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A C3 Crop (Rice) vs. a C4 Weed (Barnyardgrass)
Zhu, C., Zeng, Q, Ziska, L.H., Zhu, J., Xie, Z. and 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.

The authors write that "rice, a C3 plant, is one of the most important staple foods for human nutrition with the highest productivity associated with paddy (flooded) cropping systems," and that "barnyardgrass (a C4 plant), is well-adapted to flooded environments and is a major limitation to rice yields in eastern China." Hence, it is not a trivial exercise to attempt to determine which plant -- the crop or the weed -- will be most benefited by the ongoing rise in the air's CO2 content, which is precisely what Zhu et al. set out to do.

What was done
Zhu et al. state that "the growth of rice was examined in both monoculture and in competition with a common weed, barnyard grass, at two levels of nitrogen supply (0.357 and 1.071 mmol N/L and two levels of CO2 (ambient and ambient + 200 ppm) under field conditions in eastern China."

What was learned
The six researchers found that "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," but that "if nitrogen was low, elevated CO2 significantly reduced the proportion of leaf area and root biomass [of rice] relative to barnyard biomass."

What it means
Zhu et al. conclude that "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." As a result, there is reason to believe that with intelligent management of soil nitrogen fertility, important C3 crops will gain an advantage over major C4 weeds as the air's CO2 content continues to rise.

Reviewed 6 August 2008