As atmospheric carbon dioxide concentration ([CO₂]) increases, it is anticipated that the competitive ability of C₃ crops could be enhanced relative to C₄ weeds in agricultural systems. However, given the different nitrogen use efficiencies of C₃ and C₄ plants, it is unclear whether any effect of increasing [CO₂] on C₃/C₄ competition is nitrogen dependent. To determine the interaction of [CO₂] and N availability on species growth and competitive outcomes, the growth of rice (C₃ photosynthetic pathway) was examined in both monoculture and in competition with a common weed, barnyardgrass (C₄ photosynthetic pathway) at two levels of N supply (0.357 and 1.071 mmol N L⁻¹) and two levels of [CO₂] (ambient and ambient 200 µmol mol⁻¹) under field conditions in eastern China. In monoculture, the biomass response of rice to elevated [CO₂] depended on N supply, whereas the response of barnyardgrass to elevated [CO₂] was less dependent on nitrogen. Consequently, when grown in mixture, the proportion of rice biomass increased relative to that of barnyardgrass under elevated [CO₂] if the supply of nitrogen was adequate. However, if N was low, elevated [CO₂] significantly reduced the proportion of leaf area and root biomass relative to barnyardgrass biomass. Although data from this experiment confirm that competitiveness of rice could be enhanced relative to C₄ weeds in response to rising [CO₂] in situ, the data also indicate that such a response could be contingent on the supply of nitrogen. This suggests that, for rice cropping systems where N is in limited supply, rising atmospheric CO₂ could still exacerbate competitive losses, even from C₄ weeds.

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; rice, Oryza sativa L. ‘Japonica 9915’.