Economic Value of the AM Symbiosis
(presentation at conference in Halifax, Nova Scotia, Canada)

All available evidence indicates that the symbiosis genes conferring compatible interactions between glomalean endomycorrhizal fungi and plants arose when the association was first formed over 400 million years ago and have been conserved in all subsequent plant radiations (with modern-day nonmycorrhizal plants having lost the association through geologic time). For a conference in Halifax, I was charged with devising an analysis which placed monetary value on this symbiosis worldwide. It is widely known from substantial press that one of the main benefits conferred by the mycorrhizal association is P uptake, so that the cost of P substitution in the absence of the mycorrhizal association could serve as an economic indicator of the value of the symbiosis. Defining plant cover and usage on land worldwide was a daunting task at first, but an article by Costanza et al., (1997) saved the day.

To estimate P substitution cost, I used data from a study of red clover (Morton et al., 1990) where we were trying to determine the amount of P required to obtain equivalent growth between mycorrhizal and nonmycorrhizal plants. In the figure at right (click on figure to see larger more readable image), the data are reported in soil solution P levels to be independent of soil type. Translated, low and high values were the native soil P level of ~ 6 mg/kg^-1 (none added) and 150 mg/kg^-1 added P. Since red clover is a highly mycotrophic legume, and to be conservative in my estimates, I used 150 mg/kg^-1as the high-end input (most dependent plants), 75 mg/kg^-1 for moderately dependent plants, and 50 mg/kg^-1 for weakly dependent plants. I used triple superphosphate as the substitute fertilizer of choice at a cost (in 1998) of $8 per 10 kg.

To calcuate land cover, I estimated percentage of plant cover in each of the major biomes (Costanza et al., 1997) that might consist of mycorrhizal plants. I then assigned a dependency class (from the three arbitrarily chosen above) to each biome and assigned a level of P input (and associated P cost) consistent with that class designation. The total P cost per hectare that substituted for the mycorrhizal association, then, was the product of $P/ha and adjusted hectarage. was calculated as the product of P input rate and adjusted hectarage.

From these data, a conservative estimate of P input to compensate for absence of mycorrhizal fungi in soils would be $606 billion over 9.2 billion hectares, probably each year where fixation rates are high (most soils). This estimate does not take into account the mortality of many tree and other species so dependent on the association that fertilizer applications would be ineffective regardless of levels applied (such as in many tropical areas).

The ultimate purpose of this exercise was to ascertain that the cost of INVAM was only 0.0002% of the global economic value of the arbuscular mycorrhizal association. Of course, an economist would point out that this is strictly an academic exercise and that since most of the fungi found in most habitats are not in any danger of imminent extinction, they really don't have much real market value. I can live with that, mainly because I can't imagine the real consequences if mycorrhizal fungi were to suddenly disappear from the earth. End of all life?

REFERENCES

Constanza, R., R. d'Arge, R. de Groot, S. Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, R. V. O'Neill, J. Paruelo, R. G. Raskin, P. Sutton, and M. van den Belt. 1997. The value of the world's ecosystem services and natural capital. Nature 387:253-260.

Morton, J. B., J. E. Yarger, and S. F. Wright. 1990. Soil solution phosphorus requirement for nodulation and nitrogen fixation in mycorrhizal and nonmycorrhizal red clover (Trifolium pratense L.). Soil Biology and Biochemistry 22:128-129.