Legumes native to longleaf pine savannas exhibit capacity for high N2-fixation rates and negligible impacts due to timing of fire

Authors


Author for correspondence:R. J. Mitchell Tel: +1 229 734 4706 Fax: +1 229 734 4707 Email: rmitchel@jonesctr.org

Summary

  • N2 fixation rates of three legume species and the impact of fire regime are reported.
  • Summer, winter, and no burn treatments were applied. N 2 fixation rates ( 15 N isotope dilution) and C trade-offs with flowering and fine root turnover were examined in response to season of burn.
  • Tephrosia and Centrosema had uniformly high percentage N dfa across all treatments (74–92% N dfa ), whereas Rhynchosia showed limited N 2 fixation activity (18% and 0%). No evidence for decreased N 2 fixation due to loss of leaf area following growing season burns was found. Moreover, no consistent evidence for decreased N 2 fixation with greater flowering or fine root turnover was observed.
  • Despite species differences in response to fire regime, the following patterns emerged: when increased N 2 fixation is associated with decreased growth rates, legumes show limited N 2 fixation rates (as seen in Rhynchosia ). Alternatively, if greater N 2 fixation is related to increased growth rates, then legumes experience C limitations to N 2 fixation only in small individuals or during periods of rapid growth (as in Centrosema ). Reproduction may influence N 2 -fixation, but, as in the case of Tephrosia , the relationship was positive, opposite to patterns indicative of C trade-offs.

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