How does rising atmospheric CO2 affect marine organisms?

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Grasslands (Soil Moisture) -- Summary
As the CO2 content of the air increases, most plants exhibit modifications in various aspects of their physiology.  One common modification that occurs as a result of atmospheric CO2 enrichment is a reduction in leaf stomatal conductance.  With greater concentrations of CO2 in the air, plants don’t need to open their stomates as wide as they do at lower atmospheric CO2 concentrations to allow for the diffusion into their leaves of sufficient CO2 for photosynthesis to occur at a reasonable rate.  This phenomenon, in turn, generally leads to reduced rates of plant transpirational water loss that can result in a slower depletion of soil moisture reserves beneath CO2-enriched plants.  In this summary we thus review several papers that describe how increased atmospheric CO2 concentrations modify soil moisture contents beneath various grassland ecosystems.

In an eight-year CO2-enrichment study of a tallgrass prairie ecosystem located in Kansas, USA, Owensby et al. (1999) reported that prairie communities in open-top chambers fumigated with twice-ambient levels of atmospheric CO2 maintained greater amounts of soil moisture than communities exposed to ambient air in every year of the study.  Likewise, in an experiment lasting four years, Lutze et al. (1998) grew swards of Danthonia richardsonii at various soil nitrogen contents in glasshouses receiving atmospheric CO2 concentrations of 360 and 720 ppm.  Averaged across all nitrogen regimes, they noted that CO2-enriched swards used about 25% less water than swards grown at 360 ppm CO2, which consequently allowed the CO2-enriched microcosms to maintain greater soil water contents throughout the study.

In a two-year experiment, Arnone and Bohlen (1998) grew intact monoliths of calcareous grasslands in controlled environments fumigated with air containing 350 and 600 ppm CO2.  They found that CO2-induced reductions in transpirational water loss were likely responsible for the higher soil moisture contents observed in the CO2-enriched monoliths, which were 10 to 20% greater than those observed in monoliths exposed to ambient air.  Last of all, in a one-year study, Zaller et al. (1997) established open-top and open-bottom chambers in a species-rich grassland located near Basal, Switzerland, subjecting them to atmospheric CO2 concentrations of 350 and 600 ppm.  They reported that mean annual soil moisture contents in the CO2-enriched chambers were 10% greater than those observed in control chambers treated with air containing 350 ppm CO2.

These recent studies clearly indicate that elevated atmospheric CO2 concentrations do indeed reduce transpirational water losses, and that this phenomenon allows greater soil moisture contents to exist beneath CO2-enriched vegetation.  In addition, a trend that is evident in these studies suggests that the longer the duration of atmospheric CO2 enrichment, the greater the potential increases in community soil moisture content.  Thus, as the air’s CO2 content continues to rise, it is likely that grasslands will 1) fare better than they currently do under high temperature, 2) better cope with conditions of water stress, and 3) expand their ranges into more arid areas where low soil moisture contents currently preclude their survival.

References
Arnone III, J.A. and Bohlen, P.J.  1998.  Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2Oecologia 116: 331-335.

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.

Owensby, C.E., Ham, J.M., Knapp, A.K. and Auen, L.M.  1999.  Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2Global Change Biology 5: 497-506.

Zaller, J.G. and Arnone, J.A., III.  1997.  Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2Oecologia 111: 249-254.