What was done
The authors conducted a three-year free-air CO2-enrichment (FACE) study of paddy rice production between intervening wheat crops in a rice-wheat rotation at Wuxi, Jiangsu, China, wherein they assessed nitrogen uptake and utilization at various stages of crop development for plants grown under three levels of nitrogen (N) fertilization - low N (15 g N m^-2), medium N (25 g N m^-2), high N (35 g N m^-2) - in either ambient air or air that was enriched with an extra 200 ppm of CO2.

What was learned
Yang et al. report that "averaged across all N levels and years, shoot N uptake under FACE was enhanced by 46%, 38%, 6%, and 16% on average during the growth periods from transplanting to early-tillering (period 1), early-tillering to mid-tillering (period 2), mid-tillering to panicle initiation (period 3) and heading to grain maturity (period 5), respectively, but slightly decreased by 2% in the period from panicle initiation to heading (period 4)," such that "FACE increased final aboveground N uptake by 9% at maturity," while noting that "overall dry matter accumulation at harvest was increased on average about 16% under FACE," and that "average grain yield at harvest was stimulated ... by an average of 13% by FACE."

What it means
As a result of their detection of the vastly different abilities of rice to take up significantly different extra amounts of nitrogen at different stages of development, the eight Chinese crop and soil scientists concluded that "in order to maximize grain output in a future high CO2 environment, the recommended rates, proportion and timing across the season of N application should be altered, in order to take full advantage of strong N uptake capacity during the early growth period and facilitate N uptake after that."

With respect to the extra N that could be accommodated by the rice plants during the early stages of crop growth, however, they note that "heavy applications of N frequently lead to greater yield losses in rice due to lodging (Yoshida, 1981)," but they note that in their experiment, "FACE crops exhibited increased resistance to lodging ... while the ambient crops experienced lodging damage," which phenomenon, in their words, "coincides with our previous findings which indicated that FACE-crops were commonly higher in rooting volume, stem weight and proportion, concentration and amount of soluble carbohydrates in stem throughout the season (Yang et al., 2006a,b)." Likewise, Shimono et al. (2007) found that "lodging in rice can be alleviated by atmospheric CO2 enrichment" in experiments conducted in Japan. Hence, the outlook appears bright for the world's farmers to harvest significantly increased amounts of rice in the years and decades to come, while significantly increasing N use efficiency in the process.

References
Shimono, H., Okada, M., Yamakawa, Y., Nakamura, H., Kobayashi, K. and Hasegawa, T. 2007. Lodging in rice can be alleviated by atmospheric CO2 enrichment. Agriculture, Ecosystems and Environment 118: 223-230.

Yang, L.X., Huang, J.Y., Yang, H.J., Dong, G.C., Liu, G., Zhu, J.G. and Wang, Y.L. 2006a. Seasonal changes in the effects of free-air CO2 enrichment (FACE) on dry matter production and distribution of rice (Oryza sativa L.). Field Crops Research 98: 12-19.

Yang, L.X., Huang, J.Y., Yang, H.J., Zhu, J.G., Liu, H.J., Dong, G.C., Liu, G., Han, Y. and Wang, Y.L. 2006b. The impact of free-air CO2 enrichment (FACE) and N supply on yield formation of rice crops with large panicle. Field Crops Research 98: 141-150.

Yoshida, S. 1981. Fundamentals of Rice Crop Science. International Rice Research Institute, Los Banos, Philippines.