Effects of elevated ozone concentration and nitrogen addition on ammonia stomatal compensation point in a poplar clone.

The stomatal compensation point of ammonia (χ s ) is a key factor controlling plant-atmosphere NH 3 exchange, which is dependent on the nitrogen (N) supply and varies among plant species. However, knowledge gaps remain concerning the effects of elevated atmospheric N deposition and ozone (O 3 ) on χ s for forest species, resulting in large uncertainties in the parameterizations of NH 3 incorporated into atmospheric chemistry and transport models (CTMs). Here, we present leaf-scale measurements of χ s for hybrid poplar clone ‘546’ ( Populusdeltoides cv. 55/56 x P. deltoides cv. Imperial ) growing in two N treatments (N0, no N added; N50, 50 kg N ha −1 yr −1 urea fertilizer added) and two O 3 treatments (CF, charcoal-filtered air; E-O 3 , non-filtered air plus 40 ppb) for 105 days. Our results showed that χ s was significantly reduced by E-O 3 (41%) and elevated N (19%). The interaction of N and O 3 was significant, and N can mitigate the negative effects of O 3 on χ s . Elevated O 3 significantly reduced the light-saturated photosynthetic rate ( A sat ) and chlorophyll (Chl) content and significantly increased intercellular CO 2 concentrations ( C i), but had no significant effect on stomatal conductance ( g s ). By contrast, elevated N did not significantly affect all measured photosynthetic parameters. Overall, χ s was significantly and positively correlated with A sat , g s and Chl, whereas a significant and negative relationship was observed between χ s and C i. Our results suggest that O 3 -induced changes in A sat , C i and Chl may affect χ s . Our findings provide a scientific basis for optimizing parameterizations of χ s in CTMs in response to environmental change factors (i.e., elevated N deposition and/or O 3 ) in the future. [ABSTRACT FROM AUTHOR]