What was done
The author reviews what is known about desert biological soil crusts, which she describes as communities of cyanobacteria, eucaryotic green algae, microfungi, lichens and mosses that live on, or just beneath, the surface of desert sands.

What was learned
Belnap reports that biological soil crusts "are vital in creating and maintaining fertility in otherwise infertile desert soils," noting that "they fix both carbon and nitrogen, much of which is leaked to the surrounding soils," and that "they also capture nutrient-rich dust, and can stimulate plant growth."

Expanding on these statements, Belnap says that captured dust "can augment the levels of most plant-essential nutrients, including nitrogen, phosphorus, and potassium, up to fourfold (Reynolds et al., 2001)," and that "this increases both the fertility and water-holding capacity of soils (Verrecchia et al., 1995)."  Also, because biological soil crusts "hold water longer, enhancing mineral dissolution and freeze-thaw action," she notes that "these factors can accelerate bedrock weathering rates up to 100 times (Schwartzman and Volk, 1989)."

Belnap also reports that "polysaccharides extruded by crust organisms bind soil particles together into aggregates critical to soil functioning," noting that "aggregate surfaces are where most soil biota and nutrient transformations occur, and aggregates increase soil aeration, infiltration (Herrick and Wander, 1998), and resistance to soil erosion (McKenna-Neuman et al., 1996)."  Last of all, Belnap states that "multiple field studies show that germination and survival of native plants is either higher or unaffected in biologically crusted, as compared with uncrusted, areas," and that "several studies in both Australia and the US indicate that soil crusts can slow the germination of exotic annual [invasive] grasses (Belnap et al., 2001)."

What it means
What do these facts have to do with the ongoing rise in the air's CO2 content?  Actually, quite a lot.  Belnap states, for example, that "elevated atmospheric CO2 levels may increase crust primary production," noting that "soil lichen photosynthesis is CO2 limited."  This increase in photosynthesis would then tend to amplify and further enhance the many important ecosystem services enumerated above that are provided by soil biological crusts.

That Belnap is indeed correct in her presumption is demonstrated by the research findings of Tuba et al. (1998) and Brostoff et al. (2002).  Hence, we have reason to expect that large portions of earth's deserts will gradually be reclaimed and made more productive by the stimulatory impact of the ongoing rise in the air's CO2 content on the growth and development of desert biological soil crusts, which in turn will make arid environments much more supportive of higher plant lifeforms.

References
Belnap, J., Prasse, R. and Harper, K.T.  2001.  Influence of biological soil crusts on soil environments and vascular plants.  In: Belnap, J. and Lange, O.L. (Eds.), Biological Soil Crusts: Structure, Function, and Management.  Springer-Verlag, Berlin, Germany, pp. 281-300.

Brostoff, W.N., Sharifi, M.R. and Rundel, P.W.  2002.  Photosynthesis of cryptobiotic crusts in a seasonally inundated system of pans and dunes at Edwards Air Force Base, western Mojave Desert, California: laboratory studies.  Flora 197: 143-151.

Herrick, J.E. and Wander, M.M.  1998.  Relationships between soil organic carbon and soil quality in cropped and rangeland soils: the importance of distribution, composition and soil biological activity.  In: Lal, R., Kimble, J., Follett, R. and Stewart, B.A. (Eds.), Advances in Soil Science: Soil Processes and the Carbon Cycle.  CRC Press, Boca Raton, FL, USA, pp. 405-425.

McKenna-Neuman, C., Maxwell, C.D. and Boulton, J.W.  1996.  Wind transport of sand surfaces crusted with photoautotrophic microorganisms.  Catena 27: 229-247.

Reynolds, R., Belnap, J., Reheis, M., et al.  2001.  Aeolian dust in Colorado Plateau soils: nutrient inputs and recent change in source.  Proceedings of the National Academy of Sciences 98: 7123-7127.

Schwartzman, D.W. and Volk, T.  1989.  Biotic enhancement of weathering and the habitability of Earth.  Nature 340: 457-460.

Tuba, Z., Csintalan, Z., Szente, K., Nagy, Z. and Grace, J.  1998.  Carbon gains by desiccation-tolerant plants at elevated CO2.  Functional Ecology 12: 39-44.

Verrecchia, E., Yair, A., Kidron, G.J. and Verrecchia, K.  1995.  Physical properties of the psammophile cryptogamic crust and their consequences to the water regime of sandy soils, northwestern Negev, Israel.  Journal of Arid Environments 29: 427-437.