The effect of climate change factors on biological N-fixation by bryophyte-cyanobacteria associations in Boreal forests

Background/Question/Methods

Boreal forests serve as a significant pool of terrestrial carbon (C), and currently serve as net C sinks.  Several models predict that the productivity of boreal forests will increase in response to a variety of global change factors, which has clear implications for the global C cycle.  However, it is increasingly recognized that increases in productivity in these systems may be constrained by nitrogen (N) inputs and availability.  As such there is great interest in understanding how global change factors directly influence biological N-fixation in these systems, as well as the degree to which biological N-fixation down-regulates in response to anthropogenic N inputs.  One of the most widespread sources of biological N-fixation in boreal forests occurs through symbiotic associations between a variety of feather moss and cyanobacteria species.  Using a combination of field and indoor experiments, I will describe several studies (conducted with co-authors) showing how biological N-fixation responds to a variety of global change factors, including temperature, precipitation, and atmospheric N-deposition.

Results/Conclusions

In a controlled incubation study, we found that upper range IPCC warming predictions will likely increase N-fixation rates, given that mean summer temperatures in many boreal ecosystem are well below the enzymatic optimum of the nitrogenase enzyme.  However, this experiment also showed that the positive response of N-fixation to warming was constrained by reduction in light intensity, which may occur as a result of warming associated increases in forest canopy cover.  In a separate four-year field experiment where we manipulated precipitation regimes, we found that increased responses to experimental warming were further constrained by reductions in summer precipitation frequency.  Finally, using a 4-year stand scale simulated N-deposition experiment, we found that N-fixation sharply decreased in response to even very low rates of N-deposition (3 kg ha^-1 yr^-1), and that feather mosses served as a substantial sink of anthropogenic N-inputs that limit the uptake of this novel N source by vascular plants.  These results highlight the key role that boreal feather mosses may have in controlling N-inputs in boreal forests, which have clear implications for understanding the long term productivity and C balance of boreal ecosystems in response to climate change.