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.
KeywordsFire Molybdenum Nitrogen Nutrient limitation Biodiversity
We thank A. Barón, D. Blount, E. Coughlin, A. Johnson, A. Martin, J. Minucci, M. Patillo, C. Phillips, C. Timpone, B. Walker and S. Wilson for their assistance in the field, greenhouse and laboratory. We are grateful to D. Markewitz, J. Minucci, J. O’Brien and C. Phillips for their constructive comments on the manuscript. This research was supported by the Strategic Environmental Research and Development Program (RC-2328).
Author contribution statement
MRA, LOH, EAH and NW designed the experiments, MRA conducted the research, MRA, JAT and NW analyzed the data, MRA, JAT, LOH and NW wrote the manuscript and EAH provided editorial advice.
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