Progressive nitrogen limitation of plant and ecosystem responses to elevated CO2

This working group will address the issue: why don't most field experimental results support model predictions that progressive ecosystem nitrogen (N) limitation leads to downregulation of primary production in elevated CO2? Ecosystem models that link nutrient cycling with plant production generally predict long-term photosynthetic downregulation and reduced stimulation in plant production caused by progressive N constraints in elevated CO2. The conceptual model is that increased carbon (C) influx under elevated CO2 will immobilize more N in plant biomass and soil organic matter, progressively leading to less N available for plant uptake. However, the predicted downregulation in primary production has not been confirmed by most of the free-air CO2 enrichment (FACE) and open-top chamber (OTC) experiments. The discrepancy between model predictions and experimental data apparently results from a fundamental knowledge gap concerning ecosystem C and N interactions, which lie at the core of ecosystem ecology. The proposed working group will address the issue of progressive N limitations by (1) compilation and synthesis of experimental data from the on-going FACE and OTC projects as well as from the literature; (2) synthesis of published modeling results and comparison of mechanisms incorporated into various models; (3) identification of possible mechanisms causing the discrepancy between model predictions and experimental results; and (4) defining future research need for both experimental and modeling studies. This project has the potential to make a critical contribution to ecosystem ecology. Moreover, a solid understanding of C:N interactions extends beyond ecology. Models increasingly applied to policy questions appear to have major problems matching the aspects of experimental results. The latter are most relevant to the policy applications. The ability of the ecological community to provide useful information on the future carbon dynamics of the terrestrial biosphere (including the terrestrial sink) depends critically on resolving the question of progressive N limitation.