OOS 29-7 - Time-lags in the regulation of symbiotic nitrogen fixation measured in real time

Wednesday, August 9, 2017: 3:40 PM
Portland Blrm 257, Oregon Convention Center
Thomas A. Bytnerowicz1, Kevin L. Griffin2 and Duncan N. L. Menge1, (1)Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY, (2)Earth and Environmental Sciences, Columbia University, New York, NY

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.