Nitrogen species mediates elevated CO2 effects on plant N utilization

Background/Question/Methods

Atmospheric carbon dioxide (CO₂) enrichment generally increases plant growth but reduces plant nitrogen (N) concentration. Experimental evidence suggests that elevated CO₂ may inhibit plant photoassimilation of nitrate (NO₃^-) in C₃ plants. Since NO₃^- is the primary source of available soil N, CO₂-inhibition of plant NO₃^- uptake will likely reduce plant N uptake and N concentrations in cereal seeds. Manipulation of the relative availability of soil ammonium (NH₄⁺) and NO₃^- may significantly affect plant N acquisition, but direct experimental evidence is still lacking. We examined the effect of different N sources on wheat growth and N utilization through manipulating inputs of N species and quantity (80 vs. 160 kg N per ha.) under two different CO₂ concentrations (ambient, and ambient plus 200ppm CO₂). ¹⁵N-enriched NH₄⁺ and NO₃^- fertilizers were periodically applied to the soil to assess the effect of elevated CO₂ on plant N uptake and growth. 

Results/Conclusions

Elevated CO₂ significantly increased plants growth under both low and high N levels. However, N species significantly affected plant growth under elevated CO₂. Under elevated CO₂, plants were significantly larger when they received NH₄⁺ than NO₃^-. Under ambient CO₂, no significant differences were observed between two N sources. These results showed that different N species significantly impacted CO₂ effects on plant growth, suggesting that manipulation of soil NH₄⁺ and NO₃^- availability may optimize the stimulative effects of the rising CO₂ on plants.