Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2007
Publication Date: 1/1/2008
Citation: Zhu, C., Zeng, Q., Ziska, L.H., Zhu, J., Xie, Z., Liu, G. 2008. Effect of nitrogen supply on carbon dioxide-induced changes in competition between rice and barnyardgrass (Echinochloa crus-galli). Weed Science. 56:66-71. Interpretive Summary: Carbon dioxide is rising in the atmosphere. Carbon dioxide provides the raw material needed for plants to grow and as it increases, plant growth will be stimulated. However, if crops and weeds respond differently to rising carbon dioxide this could affect crop/weed competition and crop yields. In this experiment we examined how rice, one of the world's most important food crops, and barnyard grass, a common weed in rice production, responded as carbon dioxide increased. We found that rice growth was stimulated by rising carbon dioxide, but only if nitrogen was in plentiful supply. If nitrogen was not available, rice growth was unaffected by carbon dioxide. However, barnyard grass responded to carbon dioxide even if soil nitrogen levels were low. Our results suggest that for rice cropping systems where N is in limited supply, such as eastern China, rising carbon dioxide could result in declining productivity from increased weed competition, even as carbon dioxide increases. These data will be of interest to food scientists, policy makers and climatologists.
Technical Abstract: As atmospheric carbon dioxide concentration [CO2] increases, different plants will react differently. For agriculture it is anticipated that the competitive ability of C3 crops may be enhanced relative to C4 weeds in agricultural systems. However, given the different nitrogen use efficiencies of C3 and C4 plants, it is unclear if any impact of [CO2] on C3/C4 competition is nitrogen dependent. Using a Free-air Carbon dioxide Enrichment system (FACE), the growth of rice (Oryza sativa, C3 photosynthetic pathway) was examined in both monoculture and in competition with a common weed, barnyard grass (Echninochloa crusgalli, C4 photosynthetic pathway) at two levels of N supply (0.357 m mol N L-1and 1.071 m mol N L-1) and two levels of [CO2] (ambient and ambient + 200 µmol mol-1) in order to determine the interaction of [CO2] and N availability on species growth and competitive outcomes. In monoculture, the biomass response of rice to elevated [CO2] depended on N supply, whereas the response of E. crusgalli to elevated [CO2] was independent of nitrogen. When grown in mixture, the proportion of rice biomass increased relative to that of E. crusgalli under elevated [CO2], but only if N supply was adequate. If N was limited, elevated [CO2] led to a significantly reduced proportion of rice relative to E. crusgalli biomass. Although data from this experiment confirm that competitiveness of rice could be enhanced relative to C4 weeds in response to rising [CO2], such a response is likely to be contingent on the supply of nitrogen in rice systems. This suggests that for rice cropping systems where N is unavailable or in limited supply, rising CO2 could exacerbate competitive losses, even from C4 weeds