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Growth Response to CO2 With Other Variables -- Nitrogen (Crops: Other) -- Summary
In exploring the effects of soil nitrogen concentrations on the ability of atmospheric CO2 enrichment to stimulate the growth and development of agricultural crops, we have previously reviewed and summarized the results of multiple studies that have been conducted on rice and wheat [see Nutrients x CO2 Effects on Plants (Nitrogen - Crops: Rice and Wheat) in our Subject Index].  Here, we review what has been learned about a number of other plants in this regard.

Zerihun et al. (2000) grew sunflowers for one month in pots of three different soil nitrogen concentrations that were placed within open-top chambers maintained at atmospheric CO2 concentrations of 360 and 700 ppm.  The extra CO2 of the CO2-enriched chambers reduced average rates of root nitrogen uptake by about 25%, which reduction, by itself, would normally tend to reduce tissue nitrogen contents and the relative growth rates of the seedlings.  However, the elevated CO2 also increased photosynthetic nitrogen-use efficiency by an average of 50%, which increase normally tends to increase the relative growth rates of seedlings.  Of these two competing effects, the latter was by far the more powerful, ultimately leading to an increase in whole plant biomass.  After the one month of the study, for example, the CO2-enriched plants exhibited whole plant biomass values that were 44, 13 and 115% greater than those of the plants growing in ambient air at low, medium and high levels of soil nitrogen, respectively, thus demonstrating that low tissue nitrogen contents do not necessarily preclude a growth response to atmospheric CO2 enrichment, particularly if photosynthetic nitrogen-use efficiency is enhanced, which is typically the case, as it was in this study.  Nevertheless, the greatest CO2-induced growth increase of Zerihun et al.'s study was exhibited by the plants growing in the high soil nitrogen treatment.

Deng and Woodward (1998) grew strawberries in environment-controlled glasshouses maintained at atmospheric CO2 concentrations of 390 and 560 ppm for nearly three months.  In addition, the strawberries were supplied with fertilizers containing three levels of nitrogen.  The extra CO2 increased rates of net photosynthesis and total plant dry weight at all three nitrogen levels, but the increases were not significant.  Nevertheless, they provided the CO2-enriched plants with enough additional sugar and physical mass to support significantly greater numbers of flowers and fruits than the plants grown at 390 ppm CO2.  This effect consequently led to total fresh fruit weights that were 42 and 17% greater in the CO2-enriched plants that received the highest and lowest levels of nitrogen fertilization, respectively, once again indicating a greater growth response at higher nitrogen levels.

Newman et al. (2003) investigated the effects of two levels of nitrogen fertilization and an approximate doubling of the air's CO2 concentration on the growth of tall fescue, which is an important forage crop.  The plants with which they worked were initially grown from seed in greenhouse flats, but after sixteen weeks they were transplanted into 19-liter pots filled with potting media that received periodic applications of a slow-release fertilizer.  Then, over the next two years of outdoor growth, they were periodically clipped, divided and repotted to ensure they did not become root-bound; and at the end of that time, they were placed within twenty 1.3-m-diameter open-top chambers, half of which were maintained at the ambient atmospheric CO2 concentration and half of which were maintained at an approximately doubled CO2 concentration of 700 ppm.  In addition, half of the pots in each CO2 treatment received 0.0673 kg N m-2 applied over a period of three consecutive days, while half of them received only one-tenth that amount, with the entire procedure being repeated three times during the course of the 12-week experiment.  As to what was learned, Newman et al. report that the plants grown in the high-CO2 air photosynthesized 15% more and produced 53% more dry matter (DM) under low N conditions and 61% more DM under high N conditions.  In addition, they report that the % organic matter (OM) was little changed, except under elevated CO2 and high N, when %OM (as %DM) increased by 3%.  In this study too, therefore, the greatest relative increase in productivity occurred under high, as opposed to low, soil N availability.

Demmers-Derks et al. (1998) grew sugar beets as an annual crop in controlled-environment chambers at atmospheric CO2 concentrations of 360 and 700 ppm and air temperatures of ambient and ambient plus 3°C for three consecutive years.  In addition to being exposed to these CO2 and temperature combinations, the sugar beets were supplied with solutions of low and high nitrogen content.  Averaged across all three years and both temperature regimes, the extra CO2 of this study enhanced total plant biomass by 13 and 25% in the low and high nitrogen treatments, respectively.  In addition, it increased root biomass by 12 and 26% for the same situations.  Thus, as was the case with sunflowers, strawberries and tall fescue, elevated CO2 elicited the largest growth responses in the sugar beets that received a high, as opposed to a low, supply of nitrogen.

