Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 12, 1999
Publication Date: N/A
Interpretive Summary: The atmospheric concentration of carbon dioxide (CO2) has been increasing, and is predicted to continue to increase for many years into the future. A number of modifications in plant growth are likely to result from this change in the global environment. One important possibility is that leaf nitrogen concentration (N) in grasses will decrease, and thereby lessen the nutritive quality of the forages consumed by both domestic and wild grazing animals. Predicting changes in leaf N is difficult because most grasslands are grown under low nutrient fertility and suffer at least intermittent drought stress. This 4-y experiment, which was undertaken by USDA-ARS scientists at Gainesville, FL and Phoenix, AZ, examined the interactive responses of leaf N to increased CO2 and either drought or low N fertility. With adequate water and soil N, leaf N were not altered by increased CO2. Similarly, drought-stress was found to result in little change in leaf N. The combination of low soil N and high CO2 resulted in important decreases in leaf N. These results indicate that grasslands under future global environments may have decreased nutritive quality in regards to animal production.
Leaf N concentration is important because it is associated with the CO2 assimilatory capacity of crops, and in grasslands, it is an important determinant of forage nutritive value. This four-year research program was undertaken with wheat (Triticum aestivum) as a model system for grasses, to document the potential changes in leaf N concentration in response to global environment changes. Wheat crops grown under field conditions were subjected to elevated CO2 and either water-deficit or N-deficit treatments using large Free Air Carbon dioxide Enrichment (FACE) arrays. Surprisingly, elevated [CO2] under optimum conditions resulted in little change in leaf N concentration. Further, water- deficit treatment had little influence on leaf N concentration. Only on one date late in the season was the water-deficit treatment found to result in decreased leaf N concentration. The low N treatment in combination with elevated [CO2], however, resulted in the lowest leaf N concentration. These results point to a potential problem with grasslands in that the nutritive value of the forage consumed by animals will be decreased under future global environment changes.