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Legumes -- Summary
Nearly all of earth's plant life responds favorably to increases in the air's CO2 content by exhibiting enhanced rates of photosynthesis and biomass production under controlled experimental conditions. However, there is some concern that real-world ecosystem responses may be less than those observed for individual species in the laboratory, due to often-observed inadequacies of soil nutrients (primarily nitrogen) in the world of nature. It is important, therefore, to determine the responses of various legumes to atmospheric CO2 enrichment; for these specially-equipped plants have the ability to form symbiotic relationships with certain bacteria that take up residence in nodules on their roots and make nitrogen more readily available to them for uptake and subsequent utilization. If atmospheric CO2 enrichment stimulates legume growth, for example, the greater amounts of fixed nitrogen ultimately made available to the non-leguminous plants of the community as a consequence of this phenomenon may lead to greater whole ecosystem responses to increases in the air's CO2 concentration than might otherwise occur.

In one such legume study, Palta and Ludwig (2000) exposed the narrow-leafed lupin Lupinus angustifolius to twice-ambient levels of atmospheric CO2 and observed increases in dry matter and seed yield of 52 and 55%, respectively. Likewise, in an experiment conducted on seedlings of the leguminous tree Acacia melanoxylon, a doubling of the air's CO2 content increased photosynthetic rates by an average of 22% across a large soil nitrogen gradient, while seedling biomass was enhanced by as much as 100% (Schortemeyer et al., 1999).

In addition to increases in growth, in a FACE study conducted on lucerne (Medicago sativa L.), plants that were fumigated with air containing 600 ppm CO2 significantly increased their total tissue nitrogen contents derived from symbiotic nitrogen-fixation (Luscher et al., 2000). In fact, plants grown on soil containing high nitrogen doubled their percentage of symbiotically-derived tissue nitrogen content from 21 to 41%; while plants grown on soils containing low nitrogen increased their symbiotically-derived nitrogen contents from 82 to 88%. And in a related study performed on the same species, a doubling of the air's CO2 concentration 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 these well-watered and water-stressed conditions (De Luis et al. 1999).

In mixed-species experiments, Luscher et al. (1998) exposed 12 different grassland plants native to Switzerland to an atmospheric CO2 concentration of 700 ppm and reported finding that CO2-induced increases in biomass were greater for legumes than for non-leguminous species. Similarly, Niklaus et al. (1998) determined that artificially-constructed calcareous grassland swards were much more responsive to an atmospheric CO2 concentration of 600 ppm when legumes were present than when they were absent.

In some cases, legumes are so responsive to atmospheric CO2 enrichment that they can actually increase in abundance within mixed communities. In a review of over 165 peer-reviewed scientific journal articles dealing with pasture and rangeland responses to atmospheric CO2 enrichment, for example, Campbell et al. (2000) determined that the legume content of grass-legume swards increased by about 10% in response to a doubling of the air's CO2 content, which would ultimately make more nitrogen available to the ecosystem's non-leguminous plants.

Working with the leguminous tree commonly known as jatoba (Hymenaea courbaril) -- a late secondary/climax species that is one of the most important trees in mature tropical forests of the Americas -- Aidar et al. (2002) sprouted and grew potted seedlings within small open-top chambers they maintained at atmospheric CO2 concentrations of 360 and 720 ppm within a shaded glasshouse (to simulate the low light regime at the forest floor where the seeds typically germinate) for a period of 70 days, over which time they measured net photosynthesis rates of seedlings with and without cotyledons, which they removed from half of the plants. In doing so, they say "a marked and persistent increase (2 fold) in photosynthesis was observed in all cases (with or without cotyledons)," when the seedlings were exposed to elevated CO2. In addition, they found that the plants grown under enriched CO2 did not experience any down regulation of photosynthesis; and they observed a 35% increase in the water use efficiency of the seedlings.

In discussing the implications of their findings, the researchers remarked that "under the climatic conditions forecasted on the basis of the present carbon dioxide emissions, Hymenaea courbaril should establish faster in its natural environment and might also serve as an efficient mechanism of carbon sequestration within the forest." In addition, they note that the CO2-induced increase in water use efficiency may enable jatoba "to tolerate dryer and more open environments, which should allow them to better cope with drought stress or a more seasonal climate." Last of all, they say that the jatoba tree would likely exhibit similar positive responses to even greater emissions of CO2, for they note that light-saturated photosynthesis in jatoba seedlings continued to rise in response to increasing atmospheric CO2 concentrations well above 1,000 ppm. What is more, they say they "have measured the saturation level of some other tropical trees from the rain forest and all of them [also] saturate at relatively high CO2 concentrations." Hence, it is likely that neotropical forests in general may be better suited to much higher-than-present atmospheric CO2 concentrations, and that they would fare far better than they do today in a CO2-enriched world of the future.

