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

Abstract

Summary• N 2 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.

Identification of nitrogen sources and transformations within karst springs using isotope tracers of nitrogen

“…Natural sources include the products of microbial mineralization of organic matter and subsequent nitrification and biological dinitrogen fixation, which can contribute 28% of the total N input to pine savannas (Van Breemen et al 2002;Hiers et al 2003;Cohen 2008). In budgets calculated for springsheds in Florida, inorganic fertilizers (containing ammonium) are the largest anthropogenic source of nitrogen, however, the amount of nitrogen that actually reaches the Floridan aquifer from nonpoint sources is difficult to determine.…”

Ground layer carbon and nitrogen cycling and legume nitrogen inputs following fire in mixed pine forests

“…Though longer-term experiments evaluating frequent burning are not common, declines in tree productivity (Boyer, 2000) and N cycling (Wright and Hart, 1997) have been observed. At this longer time scale, N 2 fixation by native herbaceous legumes may play an important role in the N cycle of burned forests by replenishing or even exceeding N lost due to fire Boring, 1992, 1999;Johnson and Curtis, 2001;Caldwell et al, 2002;Hiers et al, 2003). However, the extent of N 2 fixation by legumes in southeastern forests has been examined primarily in certain forest types such as pine plantations or pine savannas Hiers et al, 2003).…”

“…At this longer time scale, N 2 fixation by native herbaceous legumes may play an important role in the N cycle of burned forests by replenishing or even exceeding N lost due to fire Boring, 1992, 1999;Johnson and Curtis, 2001;Caldwell et al, 2002;Hiers et al, 2003). However, the extent of N 2 fixation by legumes in southeastern forests has been examined primarily in certain forest types such as pine plantations or pine savannas Hiers et al, 2003). Further, interactions between fire effects on legume composition and N inputs via fixation and soil texture have not been addressed over the full range of southeastern forest types currently undergoing prescribed burning.…”

Influence of a native legume on soil N and plant response following prescribed fire in sagebrush steppe

“…In our study, we observed slightly lower N mineralisation 1 year after fire, which may have benefited Lupinus. Also, N 2 fixation is not always negatively affected by increases in soil N following fire (Hiers et al 2003). Casals et al (2005) found that both seedling and resprouting legume species derived 52-99% of their N from fixation after fire, despite increased amounts of mineral N in burned plots.…”