Also working with sugar beets were Romanova et al. (2002), who grew them from seed for one month in controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm, while fertilizing them with three different levels of nitrate-nitrogen.  In this study, the plants grown in CO2-enriched air exhibited rates of net photosynthesis that were approximately 50% greater than those displayed by the plants grown in ambient air, regardless of soil nitrate availability.  These CO2-induced increases in photosynthetic carbon uptake contributed to 60, 40 and 30% aboveground organ dry weight increases in plants receiving one-half, standard, and three-fold levels of soil nitrate, respectively.  Root weights, however, were less responsive to atmospheric CO2 enrichment, displaying 10 and 30% increases in dry weight at one-half and standard nitrate levels, but no increase at the high soil nitrate concentration.  In this study, therefore, the role of soil nitrogen fertility was clearly opposite to that observed in the four prior studies in the case of aboveground biomass production, but was mixed in the case of belowground biomass production.

Switching to barley, Fangmeier et al. (2000) grew plants in containers placed within open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 650 ppm and either a high or low nitrogen fertilization regime; and in this case, as in the case of the aboveground biomass response of the sugar beets of Romanova et al., the elevated CO2 had the greatest relative impact on yield when the plants were grown under the less-than-optimum low-nitrogen regime, i.e., a 48% increase vs. 31% under high-nitrogen conditions.

Last of all, we report the pertinent results of the review and analysis of Kimball et al. (2002), who summarized the findings of most Free-Air CO2 Enrichment (FACE) studies conducted on agricultural crops since the introduction of that technology back in the late 1980s.  In response to a 300-ppm increase in the air's CO2 concentration, rates of net photosynthesis in several C3 grasses were enhanced by an average of 46% under conditions of ample soil nitrogen supply and by 44% when nitrogen was limiting to growth.  With respect to aboveground biomass production, the differential was much larger, with the C3 grasses wheat, rice and ryegrass experiencing an average increase of 18% at ample nitrogen but only 4% at low nitrogen; while with respect to belowground biomass production, they experienced an average increase of 70% at ample nitrogen and 58% at low nitrogen.  Similarly, clover experienced a 38% increase in belowground biomass production at ample soil nitrogen, and a 32% increase at low soil nitrogen.  Finally, with respect to agricultural yield, which is the bottom line in terms of food and fiber production, wheat and ryegrass experienced an average increase of 18% at ample nitrogen, while wheat experienced only a 10% increase at low nitrogen.

In light of these several results, it can be safely concluded that although there are some significant exceptions to the rule, most agricultural crops generally experience somewhat greater CO2-induced relative (percentage) increases in net photosynthesis and biomass production when soil nitrogen concentrations are higher rather than lower.

References
Demmers-Derks, H., Mitchell, R.A.G., Mitchell, V.J. and Lawlor, D.W.  1998.  Response of sugar beet (Beta vulgaris L.) yield and biochemical composition to elevated CO2 and temperature at two nitrogen applications.  Plant, Cell and Environment 21: 829-836.

Deng, X. and Woodward, F.I.  1998.  The growth and yield responses of Fragaria ananassa to elevated CO2 and N supply.  Annals of Botany 81: 67-71.

Fangmeier, A., Chrost, B., Hogy, P. and Krupinska, K.  2000.  CO2 enrichment enhances flag leaf senescence in barley due to greater grain nitrogen sink capacity.  Environmental and Experimental Botany 44: 151-164.

Kimball, B.A., Kobayashi, K. and Bindi, M.  2002.  Responses of agricultural crops to free-air CO2 enrichment.  Advances in Agronomy 77: 293-368.

Newman, J.A., Abner, M.L., Dado, R.G., Gibson, D.J., Brookings, A. and Parsons, A.J.  2003.  Effects of elevated CO2, nitrogen and fungal endophyte-infection on tall fescue: growth, photosynthesis, chemical composition and digestibility.  Global Change Biology 9: 425-437.

Romanova, A.K., Mudrik, V.A., Novichkova, N.S., Demidova, R.N. and Polyakova, V.A.  2002.  Physiological and biochemical characteristics of sugar beet plants grown at an increased carbon dioxide concentration and at various nitrate doses.  Russian Journal of Plant Physiology 49: 204-210.

Zerihun, A., Gutschick, V.P. and BassiriRad, H.  2000.  Compensatory roles of nitrogen uptake and photosynthetic N-use efficiency in determining plant growth response to elevated CO2: Evaluation using a functional balance model.  Annals of Botany 86: 723-730.