How does atmospheric CO2 enrichment affect the water status of soils upon which plants are growing? In a review that broaches this subject, albeit peripherally, Wullschleger et al. (2002) note that plants growing in air of elevated CO2 concentration typically display reductions in leaf stomatal conductance that can lead to reductions in plant transpirational water loss. This response, in turn, commonly results in CO2-enriched plants exhibiting higher (less negative and, therefore, less stressful) leaf water potentials than plants growing in ambient air, which is indicative of the better overall plant water status enjoyed by CO2-enriched vegetation and is further suggestive of the likelihood that soil moisture contents in a CO2-enriched world of the future might well be somewhat greater than those of today.
Insam et al. (1999) conducted a study that suggests much the same thing, but for a totally different reason. They constructed artificial tropical ecosystems composed of seven C3 species of plants growing upon nutrient-poor soils and maintained them at atmospheric CO2 concentrations of 340 and 610 ppm for 530 days. At the end of that time, they found that the elevated CO2 had increased the presence of humic substances in the soils by close to 30%; and as humic substances are known to significantly increase soil water-holding capacity, this finding also suggests that soils in a future CO2-enriched world may hold more moisture than they do today.
A number of experiments have approached the subject more directly, actually measuring the water contents of soils supporting plants growing in both ambient and CO2-enriched air; and we will now review some of their findings, beginning with those derived from laboratory studies.
Seneweera et al. (2001) grew the C4 grass Panicum coloratum var. makarikiense cv. Bambatsi in environmental chambers receiving atmospheric CO2 concentrations of 360 and 1000 ppm for three weeks before withholding water from half of the plants at each CO2 concentration for a period of ten days. During this time of developing water stress, leaf water potentials and leaf relative water contents dropped at much slower rates and to a lesser degree in the CO2-enriched plants than in the plants growing in ambient air. It was also observed that the transpiration rates of the CO2-enriched plants were much less than those displayed by the ambient-treatment plants; and this phenomenon led to greater moisture contents in the soils beneath the CO2-enriched plants throughout the entire dry-down period.
Volk et al. (2000) grew plant assemblages similar to those of calcareous grasslands of northwest Switzerland in controlled environment chambers maintained at atmospheric CO2 concentrations of 360 and 600 ppm for a period of three months. In addition, the plants were subjected to four irrigation regimes in order to study the interactive effects of elevated CO2 and soil moisture content on plant growth. They found that the elevated CO2 consistently reduced plant stomatal conductances, regardless of irrigation regime. Consequently, the plant assemblages grown at 600 ppm CO2 always exhibited higher soil moisture contents than the control assemblages grown in ambient air. In fact, this effect was so pronounced that the scientists inadvertently generated eight, instead of four, irrigation regimes. Furthermore, it was found that the CO2-induced increases in biomass production they observed were augmented by the extra growth that resulted from the CO2-induced enhancements of soil moisture content.
Grunzweig and Korner (2001) constructed model grasslands representative of the semi-arid Negev of Israel and placed them within growth chambers maintained at atmospheric CO2 concentrations of 280, 440 and 600 ppm for a period of five months. They found that the extra CO2 reduced evapotranspiration rates and increased soil moisture contents in the elevated-CO2 treatments. Between two periods of imposed drought, for example, soil moisture contents were 22 and 27% higher in the communities exposed to 440 and 600 ppm CO2, respectively, than they were in the control communities exposed to pre-industrial levels of atmospheric CO2.
Arnone and Bohlen (1998) grew intact grass monoliths removed from a species-rich calcareous grassland in northwestern Switzerland and exposed them to atmospheric CO2 concentrations of 350 and 600 ppm for two full growing seasons to study the effects of elevated CO2 on various ecosystem processes. They found that the monoliths exposed to elevated CO2 exhibited soil moisture contents that were 10 to 20% greater than those exposed to ambient CO2 concentrations, most likely due to reductions in transpirational water loss facilitated by CO2-induced decreases in foliar stomatal conductance. In turn, soil microbial activity nearly doubled with the increase in soil moisture, which typically leads to enhanced mineralization of nutrients and increases their availability to plants. Thus, this chain of events likely further stimulated plant growth in these ecosystems by augmenting the primary growth response provided by the aerial fertilization effect of the increase in the air's CO2 content.
Last of all, Lutze and Gifford (1998) grew microcosms of the C3 grass Danthonia richardsonii for four years in glasshouses maintained at atmospheric CO2 concentrations of 360 or 720 ppm and three levels of soil nitrogen. They found that the elevated CO2 reduced microcosm water use by 25% across all nitrogen treatments, thereby allowing greater soil water contents to exist in the CO2-enriched microcosms. The scientists note that such CO2-induced soil water content increases may have "important implications for microcosm, and potentially ecosystem, function perhaps as important as those of CO2 directly on photosynthesis." In fact, in analyzing the smallest microcosm carbon gain (15% at low nitrogen), they suggest that if all terrestrial ecosystems responded to elevated CO2 in a similar way, this phenomenon would account for all of the "missing carbon" depicted in most global carbon cycle models.
In conclusion, this group of growth chamber studies suggests that the anti-transpirant effect of atmospheric CO2 enrichment should lead to increased soil water contents in many real-world ecosystems, which phenomenon would be expected to enhance the direct growth-increasing effect of atmospheric CO2 enrichment. Confirmation of these implications is provided by a set of field experiments we will be summarizing in next week's issue.
References
Arnone, J.A., III and Bohlen, P.J. 1998. Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2. Oecologia 116: 331-335.
Grunzweig, J.M. and Korner, C. 2001. Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2. Oecologia 128: 251-262.
Insam, H., Baath, E., Berreck, M., Frostegard, A., Gerzabek, M.H., Kraft, A., Schinner, F., Schweiger, P. and Tschuggnall, G. 1999. Responses of the soil microbiota to elevated CO2 in an artificial tropical ecosystem. Journal of Microbiological Methods 36: 45-54.
Lutze, J.L. and Gifford, R.M. 1998. Carbon accumulation, distribution and water use of Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth. Global Change Biology 4: 851-861.
Seneweera, S., Ghannoum, O. and Conroy, J.P. 2001. Root and shoot factors contribute to the effect of drought on photosynthesis and growth of the C4 grass Panicum coloratum at elevated CO2 partial pressures. Australian Journal of Plant Physiology 28: 451-460.
Volk, M., Niklaus, P.A. and Korner, C. 2000. S oil moisture effects determine CO2 responses of grassland species. Oecologia 125: 380-388.
Wullschleger, S.D., Tschaplinski, T.J. and Norby, R.J. 2002. Plant water relations at elevated CO2 - implications for water-limited environments. Plant, Cell and Environment 25: 319-331.