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C4 Weeds Competing with C3 Crops: Barnyard Grass vs. Rice
Reference
Zeng, Q., Liu, B., Gilna, B., Zhang, Y., Zhu, C., Ma, H., Pang, J., Chen, G. and Zhu, J. 2011. Elevated CO2 effects on nutrient competition between a C3 crop (Oryza sativa L.) and a C4 weed (Echinochloa crusgalli L.). Nutrient Cycling in Agroecosystems 89: 93-104.

Background
The authors write that "among the 18 most harmful weeds in the world, 14 are C4," and that "by contrast, of the 86 plant species that supply most of the world's food, only 5 are C4 (Patterson and Flint, 1995)." In addition, they indicate that "studies comparing C3 crops with C4 weeds (Wary and Strain, 1987; Patterson et al., 1984; Patterson, 1986; Patterson and Flint, 1990, 1995; Alberto et al., 1996; Fround-Williams, 1996; Ziska, 2000) ... have demonstrated that elevated CO2 favors the growth and development of C3 over C4 species."

What was done
In a further study of this nature, Zeng et al. grew rice (Oryza sativa L., a C3 crop) in competition with barnyard grass (Echinochloa crusgalli L., a C4 weed) in a standard paddy-field experiment conducted in ambient air and in air enriched with an extra 200 ppm of CO2 provided via free-air CO2-enrichment or FACE technology that was conducted at Xiaoji Village, Yangzhou City in Eastern China over a period of 120 days.

What was learned
The eight Chinese researchers and their lone Norwegian colleague report that the elevated CO2 significantly enhanced rice biomass (straw +27.3%, ears +37.6%), tillers (+20%), leaf area index (+11.7%) and net assimilation rate (+50.1%); but they found that it reduced all but the last of these characteristics of barnyard grass: biomass (straw -43.6%, ears -47.9%), tillers (-46.1%), leaf area index (-27.3%), and net assimilation rate (+14.1%, the only positive result, but much less than the +50.1% experienced by rice). In addition, they note that "the absolute uptake of C, N, P, K by rice were increased while those of barnyard grass deceased."

What it means
In a significant understatement of the implications of their results, Zeng et al. write that "rising atmospheric CO2 concentration could alter the competition between rice and barnyard grass in paddy fields in favor of rice." We would say it will undoubtedly do so.

References
Alberto, M.P., Ziska, L.H., Cervancia, C.R. and Manalo, P.A. 1996. The influence of increasing carbon dioxide and temperature on competitive interactions between a C3 crop and a C4 weed. Australian Journal of Plant Physiology 23: 795-802.

Fround-Williams, R.J. 1996. Weeds and climate change: Implications for their ecology and control. Aspects of Applied Biology 45: 187-196.

Patterson, D.T. 1986. Response of soybean (Glycine max) and three C4 grass weeds to CO2 enrichment during drought. Weed Science 34: 203-210.

Patterson, D.T. and Flint, E.P. 1990. Implications of increasing carbon dioxide and climate change for plant communities and competition in natural ecosystems. In: Kimball, B.A. (Ed.) Impact of Carbon Dioxide, Trace Gases and Climate Change on Global Agriculture. American Society of Agronomy Special Publication 53. American Society of Agronomy, Madison, Wisconsin, USA, pp. 83-110.

Patterson, D.T. and Flint, E.P. 1995. Effect of environmental stress on weed/crop interactions. Weed Science 43: 483-490.

Patterson, D.T., Flint, E.P. and Beyers, J.L. 1984. Effects of CO2 enrichment on competition between a C4 weed and a C3 crop. Weed Science 32: 101-105.

Wary, S.M. and Strain, B.R. 1987. Competition in old-field perennials under CO2 enrichment. Functional Ecology 1: 145-149.

Ziska, L.H. 2000. The impact of elevated CO2 on yield loss from a C3 and C4 weed in field-grown soybean. Global Change Biology 6: 899-905.

Reviewed 30 March 2011