Exposure to elevated nutrient load results in structural and functional changes to microbial communities associated with riparian wetland plants Phalaris arundinaceae and Veronica anagallis-aquatica

The goal of this study was to better understand the resilience of rhizosphere microbial communities located in riparian wetlands subjected to high nutrient loads following effluent or runoff exposure. Specifically, this research monitored functional profiles of mesocosm communities during an artificial high nitrogen load exposure following adaptation to high or low water quality conditions. Based on prior research, two species of wetland plants were chosen, Phalaris arundinaceae and Veronica anagallis-aquatica, both located in local riparian zones, and planted in wetland mesocosms. Rhizosphere bacterial community response was monitored for approximately three weeks after exposure to elevated nutrient loads, following prior adaptation for several months to either high or low (poor) water quality conditions. The impact of prior acclimation conditions was then assessed. Changes to rhizosphere bacterial community structure and function were monitored using PCR-denaturing gel gradient electrophoresis (DGGE) and Biolog™ Ecoplates, respectively. Plants were also sampled to assess any impact on mycorrhizal associations over the exposure period. As an additional indicator of functionality and nitrogen utilization within the mesocosm system, inorganic intermediate N species were monitored at the outflow. Significant differences were observed over time following elevated nutrient exposure in bacterial structural diversity and richness, as well as functional diversity and richness (p < 0.001). The effects of interactions between time, plant species and water quality exposure were also significant for both structural (p = 0.002) and functional (p = 0.05) community diversity. Elevated nutrient exposure resulted in changes to microbial carbon source utilization profiles, specifically, bacterial usage of amino acids (p = 0.032), polymers (p = 0.029) and carboxylic and ketonic acids (p = 0.042). Carboxylic and ketonic acid utilization by bacterial communities was positively correlated with the removal of nitrate and nitrite from experimental mesocosms. Mycorrhizal associations as measured by percent colonization also decreased significantly in P. arundinaceae following elevated nutrient exposure (p < 0.001). Furthermore, abundance of nitrogenous compounds in outflow water differed between plant species treatments, with V. anagallis-aquatica removing more nitrate from the outflow (p = 0.005) while P. arundinaceae removed higher levels of ammonia (p = 0.03). This study provides insight into how microbial communities respond to transient nutrient loads such as that experienced by overland runoff following a high rain event and will assist in the design and rehabilitation of wetlands or riparian buffer strips to manage higher nutrient loads resulting from increasing human activity impacting our freshwater systems.