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Nitrogen Fixation (Herbaceous Plants) Summary
Nearly all of earth's plants respond to increases in the air's CO2 content by exhibiting increased rates of photosynthesis and biomass production [for many examples, see Plant Growth Data].  Additionally, most leguminous species, i.e., those that form symbiotic relationships with nitrogen-fixing bacteria, also find their ability to acquire nitrogen to be enhanced when exposed to CO2-enriched air, which phenomenon could increase their positive growth responses to elevated CO2 even more, as well as ultimately make more soil nitrogen available to co-occurring species.  In this summary, we assemble the findings of some of the studies of these phenomena that have been conducted on herbaceous plants in recent years that we have reviewed on our website.

Luscher et al. (1998) exposed several different grassland species to elevated levels of atmospheric CO2 and observed that nitrogen-fixing species tended to produce more biomass than non-nitrogen-fixing species, possibly in response to CO2-induced increases in the nitrogenase activity of symbiotic nitrogen-fixing bacteria associated with their roots.  In the study of Dakora and Drake (2000), in fact, a 300-ppm increase in the air's CO2 concentration was indeed documented to increase nitrogenase activity in a C3 and a C4 wetland species by 35 and 13%, respectively; while in the study of Marilley et al. (1999), enriching the air with CO2 was observed to increase the dominance of nitrogen-fixing Rhizobium bacterial species associated with the roots of white clover.  In the study of Arnone (1999), however, atmospheric CO2 enrichment had no effect on symbiotic nitrogen fixation in Trifolium alpinum, a grassland species common to the Swiss Alps.

In a FACE study conducted on lucerne, plants fumigated with air containing 600 ppm CO2 significantly increased their total tissue nitrogen content derived from symbiotic nitrogen-fixation (Luscher et al., 2000).  In fact, plants grown on soil containing high nitrogen nearly doubled their symbiotically-derived tissue nitrogen content, which rose from 21 to 41%, while plants grown on soils containing low nitrogen increased their symbiotically-derived nitrogen content from 82 to 88%.  In a related study performed on the same species, a doubling of the air's CO2 content increased root nodule biomass by 40 and 100% in well-watered and water-stressed plants, respectively, as the CO2-enriched plants obtained 31 and 97% more total nitrogen than control plants under the same conditions (De Luis et al. 1999).

Lee et al. (2003b) investigated the effects of atmospheric CO2 concentration (365 and 700 ppm) and nitrogen fertilization (low-N field soil + 0, 4, 8, 12, 16 and 20 g N m-2 year-1) on leaf net photosynthesis, whole plant growth, and carbon and nitrogen acquisition in the N2-fixing wild lupine (Lupinus perennis) in controlled-environment chambers, where plants were grown from seed in pots for one full growing season.  They found the mean rate of leaf net photosynthesis in the CO2-enriched chambers to be 39% greater than in the ambient-air chambers, irrespective of N treatment, while total plant biomass at final harvest was 80% greater in the CO2-enriched chambers, again irrespective of N treatment.  Also, elevated CO2 increased plant total N by 57%, with the extra N coming from enhanced symbiotic N2 fixation related to an increased number and overall mass of nodules.  They additionally report that although partial photosynthetic acclimation to CO2 enrichment occurred, the plants maintained significantly higher rates of photosynthesis and more efficient carbon capture per unit leaf N (average + 60%) in elevated compared to ambient CO2, indicative of a substantial CO2-induced increase in nitrogen use efficiency.

In mixed species experiments, Niklaus et al. (1998) found that artificially-constructed calcareous grassland swards were considerably more responsive to CO2-enriched air when legumes were present than when they were absent.  In addition, they reported that elevated CO2 stimulated nitrogen fixation, particularly when soil phosphorus was not limiting to growth.  Thus, under conditions of adequate soil phosphorus, symbiotically-derived nitrogen would likely become available for the use of non-nitrogen fixing species as well.

In a similar study conducted a few years later, Lee et al. (2003a) grew the N2-fixing Lupinus perennis in monoculture and in nine-species plots exposed to ambient air and air enriched to 560 ppm CO2.  The proportion of Lupinus N derived from symbiotic N2 fixation in monoculture increased from 44% in ambient air to 57% in CO2-enriched air, while in the nine-species plots it increased from 43% in ambient air to 54% in CO2-enriched air, which combined with the CO2-induced increases in plant biomass production resulted in a doubling of N fixed per plot under elevated compared to ambient CO2.  A similar result was obtained by Hartwig et al. (2002), who observed a 70% increase in the air's CO2 concentration to roughly double the amount of nitrogen input through symbiotic N2-fixation by white clover in a clover-ryegrass mixed ecosystem.

In light of these several findings, we conclude that increases in the air's CO2 content will stimulate nitrogen fixation in most herbaceous species that form symbiotic relationships with nitrogen-fixing soil bacteria, i.e., legumes, and that this phenomenon will likely lead to increased nitrogen availability in soils, ultimately leading to large CO2-induced increases in agro- and natural ecosystem productivity.

References
Arnone, J.A., III.  1999.  Symbiotic N2 fixation in a high Alpine grassland: effects of four growing seasons of elevated CO2Functional Ecology 13: 383-387.

Dakora, F.D. and Drake, B.G.  2000.  Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland.  Plant, Cell and Environment 23: 943-953.

De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M.  1999.  Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water stress.  Physiologia Plantarum 107: 84-89.

Hartwig, U.A., Luscher, A., Nosberger, J. and van Kessel, C.  2002.  Nitrogen-15 budget in model ecosystems of white clover and perennial ryegrass exposed for four years at elevated atmospheric pCO2Global Change Biology 8: 194-202.

Lee, T.D., Reich, P.B. and Tjoelker, M.G.  2003a.  Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment.  Oecologia 10.1007/s00442-003-1309-1.

Lee, T.D., Tjoelker, M.G., Reich, P.B. and Russelle, M.P.  2003b.  Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply.  Plant and Soil 255: 475-486.

Luscher, A., Hartwig, U.A., Suter, D. and Nosberger, J.  2000.  Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2Global Change Biology 6: 655-662.

Luscher, A., Hendrey, G.R. and Nosberger, J.  1998.  Long-term responsiveness to free air CO2 enrichment of functional types, species and genotypes of plants from fertile permanent grassland.  Oecologia 113: 37-45.

Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C.  1998.  Nutrient relations in calcareous grassland under elevated CO2Oecologia 116: 67-75.

Marilley, L., Hartwig, U.A. and Aragno, M.  1999.  Influence of an elevated atmospheric CO2 content on soil and rhizosphere bacterial communities beneath Lolium perenne and Trifolium repens under field conditions.  Microbial Ecology 38: 39-49.