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Abstract
Biological nitrogen fixation (BNF) can alleviate nitrogen (N) deficiencies that inhibit ecosystem recovery, yet how BNF achieves this under frequent fire disturbance is unclear. We investigated BNFs capacity to provide N to restored longleaf pine savannas, how it is organized and regulated over time, and how it responds to fire. We conducted this study in 54 1-ha plots of longleaf pine across gradients of stand age and fire frequency at two sites in the southeastern US. We determined BNF by three functional groups of N-fixers (legumes, soil crusts, and asymbiotic bacteria), and quantified components of the N cycle. We found that BNF was maintained over time, and fire enhanced BNF from legumes. Despite this, BNF was insufficient to balance N losses from fire. Unexpectedly, differences between the two sites described the most variation in BNF, highlighting that landscape-level drivers can have dramatic influences on BNF.