What's new in the world of belowground impacts of the ongoing rise in the air's CO2 content? How about the ever-increasing production of a protein that is created by fungi that live in symbiotic association with the roots of 80% of the planet's vascular plants, which is being released to almost every soil in the world in ever greater quantities with the passage of time, and that is working ever greater wonders with a variety of processes that benefit the biosphere? It certainly has our vote as something up there near the top of the list of highly-desired biological phenomena; so let us explain how and why it works.
In a multi-faceted research program carried out at experimental sites in northern and southern California, USA, Rillig et al. (1999) studied belowground ecosystem responses to elevated atmospheric CO2 concentrations over a period of several years, focusing their attention on arbuscular mycorrhizal fungi (AMF) that form symbiotic associations with plant roots. In addition, they measured soil concentrations of an AMF-produced glycoprotein called glomalin and evaluated its response to elevated CO2, after which they evaluated the impact of glomalin on the formation of small soil aggregates and their subsequent stability.
Rillig et al.'s reason for making these multiple measurements derives from the fact that the degree of soil aggregation and the stability of soil aggregates across many different soil types is closely related to the amount of glomalin in the soil; and they wanted to see if the aboveground benefits of atmospheric CO2 enrichment would trickle down, so to speak, from plant leaves to plant roots to symbiotic soil fungi to glomalin production to soil aggregate formation and, ultimately, to an enhanced stability of soil aggregates in the presence of water.
The researchers' plan paid off. The amount of fungal-produced glomalin in the soils of the CO2-enriched treatments in all three of the ecosystems they studied was greater than that observed in the soils of corresponding ambient CO2 treatments. They also observed increases in the mass of small soil aggregates in the treatments exposed to elevated CO2; and the stability of the small soil aggregates in the CO2-enriched treatments was greater than the stability of the aggregates in the ambient CO2 treatments. In addition, in one of their studies, where six CO2 concentrations ranging from 250 to 750 ppm were imposed as treatments, they found that "the proportion of soil mass in aggregates of 0.25-1 mm showed a linear increase along the CO2 gradient," and that "glomalin concentrations followed a pattern similar to that of the small aggregate size class," indicative of ever-increasing soil structure benefits with ever-increasing concentrations of atmospheric CO2.
In a subsequent study conducted in New Zealand, Rillig et al. (2000) examined several characteristics of AMF associated with the roots of plants that had been growing for at least 20 years along a natural CO2 gradient near a CO2-emmitting spring. They found that the elevated CO2 significantly increased percent root colonization by AMF in a linear fashion - and by nearly 4-fold! - in going from 370 to 670 ppm. In addition, fungal hyphal length experienced a linear increase of over 3-fold along the same CO2 gradient, while total soil glomalin experienced a linear increase of approximately 5-fold.
What are the implications of these observations? First of all, just as more and longer roots help plants hold soil together and prevent its erosion, so too do more and longer fungal hyphae protect soil from disruption and dispersion. In addition, fungal-produced glomalin acts like a biological glue, helping to bind tiny particles of soil into small aggregates that are much more difficult to break down and blow or wash away. And to have soil glomalin concentrations increase by fully 5-fold as a consequence of less than a doubling of the air's CO2 content is a truly mind-boggling benefit.
These observations lead one to wonder if CO2-induced increases in soil-stabilizing fungal activities might lead to increases in soil carbon sequestration. A potential answer comes from another study conducted near a natural CO2 vent in New Zealand, where Ross et al. (2000) measured soil carbon (C) and nitrogen (N) contents in areas exposed to atmospheric CO2 concentrations on the order of 440 to 460 ppm and other areas exposed to concentrations on the order of 510 to 900 ppm.
In this study, it was determined that several decades of differential atmospheric CO2 exposure had increased soil organic C and total N contents by approximately 24% each, while it had increased microbial C and N contents by more than 100% each. Hence, in the words of the scientists who did the work, "storage of C and N can increase under prolonged exposure to elevated CO2." In addition, they concluded that increased storage of soil organic matter can occur "even when soil C concentrations are already high," as they were in the situation they investigated.
Consequently, as the air's CO2 content continues to rise over the years and decades ahead, the potential for soils to sequester carbon will likely prove much greater than what nearly everyone had previously anticipated. Not only will the capacity of soils to store carbon grow ever larger due to the ever-increasing aerial fertilization effect of atmospheric CO2 enrichment - which enhances plant growth and results in more carbon being transferred to the soil - it will also grow ever larger as increasingly active soil fungi help to keep ever greater portions of that carbon better preserved in increasingly more stable soils.
Yet augmented soil carbon sequestration is but the beginning of benefits that can be expected to be provided by CO2-enhanced AMF growth and glomalin production. In their report of a FACE study of sorghum conducted near Phoenix, Arizona, USA, for example, where it was found that an approximate 50% increase in the air's CO2 content increased fungal hyphae lengths by 120% and 240% in wet and dry irrigation treatments, respectively, with the mass of water-stable soil aggregates increasing by 40% and 20% in the same respective treatments, Rillig et al. (2001) noted that "soil structure and water-stable aggregation are crucial for facilitating water infiltration, soil-borne aspects of biogeochemical cycling processes, success of sustainable agriculture, and for providing resistance against erosional loss of soil (Oades, 1984; Elliott and Coleman, 1988; Van Veen and Kuikman, 1990; Bethlenfalvay and Lindermann, 1992; Daily, 1995; Arshad et al., 1996; Coleman, 1996; Jastrow and Miller, 1997; Young et al., 1998)."
