Temperate forest patterns, gradients in 15N/14N, and N fixation effects

Background/Question/Methods

Biological nitrogen fixation supplies most of the nitrogen to many terrestrial ecosystems worldwide. Disturbance events often trigger increased rates of nitrogen fixation by fostering colonization and activity of symbiotic nitrogen-fixing plants. Each of these processes - disturbance and nitrogen fixation - can alter soil nitrogen cycling and its stable isotope composition (¹⁵N/¹⁴N). Disturbance is thought to promote the preferential loss of ¹⁴N, thus increasing the ¹⁵N/¹⁴N of remaining soil. Conversely, inputs of fixed nitrogen move systems towards the atmospheric ¹⁵N/¹⁴N value, typically lowering soil ¹⁵N/¹⁴N. The balance between fractionating losses and fixation inputs during cycles of ecosystem disturbance and recovery can therefore drive transient shifts in soil ¹⁵N/¹⁴N away from the steady-state value of undisturbed ecosystems. In this talk, I will review understanding of how disturbance and biological nitrogen fixation influence the magnitude and duration of changes in soil ¹⁵N/¹⁴N. I will also use a simple dynamic simulation model of ecosystem ¹⁵N/¹⁴N to evaluate how transient versus steady-state ¹⁵N/¹⁴N dynamics respond to disturbance regimes and nitrogen fixing strategy.    

Results/Conclusions

Primary successional ecosystems with symbiotic nitrogen fixing plants often display soil ¹⁵N/¹⁴N near atmospheric values for hundreds to thousands of years. Soil ¹⁵N/¹⁴N in secondary succession is more variable, and shifts towards atmospheric values after disturbance are consistent with inputs of fixed nitrogen, though quantitative estimates are poorly constrained due to uncertainty in fractionating losses. Analysis of obligate versus facultative nitrogen fixation strategies via simulation modeling reveals potential input-driven effects on ecosystem ¹⁵N/¹⁴N. Facultative nitrogen fixation (i.e., downregulation at high N) yields less nitrogen accumulation and higher transient system ¹⁵N/¹⁴N after multiple disturbances than does obligate fixation. This pattern is consistent with elevated soil ¹⁵N/¹⁴N in low-latitudes, where facultative fixers are thought to be widespread. Steady-state ¹⁵N/¹⁴N is reached more slowly in systems with facultative than obligate fixers, and fixation strategy may indirectly shape steady-state ¹⁵N/¹⁴N by altering nitrogen accumulation and loss. A wide range of natural and human activities can induce shifts in the nitrogen cycle. These examples with disturbance and nitrogen fixation highlight how considering transient ecosystem ¹⁵N/¹⁴N dynamics can improve interpretation of changes to the nitrogen cycle at multiple scales.