Moving a bit closer to the present, Lee et al. (2003) grew the N2-fixing legume Lupinus perennis L. in monoculture and in nine-species plots at ambient and enriched (to 560 ppm) atmospheric CO2 concentrations, finding that the proportion of Lupinus nitrogen derived from symbiotic N2 fixation increased from 44% to 57% in monoculture and 43% to 54% in nine-species plots, which combined with the CO2-induced increases in plant biomass production resulted "in a doubling of nitrogen fixed per plot under elevated compared to ambient CO2." In a CO2-enriched world of the future, these results suggest that legumes like Lupinus will likely provide the ecosystems within which they occur with considerably more nitrogen than is currently available to them, which should enhance the ability of co-occurring species to grow and develop to their fullest potential if they are currently inhibited from doing so by a lack of available nitrogen in the soils in which they grow.

One year later, Pal et al. (2004) grew well-watered and fertilized berseem -- an important forage legume crop that is grown in the winter throughout India -- from germination onwards in open-top chambers maintained at atmospheric CO2 concentrations of 360 and 600 ppm for a period of 80 days. During this period, they recorded the following percent increases in the following plant growth parameters at 40, 60 and 80 days, respectively, after initial exposure to the 67% increase in the air's CO2 content: plant height (47%, 36%, 42%), stem dry weight (77%, 50%, 51%), total shoot mass (98%, 42%, 40%), leaf area (98%, 79%, 47%), number of leaves (23%, 30%, 43%), and leaf dry weight (118%, 29%, 12%). With respect to leaf nutrient concentrations, however, they recorded reductions of 29%, 27% and 33% for nitrogen, but increases of 12%, 8% and 9% for phosphorus, as well as no significant changes in calcium and iron.

The single negative aspect of the study -- the decrease in leaf nitrogen concentration -- may not be as serious as it first appears when viewed in another light. Pal et al. report, for example, that "if calculated on a unit land area basis, an increase of 20-25% in nitrogen content occurs." Then there is another positive implication not revealed in the data. In addition to the intrinsic value of the huge increases in growth produced by the increase in atmospheric CO2 concentration, Pal et al. note that this "acceleration in biomass (fodder) production" may "lead to more than one cutting of berseem fodder if plants are grown under high CO conditions throughout the crop season," which would provide an even bigger additional boost to the total growing-season production of both harvestable biomass and nitrogen.

In a contemporaneous study, Pangga et al. (2004) grew well-watered and fertilized seedlings of the pencilflower (Stylosanthes scabra cv. Fitzroy) -- which is an important legume crop that is highly susceptible to anthracnose disease caused by Colletotrichum gloeosporioides -- within a controlled environment facility maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, where they inoculated six-, nine- and twelve-week-old plants with conidia of C. gloeosporioides. Then, ten days after inoculation, they counted the anthracnose lesions on the plants and classified them as either resistant or susceptible. In doing so, they found that "the mean number of susceptible, resistant, and total lesions per leaf averaged over the three plant ages was significantly greater at 350 ppm than at 700 ppm CO2, reflecting the development of a level of resistance in susceptible cv. Fitzroy at high CO2." In fact, with respect to the plants inoculated at twelve weeks of age, they say that those grown "at 350 ppm had 60 and 75% more susceptible and resistant lesions per leaf, respectively, than those at 700 ppm CO2."

In terms of infection efficiency (IE), the Australian scientists say their work "clearly shows that at 350 ppm overall susceptibility of the canopy increases with increasing age because more young leaves are produced on secondary and tertiary branches of the more advanced plants." However, they report that "at 700 ppm CO2, IE did not increase with increasing plant age despite the presence of many more young leaves in the enlarged canopy," which finding, in their words, "points to reduced pathogen efficiency or an induced partial resistance to anthracnose in Fitzroy at 700 ppm CO2." As the air's CO2 content continues to rise, therefore, it would appear that the Fitzroy cultivar of the pasture legume they studied will indeed acquire a greater intrinsic resistance to the devastating anthracnose disease.

In yet another report from the same year, which described an experiment conducted within open-top-chambers that were designed to study the response of a native scrub-oak community in central coastal Florida (USA) to a 350-ppm increase in the air's CO2 concentration, Hungate et al. (2004) tell how they measured nitrogen fixation by the leguminous vine Galactia elliotti over a period of seven years. During this time, they found that the extra CO2 "initially increased but later suppressed N fixation, contrary to the expected response." As for the reason for this anomalous behavior, they state that "reduced availability of the micro-nutrient molybdenum [Mo], a key constituent of nitrogenase, best explains this reduction in N fixation." More specifically, they note that "Mo deficiency in N-fixing plants has been documented, particularly in sandy acidic soils similar to the scrub-oak soil," and that "elevated CO2 increased nutrient accumulation in oak biomass and in organic forms in the soil, potentially reducing their availability to G. elliotti." What is more, they report that elevated CO2 did indeed substantially decrease foliar concentrations of Mo in G. elliotti.

