Do relationships with certain microbial communities enable perennial grasses to thrive in low-nitrogen (N) conditions? Specifically, N2 fixation by microbes in loose association with plants could provide a substantial portion of the N required by perennial grasses, but recent studies suggest that this process is episodic and challenging to predict. To recognize how this process impacts global biogeochemical cycling, we need to understand the physicochemical conditions that facilitate associative nitrogen fixation (ANF).
Using switchgrass (Panicum virgatum) as a model perennial grass, we examined how precipitation and drought affected ANF in June−July 2019. Specifically, we manipulated drying and wetting cycles with rainout shelters such that each plot received the same amount of water (~60 mm or 2”) over 20 days, but at different frequencies. Constant moisture plots received 15 mm of rainwater (i.e., collected from the gutters of the rainout shelters) every 5 days, while moderate drying plots received 30 mm every 10 days and the extended drying plots received 60 mm on the 20th day. Intact soil cores were sampled before and after each wetting event as well as on day 22, 25, and 30, and assessed for ANF using 15N2 incorporation. Our hypotheses were: 1) wetting events will enhance ANF by simultaneously stimulating carbon mineralization and reducing soil oxygen availability, and 2) moderate drying plots will exhibit higher ANF because the longer 20-day drying interval would stress the N2-fixers.
Results/Conclusions
Wetting stimulated ANF, and the response of ANF to wetting increased with volume of water added and decreased with N addition, even though N concentrations in the water were low (0.4−0.8 mg L−1). Additionally, ANF generally increased with post-wetting carbon consumption, thus supporting our hypothesis that wetting, carbon mineralization, and ANF are linked. In addition, the extended drying plots exhibited the highest amount of ANF over the 10-day period following the final wetting event, which might represent the episodic conditions under which significant ANF inputs occur. Our preliminary results also suggest that plant phenology may be interacting with the effects of wetting/drying cycles. A better understanding of ANF in switchgrass will shed light on how important this process is for perennial grasses and whether ANF is the key to their persistence in low-nutrient environments.