Summary• We studied the relationships of plant and AMF (arbuscular mycorrhizal fungi) species richness and community composition to each other and gradients in soil texture, nutrient content, and light availability in three oak savannas in southern Wisconsin, USA.• Sixty-three samples were analysed for plant and AMF composition along sun-shade and sand-loam gradients. Samples consisted of plant community composition at quadrat and point scales, point-scale AMF community composition, canopy transmittance, soil Kjeldahl nitrogen (N) content, available phosphorus (P), and texture.• Numbers of AMF and plant species at the point scale were positively correlated with each other and increased with soil texture/N content. The compositions of plant and AMF communities were also significantly correlated with each other and the soils gradient. The paradoxical increase in AMF richness on the most fertile soils may reflect their small soil pores or low P : N ratios.• Plant and AMF communities appear to respond to underlying environmental gradients in similar ways, perhaps reflecting similar responses to soil conditions by both groups, effects of one group on the other or both.

Summary• Existing models of nutrient transfer in arbuscular mycorrhizal (AM) symbioses are inadequate as they do not explain the range of real responses seen experimentally. A computer simulation model was used to evaluate the novel hypotheses that mycorrhizal nutrient transfers were based solely on symbionts' internal needs, and that carbon and phosphorus transfers were quantitatively unlinked. To be plausible, simulated mycorrhizal plants would show a ±50% variation in weight vs nonmycorrhizal controls, with a normal response distribution (mimicking a real data set).• One plant and one arbuscular mycorrhizal fungus (AMF) growing in a soil volume were simulated, using C, P and nitrogen nutrient cycling and stoichiometry. C-and P-exchange rates were independent and could be varied at will. The model was tested at realistic nutrient concentrations and a full range of nutrient exchange rates.• The model showed -20% to +55% range in mycorrhizal plant weight distributed close to normal, suggesting that the hypotheses were plausible.• The model suggests that theoretical assumptions about mycorrhizas should be reassessed. The model worked only because the symbionts possessed incomplete information on their partner and environmental conditions. Conventional cost-benefit models do not work under these circumstances, but both mutualistic and parasitic interactions were successfully simulated.

Oligonucleotide microarray based on ITS2 rDNA sequences would be extremely useful in identifying fungi within soil samples. However ITS2 contains phylogenetic information and duplication of sequences among taxa make false positive detections likely unless a way could be found to identify taxon-specific portions of the ITS2 sequence a priori. Examination of component ITS2 sequences suggested one method of identifying species-specific probes. Analysis of 168 fungal ITS2 sequences showed that all 168 ITS2 rRNA sequences could be folded to produce similar secondary structures of 3-4 loops. Unique probes occurred most often in the second loop. While the loop 2 sequence was unique in all taxa, there were partial congeneric and intergeneric duplicates. Evidence for a decrease in duplicates with increasing phylogenetic distance was mixed. From the evidence, 2 or 3 disjunct oligonucleotide probes from the loop 2 sequence might be sufficient to identify most fungal species. This combination appears minimally susceptible to false positives and conceivably could be extended to design probes to identify any eukaryotic species.

To explain the complex community composition found in Wisconsin (USA) oak savannas, we investigated potentially interacting effects of light gradients and arbuscular mycorrhizal fungi (AMF) on community composition in the greenhouse, using a fully randomized block experimental design. We used plant species, soil, and AMF from a remnant sand savanna in setting up the experiment, using two light and five AMF treatments. Eleven plant species were seeded into 80 microcosms, and they were grown together for 20 weeks. Plant numbers and biomass were measured, and Simpson's index was calculated for both. Data were analyzed using ANOVA and nonparametric ANOVA. We found significant light effects on biomass and on numbers of four species. There were no treatment effects on Simpson's index, and only Schizachyrium numbers showed a significant AMF effect. These findings are consistent with results from other studies of the sand savanna, and, collectively, these data suggest that plant community composition in this species-rich savanna is not strongly influenced by arbuscular mycorrhizae. This is a novel finding with important implications for understanding interactions between plant and AMF diversity in wild communities.

Wisconsin (USA) oak savannas are endangered plant communities that have remarkably high plant species diversity. To investigate factors underlying this richness, we experimentally investigated the potentially interacting effects of light gradients and arbuscular mycorrhizal fungi (AMF) on plant competition in the greenhouse, using a fully randomized block design. We used four plant species, soil, and AMF from a remnant sand savanna, under two light and five AMF treatments. Plants were grown four per pot under two competition treatments (either one or four species per pot) for 20 weeks. Using ANOVA, we found that all species showed significant treatment effects on total and shoot biomass, primarily due to differences in competition and light, less to AMF. However, effects were the opposite of predictions. Putatively mycorrhizal plants showed neutral to negative responses to AMF, and a nonmycorrhizal species outcompeted AMF species in infected pots. We concluded that our experimental setup of small pots, sandy soil, and long growing period had induced parasitism by the AMF on susceptible hosts. This unexpected result is consistent with field data from the sand savanna, and may help explain how nonmycorrhizal plants can compete successfully with AMF species in established, species-rich communities.