Nitrogen fixation does not balance fire-induced nitrogen losses in longleaf pine savannas.

Fire is a critical force in structuring ecosystems, but it also removes substantial amounts of nitrogen (N), which can limit plant growth. Biological N fixation (BNF) may alleviate fire-induced N deficiencies that inhibit ecosystem recovery, yet if and how BNF achieves this under frequent fire is unclear. This problem is further complicated in the context of modern human influences (such as land-use history and atmospheric N deposition), which may confound the relationship between fire and fixation. Here, we investigate whether BNF supplies the N necessary to replace fire-induced N losses in restored longleaf pine savannas, and, if so, what factors control fixation. We established 54 1-ha plots of longleaf pine capturing 227 yr of forest recovery and a broad gradient of fire return interval (1.5-20 yr) at two sites in the southeastern United States. We quantified N fixation from three functional groups (herbaceous legumes, soil crusts, and asymbiotic N fixing bacteria), N losses from individual fire events and ecosystem dynamics of N supply and demand. We found that BNF rates were low but sustained over stand age but were substantially below estimated rates of atmospheric N deposition. While fire temporarily stimulated BNF from herbaceous legumes, neither BNF nor atmospheric N deposition were sufficient to balance N losses from fire and soil N stocks declined over stand age. However, rates of N mineralization were surprisingly high and tree productivity was unrelated to N availability, questioning the importance of N limitation in these temperate savannas. While it is possible that progressive N losses signal a decline in ecosystem resiliency, N enrichment from multiple land-use transitions and anthropogenic N deposition may suppress rates of BNF or diminish its importance as a long-term N balancing source in these pyrogenic ecosystems. In this case, fire may be acting as relief mechanism, critical for returning the modern longleaf pine landscape to its historical oligotrophic condition.

Phosphorus and species regulate N2 fixation by herbaceous legumes in longleaf pine savannas.

Longleaf pine savannas house a diverse community of herbaceous N2-fixing legume species that have the potential to replenish nitrogen (N) losses from fire. Whether legumes fill this role depends on the factors that regulate symbiotic fixation, including soil nutrients such as phosphorus (P) and molybdenum (Mo) and the growth and fixation strategies of different species. In greenhouse experiments, we determined how these factors influence fixation for seven species of legumes grown in pure field soil from two different regions of the southeastern US longleaf pine ecosystem. We first added P and Mo individually and in combination, and found that P alone constrained fixation. Phosphorus primarily influenced fixation by regulating legume growth. Second, we added N to plants and found that species either downregulated fixation (facultative strategy) or maintained fixation at a constant rate (obligate strategy). Species varied nearly fourfold in fixation rate, reflecting differences in growth rate, taxonomy and fixation strategy. However, fixation responded strongly to P addition across all species in our study, suggesting that the P cycle regulates N inputs by herbaceous legumes.