Interacting effects of CO2 concentrations and nitrogen-form on root hydraulic conductivity in conifer seedlings

Background/Question/Methods

Despite the widespread recognition that root hydraulic conductivity (Lp) is a major determining factor of whole plant water loss and/or status, there is very little information on how it responds to rising levels of atmospheric CO₂. Studies of plant water status responses to high CO₂ often use stomatal conductance (gs) as a critical mechanism but it is currently unknown if Lp is coordinated or decoupled from shoot conductances. Therefore, in this study we examined Lp of four contrasting conifers to CO₂ enrichment. Because Lp has been shown to be highly sensitive to availability and/or N-form we also examined if the potential CO₂ effects are mediated by nitrate-N or ammonium-N nutrition. One year-old seedlings of Colorado spruce (Picea pungens), Douglas-fir (Pseudotsuga menziesii), Frasier-fir (Abies fraseri) and Norway spruce (Picea abies) were grown at near ambient (400 ppm) or elevated (600 or 800 ppm) CO₂ concentrations under identical environmental regimes. Plants were fertilized using a modified Hoagland solution differing in N-form: nitrate-N, ammonium-N or ammonium nitrate as control. After six months of CO₂ and N-form treatments, Lp was measured using intact detopped root systems and a pressure chamber.

Results/Conclusions

CO₂ enrichment significantly affected Lp but this effect was highly species-specific. For example, across N treatments, Lp increased by an average of 61% in Colorado spruce and Norway spruce while it decreased by 25% in Douglas-fir and Frasier-fir with CO2 enrichment. In general, there was no consistent effect of inorganic N-form on Lp. However, the CO₂ induced changes in Lp was dependent on N-form. For example, in Norway spruce increased CO₂ from 400 to 800 ppm resulted in 44% increase in Lp in control and nitrate-fed plants but not in ammonium-fed plants. Using a covariate analysis we examined Lp responses to CO₂ enrichment simultaneously with changes in total leaf area. The results indicate that in Colorado spruce and Douglas-fir Lp responses to changes in CO₂ do scale to changes in leaf area, but not in Norway spruce and Frasier-fir. Therefore, Lp responses to high CO₂ and the coordination with canopy demand for water are species-dependent. To our knowledge, this is the first study that examines Lp responses of multiple species to CO₂ enrichment under different N-forms. The results show a strong effect of CO₂ on Lp which appears to be partially decoupled from canopy conductivity. This marked effect of CO₂ on root water transport properties is highly regulated by inorganic N-form.