A number of recent modelling studies have demonstrated the importance of considering nitrogen dynamics for projecting the responses to the terrestrial carbon cycle to increasing atmospheric [CO₂] and climatic changes [1-4]. However, there are considerable differences in the regional and global responses of individual models concerning the strength and even the sign of the effect of N dynamics of the dynamics of the terrestrial carbon cycle. Here, the implications of alternative hypotheses on key nitrogen cycle characteristics that determine vegetation responses are tested to assess the reliability of the modelled responses. For this purpose, the terrestrial biosphere model O-CN model, derived from the land-surface scheme ORCHIDEE of the IPSL Earth system model, is used in different configurations, namely the elasticity of the plant’s C:N stoichiometry, and the capacity of vegetation to increase biological nitrogen fixation as a function of N demand and C excess.

Results/Conclusions

The different model setups result in perceivable differences in the regional responses of the land C balance to elevated atmospheric [CO₂] and artificial soil warming. Results from Free Air CO₂ Enrichments (FACE) in the ORNL and Duke Forests, as well as soil warming experiments are used to corroborate the modelled responses. Those measurements and parameters are identified that are most useful to evaluate the models; both in terms of different trajectories at the site level as well as different average responses in boreal, temperate and tropical climates.

The alternative hypotheses result in substantially different projected land C storage by the year 2100. However, they do not prevent i) that there is a significant reduction of the net land C storage resulting from CO₂ fertilisation compared to the model version not accounting for terrestrial N dynamics; and ii) that on the global scale the limiting effect of N dynamics on the CO₂ fertilisation response is stronger than the stimulating effect of increased N release from soil organic matter decomposition in a future warmer climate.

References
1. Sokolov, A.P., et al., Journal of Climate, 2008. 21(15): p. 3776-3796.
2. Jain, A., et al., Global Biogeochemical Cycles, 2009. 23: GB4028, doi:10.1029/2009GB003519.
3. Thornton, P.E., et al., Biogeosciences, 2009. 6: p. 2099-2120.
4. Zaehle, S., P. Friedlingstein, and A. Friend, Geophysical Research Letters, 2010. 37: L01401, doi:10.1029/2009GL041345.