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CO2 and Soil Fungi: A Powerful Combination that Helps Plants Sequester More Carbon
Writing about the potential for soils to sequester carbon and thereby help mitigate global warming, Swift (2001) notes that to be able to accomplish this objective, soils must first be stabilized and protected from the ravages of wind and running water, which can cause severe erosion leading to enormous carbon losses.  He further notes there is a good correlation between soil aggregate stability and soil organic matter content across a wide range of soil types, suggesting that whatever enhances soil stability will enhance the likelihood that carbon delivered to the soil as a consequence of plant growth and decay will stay sequestered there for the longest time possible.

In this regard, the research results of Rillig et al. (2000) are real eye-openers.  Working along a naturally-occurring gradient of atmospheric CO2 concentration in the vicinity of a CO2-emitting spring in New Zealand, they studied the effects of elevated CO2 on several properties of soil fungi that were living in a mutually beneficial or symbiotic association with the roots of plants that had been growing there for at least twenty years.  Their findings?

First of all, as the air's CO2 concentration increased by 300 ppm in going from the normal background level of 370 ppm to 670 ppm, percent root colonization by the soil fungi increased in essentially linear fashion ... and by nearly 4-fold.  Second, the total length of fungal filaments or hyphae also experienced a linear increase with increasing levels of atmospheric CO2, in this case rising by more than 3-fold in response to the 300 ppm increase in the air's CO2 concentration.  Third, root-zone concentrations of a fungal-produced protein called glomalin exhibited yet another linear increase with increasing atmospheric CO2, rising by approximately 5-fold as the air's CO2 content climbed from 370 to 670 ppm.

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, if you will, 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.

Prior to the New Zealand study with its new and novel findings, Rillig et al. (1999) had also demonstrated the existence of the same CO2-induced soil stabilization phenomenon in several multi-year experiments conducted in northern and southern California.  In all of the ecosystems he and his colleagues studied in these western US locations, they always observed increases in the mass of small soil aggregates in the portions of the ecosystems exposed to elevated CO2; and the stability of those soil aggregates was always greater than that of the soil aggregates found in the portions of the ecosystems exposed to ambient air.  In addition, in one of their studies (where six CO2 concentrations ranging from 250 to 750 ppm were employed), they reported that "glomalin concentrations followed a pattern similar to that of the small aggregate size class," i.e., both soil glomalin concentration and small soil aggregate stability increased hand-in-hand as the air's CO2 content rose across the entire concentration gradient.

So what's the bottom line?  Do these CO2-induced increases in soil-stabilizing fungal activities actually lead to real-world increases in soil carbon sequestration?  An 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 coming years and decades, the potential for soils to sequester carbon will likely prove much greater than what nearly everyone has anticipated.  Not only will the soil's capacity 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.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
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.

Swift, R.S.  2001.  Sequestration of carbon by soil.  Soil Science 166: 858-871.