Now a climate alarmist might suggest that these observations "caution against expecting increased biological N fixation to fuel terrestrial carbon accumulation," as Hungate et al. did. However, it is most interesting to note that, in the very same issue of Science in which Hungate et al.'s paper was published, Palmer et al. (2004) report that "fossil fuel combustion and fertilizer production have doubled the global rate of nitrogen fixation .. fertilizing remote portions of the planet," and they say that "because our planet will be over-populated for the foreseeable future and natural resource consumption shows no signs of slowing," this human modification of the environment "will only increase." Consequently, even if there may be isolated instances of CO2-induced decreases in nitrogen fixation by certain plants growing on certain soils, humanity is more than compensating for this phenomenon on a worldwide basis, in terms of supplying the extra nitrogen that will be needed to keep earth's plants sequestering ever more carbon in their tissues and the soils upon which they grow as the air's CO2 content continues to rise.

Moving on, in conjunction with the BioCON FACE experiment at Minnesota's Cedar Creek Natural History Area, West et al. (2005) studied the growth and nitrogen fixation responses of four leguminous plants grown in monoculture -- Amorpha canescens Pursh, Lespedeza capitata Mich., Lupinus perennis L., and Petalostemum villosum Nutt. -- to an extra 192 ppm of atmospheric CO2 and an extra 4 g of soil nitrogen (N, in the form of NH4NO3) per m2 per year. With respect to the proportion of N derived from the atmosphere (Ndfa), they report that "within the ambient N treatment, Amorpha and Lespedeza showed increases in Ndfa with elevated CO2, whereas Lupinus and Petalostemum showed decreases." Within the elevated N treatment, on the other hand, they report that "Amorpha Ndfa decreased with increased CO2 ... and the other three species exhibited little or no response to elevated CO2."

With respect to growth, aboveground biomass production in the ambient N treatment exhibited a hodgepodge of responses to elevated CO2, decreasing by about 30% in Amorpha, showing no effect in Petalostemum, rising by some 15% in Lupinus and skyrocketing by 250% in Lespedeza. Within the elevated N treatment, on the other hand, all responses were positive, with modest increases in Lupinus and Petalostemum (12 and 23%, respectively), a large increase in Amorpha (125%), and a monstrous increase in Lespedeza (420%). As a result of these diverse findings, West et al. concluded that "legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric CO2," and that "species identity may be an important factor controlling the response of N fixation to global change." In terms of all-important growth, there also appears to be a wide range of responses among species; but it is encouraging to see that in the face of soil nitrogen additions, such as are likely to occur in response to continued atmospheric N deposition, the four leguminous plants of this study all exhibited increases in biomass production in response to atmospheric CO2 enrichment, with some of the increases being extremely large.

One year later, Rogers et al. (2006) reported on a two-year study of soybeans grown without N fertilization from germination to senescence at the SoyFACE facility in Champaign, Illinois, USA, under fully open-air conditions. In this experiment they found that the plants growing at elevated CO2 (552 vs. 370 ppm) "had a c. 25% increase in the daily integral of photosynthesis and c. 58% increase in foliar carbohydrate content." They also say that plants of both treatments "had a low leaf N content at the beginning of the season, which was a further c. 17% lower at elevated CO2." At mid-season, however, they found that "total amino acid and N content increased markedly, and the effect of elevated CO2 on leaf N content disappeared," as the CO2-enriched plants "overcame an early-season N limitation." Last of all, the seven scientists report that the plants exposed to the extra 49% of atmospheric CO2 "showed a c. 16% increase in dry mass at final harvest and showed no significant effect of elevated CO2 on leaf N, protein or total amino acid content in the latter part of the season."

In discussing their observations, Rogers et al. remark that "one possible explanation for these findings is that N fixation had increased, and that [the] plants had acclimated to the increased N demand at elevated CO2." In addition, they say their results suggest that "in N-poor ecosystems, growth at elevated CO2 favors legumes that tend to avoid limitations of photosynthetic capacity and dominate non-leguminous species (Hanley, Trofimov and Taylor, 2004; Winkler and Herbst, 2004)," and they further suggest that "in the absence of other major limitations, field-grown soybean, and legumes in general, will show a continued increase in productivity with rising CO2."

