In exploring this question, Weerakoon et al. (1999) grew seedlings of two rice cultivars for 28 days in glasshouses maintained at atmospheric CO2 concentrations of 373, 545, 723 and 895 ppm under conditions of low, medium and high soil nitrogen content.  After four weeks of treatment, photosynthesis was found to significantly increase with increasing nitrogen availability and atmospheric CO2 concentration.  Averaged across all nitrogen regimes, plants grown at 895 ppm CO2 exhibited photosynthetic rates that were 50% greater than those observed in plants grown at ambient CO2.  Total plant dry weight also increased with increasing atmospheric CO2.  In addition, the percentage growth enhancement resulting from CO2 enrichment increased with increasing soil nitrogen; from 21% at the lowest soil nitrogen concentration to 60% at the highest concentration.

Using a different CO2 enrichment technique, Weerakoon et al. (2000) grew rice in open-top chambers maintained at atmospheric CO2 concentrations of approximately 350 and 650 ppm during a wet and dry growing season and under a range of soil nitrogen contents.  Early in both growing seasons, plants exposed to elevated atmospheric CO2 concentrations intercepted significantly more sunlight than plants fumigated with ambient air, due to CO2-induced increases in leaf area index.  This phenomenon occurred regardless of soil nitrogen content, but disappeared shortly after canopy closure in all treatments.  Later, mature canopies achieved similar leaf area indexes at identical levels of soil nitrogen supply; but mean season-long radiation use efficiency, which is the amount of biomass produced per unit of solar radiation intercepted, was 35% greater in CO2-enriched vs. ambiently-grown plants and tended to increase with increasing soil nitrogen content.

Utilizing yet a third approach to enriching the air about a crop with elevated levels of atmospheric CO2, Kim et al. (2003) grew rice crops from the seedling stage to maturity at atmospheric CO2 concentrations of ambient and ambient plus 200 ppm using FACE technology and three levels of applied nitrogen -- low (LN, 4 g N m^-2), medium (MN, 8 and 9 g N m^-2) and high (HN, 15 g N m^-2) -- for three cropping seasons (1998-2000).  They report that "the yield response to elevated CO2 in crops supplied with MN (+14.6%) or HN (+15.2%) was about twice that of crops supplied with LN (+7.4%)," confirming the importance of nitrogen availability to the response of rice to atmospheric CO2 enrichment previously determined by Kim et al. (2001) and Kobaysahi et al. (2001).

In light of these observations, it would appear that the maximum benefits of elevated levels of atmospheric CO2 for the growth and grain production of rice cannot be realized in soils that are highly deficient in nitrogen, but that increasing nitrogen concentrations above what is considered adequate may not result in proportional gains in CO2-induced growth and yield enhancement.

References
Kim, H.-Y., Lieffering, M., Kobayashi, K., Okada, M., Mitchell, M.W. and Gumpertz, M.  2003.  Effects of free-air CO2 enrichment and nitrogen supply on the yield of temperate paddy rice crops.  Field Crops Research 83: 261-270.

Kim, H.-Y., Lieffering, M., Miura, S., Kobayashi, K. and Okada, M.  2001.  Growth and nitrogen uptake of CO2-enriched rice under field conditions.  New Phytologist 150: 223-229.

Kobayashi, K., Lieffering, M. and Kim, H.-Y.  2001.  Growth and yield of paddy rice under free-air CO2 enrichment.  In: Shiyomi, M. and Koizumi, H. (Eds.), Structure and Function in Agroecosystem Design and Management.  CRC Press, Boca Raton, FL, USA, pp. 371-395.

Weerakoon, W.M.W., Ingram, K.T. and Moss, D.D.  2000.  Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice.  Plant and Soil 220: 99-106.

Weerakoon, W.M., Olszyk, D.M. and Moss, D.N.  1999.  Effects of nitrogen nutrition on responses of rice seedlings to carbon dioxide.  Agriculture, Ecosystems and Environment 72: 1-8.