Mycorrhizal fungi and grassland community structure

Relationship between mycorrhizal infection rates and the log-transformed response of species abundance to grazing.

The structuring of plant communities is complex. There are a myriad of proximal and distal factors that can influence the abundance of species. The role of mycorrhizal fungi in structuring grassland communities has always been opaque. In temperate grasslands, many of the species are dependent on arbuscular mycorrhizal fungi, yet many non-mycorrhizal species are found throughout the grasslands. Whether these non-mycorrhizal species tap unique pools or even are facilitated by the mycorrhizal species is really unknown.

Over a decade ago, Wilson and Hartnett (1998) quantified the dependence of ~100 grassland species on mycorrhizal fungi. There had never been a screening study like it. Nor has there been one since. Their work largely compared different functional groups, with the conclusion that C4 grasses are the most dependent on mycorrhizal fungi and legumes the least. The work implied that success at Konza would be dependent on the ability to utilize mycorrhizal fungi, but this was never quantified.

Recently, we've compared the screening data with actual abundances from Konza. It turns out that there is no relationship between abundance and mycorrhizal responsiveness or infection rates. As such, mycorrhizal symbioses are likely not necessary for success.

That said, mycorrhizal symbioses do determine which species perform better under certain conditions. For example, almost 25% of the variation in the response of species abundance to the presence of grazers (bison) was explained by the mycorrhizal infection rate. Grazing promoted non-mycorrhizal species. Similarly, suppression of fire promotes non-mycorrhizal species (data not shown).

In both cases, fire suppression and grazing increase the availability of nutrients relative to other resources. How to think of the role of mycorrhizal under different burning or grazing regimes is still not clear. It's easy to say that fire suppression or grazing increases nutrient availability, which decreases the need for mycorrhizal fungi. But why? Is it because they are too much of a carbon drain? Many of the high-fire, low-grazing species just do not grow at all in the absence of mycorrhizal fungi, so it is unlikely to be associated with competition for nutrients. And why would mycorrhizal responses/infection predict just the responses to grazing/fire, but not abundance overall. In contrast, we see traits like leaf tissue density--which I think of as being associated with low nutrient availability--prediction abundance across Konza, but not the responses to fire and grazing. 

How to proceed on the issue is not easy, but it's a curious pattern to line up with a number of others in understanding how grassland communities are structured.

Wilson, G. W. T. and D. C. Hartnett. 1998. Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie. American Journal of Botany 85:1732-1738.

Arbuscular mycorrhizal fungi and nitrogen acquisition

How plants acquire nitrogen is one of the fundamental questions in understanding plant adaptations to low nutrient availability. For many years, plants were understood to associate with mycorrhizal fungi in order to acquire P. Later it was understood that some types of mycorrhizal fungi (ectomycorrhizal and ericoid) acquired organic N and transferred that to the host plant. Yet, for many years it was unknown whether arbuscular mycorrhizal fungi were important in the N nutrition of plants.

Leigh, Hodge, and Fitter (2009) take a big step in showing that arbuscular mycorrhizal fungi acquire N from the soil and transfer the N to plants. Building on earlier work by Hodge, the authors grew Plantago lanceolata in microcosms with two compartments. In some, plant roots had access to the second compartment which contained 13C and 15N enriched shoots. In others, only arbuscular mycorrhizal fungi could access the compartment. The authors show that the arbuscular mycorrhizal access inorganic N derived from the shoot material and transfer a large fraction of it to the plants. In one case, ~20% of the plant’s N came from the AM fungi.

In showing the ability of AM to acquire N and transfer large amounts of it to the plant, the study is an important one. The degree to which this potential is ecologically important remains to be seen. In this study, the AM fungi had access to a large, N-rich patch that the roots did not. In nature, it doesn't seem too often that an N-starved plant find's itself on the other side of a goretex barrier from a rotting carcass. Yet, if there ever is a race for a patch, can AM fungi find it before roots? Do they proliferate and therefore compete better than roots? Can they access small, rich patches that roots cannot? The experiment by Leigh, Hodge, and Fitter is an important step in our understanding. The next experiment might be the really important step.

Leigh, Hodge, and Fitter. 2009. “Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material” New Phytologist, 181: 199-207.