Soil Chemical Analysis

High variability in plant-growth response to the presence of different mycorrhizal fungi can be a major determinant of local plant species diversity. Multiple species of arbuscular mycorrhizal fungi can coexist in terrestrial ecosystems, and co-occurring plants can differ in their response to colonization by these different fungi. However, the range of mycorrhizal plant-growth responses that can occur within communities has not been determined. In the present study, I crossed a large number of plant and fungal species that co-occur to determine the range of responses that can exist within an ecosystem. I also crossed exotic fungal isolates vs. local plant isolates and local fungal isolates vs. exotic plant isolates to determine whether the range of plant growth responses differs when using foreign genotypes. The data indicate that plant growth responses to mycorrhizal inoculation within an ecosystem can range from highly parasitic to highly mutualistic. In this study, the direction and magnitude of the response depended on the combination of plant and fungal species. No plant did best with the same fungal isolate. The range of responses was greatest when using local plants and fungi. Whereas parasitic and mutualistic responses were also detected when using foreign plant or fungal genotypes, the range of responses was significantly reduced, as was the relative frequency of positive responses. Overall, this study suggests that, within ecosystems, arbuscular mycorrhizal fungi can function along a continuum from parasitism to mutualism, and that extreme responses are more common when using locally adapted plants and fungi. This high variation in plant growth response may be a large contributor to plant species coexistence and the structure of plant communities.

Variation in plant response to native and exotic arbuscular mycorrhizal fungi

Understanding how terrestrial ecosystems function requires a combined aboveground–belowground approach, because of the importance of feedbacks that occur between herbivores, producers, and the decomposer subsystem. In this paper, we identify several mechanisms by which herbivores can indirectly affect decomposer organisms and soil processes through altering the quantity and quality of resources entering the soil. We show that these mechanisms are broadly similar in nature for both foliar and root herbivory, regardless of whether they operate in the short term as a result of physiological responses of individual plants to herbivore attack or long-term following alteration of plant community structure by herbivores and subsequent changes in the quality of litter inputs to soil. We propose that a variety of possible mechanisms is responsible for the idiosyncratic nature of herbivore effects on soil biota and ecosystem function; positive, negative, or neutral effects of herbivory are possible depending upon the balance of these different mechanisms. However, we predict that positive effects of herbivory on soil biota and soil processes are most common in ecosystems of high soil fertility and high consumption rates, whereas negative effects are most common in unproductive ecosystems with low consumption rates. The significance of multiple-species herbivore communities is also emphasized, and we propose that if resource use complementarity among herbivore species or functional groups leads to greater total consumption of phytomass, and thus greater net herbivory, then both positive and negative consequences of increasing herbivore diversity for belowground properties and processes are theoretically possible. Research priorities are highlighted and include a need for comparative studies of herbivore impacts on above- and belowground processes across ecosystems of varying productivity, as well as a need for experimental testing of the influence of antiherbivore defense compounds on complex multitrophic interactions in the rhizosphere and the significance of multiple herbivore species communities on these plant–soil interactions.

Herbivore-mediated linkages between aboveground and belowground communities

Publisher Summary This chapter discusses the ecological implications of mycorrhizal associations in natural ecosystems and the role of soil or environmental factors, mycorrhizal fungus characteristics or host plant properties. Mycorrhizal associations are regulated by features of the host plant and mycorrhizal fungus, as well as by soil conditions and environmental factors. Factors which can influence the occurrence and effectiveness of mycorrhizal associations include (1) root properties (2) edaphic or climatic factors (3) soil organisms, (4) soil disturbance, and (5) host-fungus compatibility. Several other complex ecological topics also discussed in the chapter include (1) mycorrhizal phenology , (2) factors responsible for varying degrees of mycorrhizal dependency in host plants, (3) the role of mycorrhizal hyphae in soil (4) nutrient competition involving mycorrhizal and non-mycorrhizal plants, and (5) mycorrhizal interactions involving pollution and other stresses, the rhizosphere, soil properties and allelopathy. Finally, the chapter also discusses the population ecology of mycorrhizal fungi and the influence of their associations on plant population ecology.

