Biological nitrogen fixation (BNF), a process driven by soil microorganisms, can alleviate nitrogen (N) deficiencies that inhibit ecosystem recovery. BNF may be particularly important in ecosystems recovering from land-use change and perturbations from fire, as these disturbances can exacerbate N limitation. In this study, we investigated how BNF dynamics change throughout ecosystem development in restored longleaf pine savannas, and how BNF responds to fire. We conducted this study in 59 1-ha plots of longleaf pine distributed across gradients of stand age and fire frequency at two sites in the southeastern US. We determined BNF contributions by three functional groups of N2-fixers (herbaceous legumes, biological soil crusts, and asymbiotic N2-fixing bacteria) by quantifying their abundances, assessing nitrogenase activity with acetylene reduction assays, and scaling these estimates up to the plot-level. To determine aboveground nitrogen demands, we measured tree growth using diameter increments and allometric equations paired with tissue-specific N concentrations. We fit linear mixed-effects models to determine the effects of stand age and time since fire on BNF and N demands throughout ecosystem development, and performed a separate analysis on mature stands to determine how fire return interval affects BNF.
Results/Conclusions
We observed distinct temporal patterns of N2-fixation across ecosystem development among the three groups of N2-fixers. N2-fixation by legumes remained low until stands reached maturity, while N2-fixation by biological soil crusts (BSCs) was exceptionally high in juvenile stands and decreased with stand age. Fixation supported by asymbiotic bacteria was relatively minor compared to legumes and BSCs. These patterns suggest a compensatory shift in the importance of BSCs and legumes throughout ecosystem development, and raise the possibility that contributions from BSCs are critical for meeting N demands when physical disturbances may hinder legume establishment. N2-fixation by BSCs and asymbiotic bacteria throughout stand development was not affected by time since fire, but legume abundance increased the year following fire, suggesting a recovery mechanism provided by this group. In mature stands, high fire frequency supported more N2-fixation from legumes than from BSCs and the reverse was true for low fire frequency. Our findings suggest that BSCs are the most important source of new N in the early phases of ecosystem restoration. In contrast, legumes appear to be critical in mature longleaf pine stands that burn frequently, and particularly for supplying new N in the year following a fire event.