Last of all, Tang et al. (2006) grew three C3 grasses (Poa annua L., Lolium perenne L., and Avena fatua L.), three C4 grasses (Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.), and Setaria glauca (L.) P. Beauv.), three C3 forbs (Veronica didyma Ten., Plantago virginica L., and Gnaphalium affine D.Don.), and three legumes (Vicia cracca L., Medicago lupulina L., and Kummerowia striata (Thunb.) Schindl.) from seed to maturity under well watered conditions within controlled-environment chambers (maintained at a mean atmospheric CO2 concentration of either 350 or 700 ppm) in pots containing 2.5 kg of soil that was low in extractable P content. As purely a matter of interest, this protocol revealed that the doubling of the air's CO2 concentration increased total aboveground plus belowground plant biomass by an average of 9.92% in the group of C3 grasses, 12.27% in the group of C4 grasses, 35.61% in the group of C3 forbs, and 41.48% in the group of legumes.

In light of these several findings, it should be abundantly clear that increases in the air's CO2 content will significantly stimulate the growth and nitrogen-fixing capacities of earth's legumes, and that this phenomenon should lead to a significantly increased availability of soil nitrogen in naturally-occurring ecosystems, spurring significant CO2-induced increases in community-level productivity, which is something the world is desperately going to need in the decades ahead, as the global demand for food fully doubles over the next half-century, and as ever more land and freshwater resources are taken from the world of nature and used to provide that for which humanity hungers.

References
Aidar, M.P.M., Martinez, C.A., Costa, A.C., Costa, P.M.F., Dietrich, S.M.C. and Buckeridge, M.S. 2002. Effect of atmospheric CO2 enrichment on the establishment of seedlings of jatoba, Hymenaea courbaril L. (Leguminosae, Caesalpinioideae). Biota Neotropica 2 (1): biotaneotropica.org.br/v2n1/pt/abstract?article+BN01602012002.

Campbell, B.D., Stafford Smith, D.M., Ash, A.J., Fuhrer, J., Gifford, R.M., Hiernaux, P., Howden, S.M., Jones, M.B., Ludwig, J.A., Manderscheid, R., Morgan, J.A., Newton, P.C.D., Nosberger, J., Owensby, C.E., Soussana, J.F., Tuba, Z. and ZuoZhong, C. 2000. A synthesis of recent global change research on pasture and rangeland production: reduced uncertainties and their management implications. Agriculture, Ecosystems and Environment 82: 39-55.

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.

Hanley, M.E., Trofimov, S. and Taylor, G. 2004. Species-level effects more important than functional group-level responses to elevated CO2: evidence from simulated turves. Functional Ecology 18: 304-313.

Hungate, B.A., Stiling, P.D., Dijkstra, P., Johnson, D.W., Ketterer, M.E., Hymus, G.J., Hinkle, C.R. and Drake, B.G. 2004. CO2 elicits long-term decline in nitrogen fixation. Science 304: 1291.

Lee, T.D., Reich, P.B. and Tjoelker, M.G. 2003. 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.

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 CO2. Global 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 CO2. Oecologia 116: 67-75.

Pal, M., Karthikeyapandian, V., Jain, V., Srivastava, A.C., Raj, A. and Sengupta, U.K. 2004. Biomass production and nutritional levels of berseem (Trifolium alexandrium) grown under elevated CO2. Agriculture, Ecosystems and Environment 101: 31-38.

Palmer, M., Bernhardt, E., Chornesky, E., Collins, S., Dobson, A., Duke, C., Gold, B., Jacobson, R., Kingsland, S., Kranz, R., Mappin, M., Martinez, M.L., Micheli, F., Morse, J., Pace, M., Pascual, M., Palumbi, S., Reichman, O.J., Simons, A., Townsend, A. and Turner, M. 2004. Ecology for a crowded planet. Science 304: 1251-1252.

Palta, J.A. and Ludwig, C. 2000. Elevated CO2 during pod filling increased seed yield but not harvest index in indeterminate narrow-leafed lupin. Australian Journal of Agricultural Research 51: 279-286.

Pangga, I.B., Chakraborty, S. and Yates, D. 2004. Canopy size and induced resistance in Stylosanthes scabra determine anthracnose severity at high CO2. Phytopathology 94: 221-227.

Rogers, A., Gibon, Y., Stitt, M., Morgan, P.B., Bernacchi, C.J., Ort, D.R. and Long, S.P. 2006. Increased C availability at elevated carbon dioxide concentration improves N assimilation in a legume. Plant, Cell and Environment 29: 1651-1658.

Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 1999. The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylon. Australian Journal of Plant Physiology 26: 737-774.

Tang, J., Chen, J. and Chen, X. 2006. Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil. Ecological Research 21: 664-670.

West, J.B., HilleRisLambers, J., Lee, T.D., Hobbie, S.E. and Reich, P.B. 2005. Legume species identity and soil nitrogen supply determine symbiotic nitrogen-fixation responses to elevated atmospheric [CO2]. New Phytologist 167: 523-530.

Winkler, J.B. and Herbst, M. 2004. Do plants of a semi-natural grassland community benefit from long-term CO2 enrichment? Basic and Applied Ecology 5: 131-143.

Last updated 10 September 2008