Perennial grasses thrive in low-nutrient soils, possibly in part because they receive fixed nitrogen (N) from rhizosphere diazotrophs. The magnitude of fixed N inputs to grasslands is uncertain, in part because estimates diverge widely among studies. Some of that variation may be due to seasonality, with timing of sample collection influencing N fixation estimates. Depending on their phenological stage, plants may alter exudation of labile carbon (C) compounds, which can fuel energy-intensive fixation. Plants can also locally deplete soil N during intensive growth or reproductive periods, which can also stimulate fixation. We hypothesized that N fixation would be maximized during late June and early July, when the plants are growing fastest thereby creating N demand and exuding abundant carbon.
We used switchgrass (Panicum virgatum L.) as a model perennial grass species. We tested our hypothesis by measuring N fixation in switchgrass plots 10 times during the 2019 growing season, with our sampling encapsulating key phenological time points (pre-emergence, tiller stage, stem elongation, flowering, and senescence). We measured fixation by incubating intact rhizosphere cores with 15N2 and measuring incorporation into soil and root biomass. Explanatory variables included soil moisture, soil N, soil C mineralization, air temperature, and the previous day's rainfall.
Results/Conclusions
N fixation was detectable during all time points, with rates ranging from 0.24 ± 0.14 (SE) mg N m-2 d-1 during the early tiller stage (late May) to 1.8 ± 0.62 (SE) mg N m-2 d-1 during flowering (mid-July). The highest rates occurred during stem flowering and senescence, but in all cases the highest fixation rates occurred after a rainfall event of at least 14 mm. The previous day's rainfall explained 32% of the variation in N fixation rates, with soil N, soil C, and soil moisture not adding any additional predictive power. We conclude that N fixation responds positively to rain events, particularly during times of year when C is abundant as a result of root exudation.