community composition of denitrifier assemblages, but no change in the ammonia oxidizer communities. In addition, potential denitrification rates significantly increased under wetter conditions (upland: 62-118% increase; wetland: 78-96% increase), whereas potential nitrification rates generally showed no significant change following experimental drying/flooding treatments, regardless of the hydrologic history. Based on these results, fluctuations in soil moisture are expected to influence denitrification rates to a greater extent than nitrification rates, ultimately influencing nitrate pools in this wetland. This imbalance in microbial functional response may result in a shift in dominant nitrogen cycling transformations within a wetland as a consequence of the different responses of these nitrogen-cycling functional guilds to changes in soil moisture. A shift in nitrogen transformations can be most noticeable under fluctuating hydrologic conditions, and there is potential for the wetland to be resilient to hydrologic change if soil microbes are exposed to dynamic hydrology over the long-term.