In addition to these benefits, Gonzalez-Chavez et al. (2004) report that "glomalin participates in the sequestration of different PTEs [potentially toxic elements]," that "the glomalin pool in the soil may have a potential to sequestrate PTEs, not only by the colonized roots, but also by the hyphae and through deposition of glomalin in soil," and that "this glycoprotein may be stabilizing PTEs, reducing PTE availability and decreasing the toxicity risk to other soil microorganisms and plants." That these benefits have enormous significance is vouchsafed by the fact, to quote them again, that "glomalin is ... copiously produced by all AMF tested to date (Wright et al., 1996, 1998; Nichols, 2003)," that "AMF colonize 80% of vascular plant species (Trappe, 1987)," and that AMF "are found worldwide in almost every soil."
In light of these many observations, it should be evident that the ongoing rise in the air's CO2 content must be ever so subtly having a tremendous positive impact on the biosphere via a suite of mechanisms linked to a fungal-produced protein that only a decade ago was largely unknown - even to most plant and soil scientists - and similarly unappreciated.
Arshad, M.A., Lowery, B. and Grossman, B. 1996. Physical tests for monitoring soil quality. In: Methods for Assessing Soil Quality, SSSA Special Publication 49. Soil Science Society of America, Madison, Wisconsin, USA, pp. 123-141.
Bethlenfalvay, G.J. and Linderman, R.G. 1992. Mycorrhizae in Sustainable Agriculture. ASA Special Publication 54. American Society of Agronomy, Madison, Wisconsin, USA.
Coleman, D.C. 1996. Fundamentals of Soil Ecology. Academic Press, San Diego, California, USA.
Daily, G.C. 1995. Restoring value to the world's degraded lands. Science 269: 350-354.
Elliott, E.T. and Coleman, D.C. 1988. Let the soil work for us. Ecological Bulletin 39: 23-32.
Gonzalez-Chavez, M.C., Carrillo-Gonzalez, R., Wright, S.F. and Nichols, K.A. 2004. The role of glomalin, a protein produced by arbuscular mycorrhizal fungi, in sequestering potentially toxic elements. Environmental Pollution 130: 317-323.
Jastrow, J.D. and Miller, R.M. 1997. Soil aggregate stabilization and carbon sequestration: feedbacks through organomineral associations. In: Lal, R. et al., Eds. Soil Processes and the Carbon Cycle. CRC Press, Boca Raton, Florida, USA, pp. 207-223.
Nichols, K. 2003. Characterization of Glomalin - A Glycoprotein Produced by Arbuscular Mycorrhizal Fungi. PhD Dissertation, University of Maryland, College Park, Maryland, USA.
Oades, J.M. 1984. Soil organic matter and structural stability: mechanisms and implications for management. Plant and Soil 76: 319-337.
Rillig, M.C., Hernandez, G.Y. and Newton, P.C.D. 2000. Arbuscular mycorrhizae respond to elevated atmospheric CO2 after long-term exposure: evidence from a CO2 spring in New Zealand supports the resource balance model. Ecology Letters 3: 475-478.
Rillig, M.C., Wright, S.F., Allen, M.F. and Field, C.B. 1999. Rise in carbon dioxide changes soil structure. Nature 400: 628.
Ross, D.J., Tate, K.R., Newton, P.C.D., Wilde, R.H. and Clark, H. 2000. Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring. Global Change Biology 6: 779-790.
Trappe, J.M. 1987. Phylogenetic and ecological aspects of mycotrophy in the angiosperms from an evolutionary standpoint. In: Safir, G.R. (Ed.). Ecophysiology of VA Mycorrhizal Plants. CRC Press, Boca Raton, Florida, USA, pp. 5-25.
Van Veen, J.A. and Kuikman, P.J. 1990. Soil structural aspects of decomposition of organic matter by micro-organisms. Biogeochemistry 11: 213-233.
Wright, S.F., Franke-Snyder, M., Morton, J.B. and Upadhyaya, A. 1996. Time-course study and partial characterization of a protein on hyphae of arbuscular mycorrhizal fungi during active colonization of roots. Plant and Soil 181: 193-203.
Wright, S.F., Upadhayaya, A. and Buyer, J.S. 1998. Comparison of N-linked oligosaccharides of glomalin from arbuscular mycorrhizal fungi and soils by capillary electrophoresis. Soil Biology and Biochemistry 30: 1853-1857.
Young, I.M., Blanchart, E., Chenu C. et al. 1998. The interaction of soil biota and soil structure under global change. Global Change Biology 4: 703-712.
Last updated 25 January 2006