Does the dominant nitrogen-fixing tree, Robinia pseudoacacia, enhance phosphorus cycling in eastern temperate forests?

Background/Question/Methods

Nitrogen has historically limited eastern temperate forest productivity, leaving a critical biogeochemical role for the dominant nitrogen-fixing tree, Robinia pseudoacacia (black locust). However, increased nitrogen deposition may be shifting nutrient limitation towards phosphorus in eastern forests. Interestingly, R. pseudoacacia has some of the highest growth and recruitment responses to nitrogen deposition relative to other eastern tree species, which contradicts the theory that nitrogen-fixing trees would be less competitive in nitrogen-rich environments. We hypothesized that R. pseudoacacia may have advanced strategies for acquiring other nutrients such as phosphorus due its ability to overcome individual-level nitrogen limitation, which may explain its success in response to nitrogen deposition. Nitrogen-fixing trees in other ecosystems have been found to utilize strategies such as enhanced rock weathering by lowering soil pH, increased root phosphatase activity, and increased investment in mycorrhizae relative to non-fixing trees. To test hypotheses comparing phosphorus acquisition strategies of R. pseudoacacia with other commonly co-occurring arbuscular and ectomycorrhizal temperate trees, we designed 1) a field study comparing six temperate tree species’ phosphorus acquisition strategies across three forest sites in New York, and 2) a greenhouse experiment comparing seven temperate species’ growth and phosphorus acquisition strategies across a nitrogen fertilization gradient.

Results/Conclusions

In both the field and greenhouse studies, we found that the most prominent difference in phosphorus acquisition strategies was the higher root phosphatase activity of R. pseudoacacia compared to other temperate species. Across our field study, soil pH was not lower under R. pseudoacacia as hypothesized, but R. pseudoacacia did have some of the highest rates of mycorrhizal colonization. In the greenhouse, R. pseudoacacia grew the fastest compared to all other temperate species, but we did not find strong differences in rock weathering rates. Root phosphatase activity of R. pseudoacacia was more than double that of the other temperate species. In summary, we found support for our hypothesis that R. pseudoacacia is utilizing enhanced phosphorus acquisition strategies relative to other species, but the primary strategy is enhanced phosphatase production, rather than enhanced rock weathering. These results suggest that R. pseudoacacia is primarily accelerating the recycling of organic phosphorus rather than enhancing weathering inputs from mineral sources. High levels of root phosphatase activity may explain R. pseudoacacia’s ability to capture soil phosphorus in an increasingly phosphorus-limited environment under elevated nitrogen deposition. Further quantification of phosphorus fluxes will elucidate the extent to which nitrogen fixers can overcome phosphorus limitation in temperate forests.