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Mitigating Soil Salinity Stress by Atmospheric CO2 Enrichment
Volume 14, Number 36: 7 September 2011

Writing in the Pakistan Journal of Botany a few years back, Azam et al. (2005) stated that "in agro-ecosystems, green manuring legumes occupy a key position in maintaining/improving soil fertility and productivity," noting that the important role of these plants as a source of nitrogen has increased further due to economic and pollution concerns associated with nitrogen supplied by chemical fertilizers. And they state, in this regard, that "species of sesbania have generally been considered as most important for green manuring, especially in wheat-rice rotation systems."

In many situations, however, the growth of sesbania is negatively impacted by varying degrees of water stress and salinity; but the five Pakistani scientists remind us that "elevated CO2 favors different physiological processes of plants, thereby leading to increased biomass production and ecosystem functioning," citing the studies of Drake and Leadley (1991), Idso and Idso (1994) and Azam and Farooq (2001). And they report that this effect is more pronounced for plants facing stresses imposed through the soil or atmosphere, citing the collection of papers compiled and edited by Koch and Mooney (1996). Thus, they go on to explore the possibility that rising atmospheric CO2 concentrations might mitigate salinity stress in sesbania, enabling the rotation cover-crop to more effectively "fix" atmospheric nitrogen and deposit the plant-usable form of it in the soil, where it can help promote the growth of such important agricultural staples as wheat and rice.

Conducting greenhouse experiments designed to assess the effects of elevated atmospheric CO2 concentrations on growth and nitrogen fixation in Sesbania aculeata exposed to different salinity and water regimes, Azam et al. determined that "elevated CO2 favored N2 fixation leading to a greater contribution of fixed N to the total plant N." In addition, they report that "biological nitrogen fixation decreased with salinity," but they say that "elevated CO2 arrested the decrease to a significant extent."

These findings, in their words, "are in conformity with those of Yu et al. (2002), who showed stimulation of symbiotic N2 fixation at higher levels of CO2," as well as those of Zanetti et al. (1997), who reported that "the total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in Trifolium repens under elevated CO2." And they add that in studies of several different kinds of plants, Luscher et al. (1998) "found legumes to be the most responsive to elevated CO2."

In concluding, the five researchers say "it is possible, therefore, to enhance the biomass yield of this green manuring crop [sesbania] by elevating the level of CO2 in the plant canopy." One low-tech way of doing so is suggested by their statement that "plant residues decomposing on the soil surface following mulching may help elevate the level of CO2 and thus the plant growth," while we suggest that a longer-term strategy would be for the nations of the earth to not overtly strive to reduce anthropogenic CO2 emissions associated with the burning of fossil fuels; for CO2 is "the elixir of life."

Sherwood, Keith and Craig Idso

References
Azam, F., Aziz, F., Sial, M.H., Ashraf, M. and Farooq, S. 2005. Mitigation of salinity effects on Sesbania aculeata L., through enhanced availability of carbon dioxide. Pakistan Journal of Botany 37: 959-967.

Azam, F. and Farooq, S. 2001. Impact of elevated atmospheric CO2 on crop plants -- an overview. Pakistan Journal of Biological Sciences 4: 220-22.

Drake, B.G. and Leadley, P.W. 1991. Canopy photosynthesis of crops and native plant communities exposed to long-term elevated CO2. Plant, Cell & Environment 14: 853-860.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.

Koch, G.W. and Mooney, H.A. (Eds.) 1996. Carbon Dioxide and Terrestrial Ecosystems. Academic Press, San Diego, California, USA.

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.

Yu, M., Gao, Q. and Shaffer, M.J. 2002. Stimulating interactive effects of symbiotic nitrogen fixation, carbon dioxide elevation and climatic change on legume growth. Journal of Environmental Quality 31: 634-641.

Zanetti, S., Hartwig, U.A., Luscher, A., Hebison, T., Frehner, M., Fischer, B.U., Hendry, G.R., Blum, H. and Nosberger, J. 1997. Stimulation of symbiotic N2 fixation in Trifolium repens L. under elevated atmospheric pCO2 concentration in a grassland ecosystem. Plant Physiology 112: 575-583.