Summary
Theory has shown that time lags in the regulation of symbiotic nitrogen (N) fixation (SNF) can be important to the competitive dynamics and ecosystem consequences of N-fixing trees, but measurements of these time lags are lacking.
Here, we used a novel method to measure SNF in seedlings of four N-fixing tree species that represent tropical and temperate origins and actinorhizal and rhizobial symbiotic associations, each grown under warm and cold temperature regimes. We added N to previously N-poor pots to induce downregulation and flushed N out of previously N-rich pots to induce upregulation.
It took 31–51 d for SNF to decline by 95%, with faster downregulation in temperate species and at warm temperatures. Upregulation by 95% took 108–138 d in total, including 21–57 d after SNF was first detectable. SNF started earlier in rhizobial symbioses, but increased faster once it started in actinorhizal symbioses.
These results suggest that time lags in regulating SNF represent a significant constraint on facultative SNF and can lead to large losses of available N from ecosystems, providing a resolution to the paradox of sustained N richness.
