Photosynthetic responses of 14 grassland species to 19 years of free-air CO2 enrichment and nitrogen addition

Background/Question/Methods

Elevated atmospheric carbon dioxide (eCO₂) is known to stimulate photosynthesis and reduce stomatal conductance. Enhanced carbon sequestration by plants under eCO₂ could represent a negative feedback to climate warming, however the magnitude and persistence of this response is uncertain. Physiological adjustments by plants under eCO₂ may occur in response to nitrogen limitation, reducing photosynthetic responses over time. Furthermore, it is unclear how much responses vary among plant species. If plants grouped together according to similar functional attributes respond similarly to eCO₂, functional group classifications could be useful in large-scale models of eCO₂ effects. To address these issues, we measured long-term (19 years) leaf gas exchange in monocultures of 14 perennial grassland species exposed to free-air CO₂ enrichment (+180 ppm above ambient) and nitrogen addition (4 g N m^-2 y^-1), rendering this the longest-term study of its type. Species were classified into four functional groups – C3 grasses, C4 grasses, C3 non-leguminous forbs, and C3 legumes. We addressed the following questions: 1) What is the magnitude of eCO₂-induced responses in leaf-level physiology and do responses persist through time?; 2) Are responses enhanced or sustained longer with increased N supply?; 3) Do species respond in predictable ways based on their functional attributes?

Results/Conclusions

Over nearly two decades, we found that eCO₂ stimulated photosynthesis by 10.4% on average across 14 grassland species compared to plants exposed to ambient CO₂, which is much lower than rates reported in shorter-term studies that included fewer species. eCO₂ stimulated photosynthesis the most in C3 legumes (+13.3%) and C3 forbs (+12.9%), followed by C3 grasses (+10.1%), and to the smallest extent in C4 grasses (+5.0%). Thus, while C3 and C4 species showed predictably different photosynthetic responses to eCO₂ on average, narrower functional classifications within the C3 photosynthetic type were less useful. In contrast to other studies, the eCO₂-induced stimulation of photosynthesis did not depend on soil nitrogen supply or decrease over time. The reduction in stomatal conductance under eCO₂ compared to ambient CO₂ was relatively consistent among functional groups and similar to other studies (-22.8% on average). This response, combined with enhanced photosynthesis, led to increased intrinsic water-use efficiency under eCO₂ (+45.6% on average). Together, these results suggest that enhancement of photosynthesis under eCO₂ may be more modest than current models simulate, but may persist over time regardless of nitrogen availability, and that the use of functional classifications based on photosynthetic pathway may improve predictions of eCO₂ effects.