Mycorrhizas in Natural Ecosystems

Almost all natural plant communities contain arbuscular mycorrhizal fungi (AMF). We hypothesized that the species composition of AMF communities could have the potential to determine plant community structure if the growth response to different AMF species or to communities of AMF species varies among plant species. To test the existence of such a differential response we conducted a pot experiment where each of three plant species, Hieracium pilosella, Bromus erectus, and Festuca ovina were inoculated with each of four AMF species, or with a mixture of these four AMF species, or were uninoculated. The AMF species originated from a calcareous grassland in which the three plant species also coexisted. We obtained three pieces of evidence suggesting that AMF have the potential to de- termine plant community structure. First, plant species differed in their dependency on AMF, thus varying in degree of benefit received. Second, specific AMF species and a mixture of these AMF species had significantly different effects on several plant growth variables, and these effects were not the same on each plant species. Third, the amount of variation in the growth response of a plant species to four AMF species and to the mixture of AMF species differed among the plant species. Hieracium differed greatly in its growth response to several AMF species while Bromus did not exhibit much variation in its response to different AMF species. The varying mycorrhizal dependency of different plant species has previously been proposed as a mechanism determining plant community structure. However, we found that the mycorrhizal dependency of a plant species can vary greatly because of differential growth responses to specific AMF species compared to the growth of the uninoculated plants. Consequently mycorrhizal dependency, as a measure indicating how much a plant depends on AMF for its growth, is not necessarily a fixed value and therefore cannot be used as a definitive term. In addition, those plant species with highly variable responses to single AMF species or to combinations of AMF species (AMF communities) will be strongly affected by the specific species of AMF that occupy their roots, in contrast to plant species that do not respond differently to different AMF species. We conclude that, through their differential effects on plant growth, AMF species that co-occur as natural AMF communities have the potential to determine plant community structure, and that future studies on plant population and community structure need to consider the strength of their role as a determinant.

Different arbuscular mycorrhizal fungal species are potential determinants of plant community structure

Sorghum [Sorghum bicolor (L.) Moench] plants were grown in growth chambers at 20, 25 and 30°C in a low P Typic Argiudoll (3.65 µg P g−1 soil, pH 8.3) inoculated with Glomus fasciculatum, Glomus intraradices, and Glomus macrocarpum to determine effects of vesicular-arbuscular mycorrhizal fungi (VAMF) species on plant growth and mineral nutrient uptake. Sorghum root colonization by VAMF and plant responses to Glomus species were temperature dependent. G. macrocarpum colonized sorghum roots best and enhanced plant growth and mineral uptake considerably more than the other VAMF species, especially at 30°C. G. fasciculatum enhanced shoot growth at 20 and 25°C, and mineral uptake only at 20°C. G. intraradices depressed shoot growth and mineral uptake at 30°C. G. macrocarpum enhanced shoot P, K, and Zn at all temperatures, and Fe at 25 and 30°C above that which could be accounted for by increased biomass. Sorghum plant growth responses to colonization by VAMF species may need to be evaluated at different temperatures to optimize beneficial effects.

Effects of species of VA-mycorrhizal fungi on growth and mineral uptake of sorghum at different temperatures

Sorghum [Sorghum bicolor (L.) Moench] was grown in a greenhouse in a low P (3.6 mg kg−1) soil (Typic Argiudolls) inoculated with the vesicular-arbuscular mycorrhizal fungi (VMAF) Glomus fasciculatum and P added at 0, 12.5, 25.0, and 37.5 mg kg−1 soil to determine the effects of VAMF-root associations on plant growth, benefit and cost analysis, and P efficiency (dry matter produced/unit P absorbed). Root colonization with V A M F and shoot growth enhancements decreased with increased soil P applications. Mycorrhizal plants were less P efficient than nonmycorrhizal plants. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants were considered the benefit derived by plants from VAMF-root associations. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants with similar P concentrations were considered the costs paid by plants for VAMF-root associations. Values of benefit and cost analysis for VAMF-root associations were highest when soil P was lowest and decreased with increasing P applications. Genotypic differences for calculated costs were pronounced, but not benefits. Benefit and cost analysis may be helpful to evaluate host plant genotypes and VAMF species to optimize efficiencies of VAMF symbiosis in different soil environments.

Benefit and cost analysis and phosphorus efficiency of VA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels

J. R. Ellis

Papers

Effects of species of VA-mycorrhizal fungi on growth and mineral uptake of sorghum at different temperatures

Benefit and cost analysis and phosphorus efficiency of VA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels