Symbiotic nitrogen fixation (SNF) often plays a key role in patterns of biogeochemical cycling and in forest regeneration. However, there is a stark difference in the abundance of symbiotic nitrogen fixing trees (hereafter, N fixers) across latitude. The “differential regulation hypothesis” suggests that this pattern may be explained by the degree to which N fixers regulate SNF. The rate at which N fixers adjust SNF to changing conditions (such as soil N availability) is expected to impact their competitive ability and patterns of ecosystem N richness. To determine the timescales of SNF regulation we developed a novel system to measure relative SNF rates non-destructively and in real time on N fixer seedlings (Acetylene Reduction Assay by Cavity ring-down laser Absorption Spectroscopy). Robinia pseudoacacia seedlings were inoculated with bacteria from field-collected nodules and grown under conditions that alternated between high (30 g m-2 y-1) and low (1.5 g m-2 y-1) N. SNF rates were then measured at multiple time points that covered a range of timescales (hours, days, weeks) following the switch in N availability. We asked, what are the timescales of SNF regulation and do plants up- or down-regulate SNF at different rates?
Once plants equilibrated to their N conditions, SNF rates were 14 times greater in seedlings grown at low N than seedlings grown at high N (p<0.001). Both significant up- and down-regulation was observed within 3 days of changing N supply, however the rate of down-regulation was faster than the rate of up-regulation. Down-regulation by 50% occurred within days, while the same change in SNF by up-regulation took weeks. Our results suggest that down-regulation and up-regulation occur on different timescales, which is likely due to inherent differences in the time it takes to sanction N fixing bacteria when costs of SNF outweigh its benefits vs. the time to build functional nodules when a plant becomes N limited. We conclude that immediate down-regulation in response to fluctuating N conditions may have been selected against because of the time and costs of building new nodules. However, down-regulation is still relatively rapid because continuing SNF when a plant has met its N demand is likely to reduce its competitive ability.