Phosphorus controls symbiotic nitrogen fixation in fire-dependent longleaf pine savannas

Symbiotic nitrogen (N) fixation has the potential to replenish
fire-induced N losses in frequently burned ecosystems. A strong
relationship between fire and fixation may exist because fire volatilizes
N and mineralizes phosphorus (P), creating N-poor, P-rich soils that favor
plants capable of N-fixation. However, human activities have enriched
ecosystems with N, which may complicate the interplay among fire,
fixation, and soil P. We evaluated how N and P modulate the relationship
between fire and symbiotic N fixation in longleaf pine savannas, where it
was previously documented that N fixation fails to replenish N losses from
fire. Across gradients of stand age and fire frequency, we investigated
how N and P availability influence fixation, and we established a nutrient
addition experiment to evaluate the effects of N and P on legume growth,
fixation, and mycorrhizal investment. We uncovered a clear signal of P
limitation of herbaceous legumes. Legume growth and fixation were linked
to the availability of soil mineral P and were further stimulated by P
additions. In contrast, neither soil N availability nor N additions
affected legume growth or fixation. Synthesis: Our findings suggest that
symbiotic N fixation in sandhill longleaf pine savannas is controlled by
soil P availability, which varies according to soil age and parent
material. Therefore, fixation may only counterbalance N losses from fire
if there is enough P in the soil to support this process. Nonetheless,
recent N enrichment in contemporary longleaf pine ecosystems may have
reduced the importance of N fixation as a post-fire recovery mechanism.