Differentiation of biotic soil communities
The biotic soil communities associated with two co-existing prairie species had variable effects on the growth of those species under greenhouse conditions. The seed survival and biomass, as indicated by leaf number, for C. fasciculata was reduced in its own soil by microwaving, suggesting a positive association between C. fasciculata and its own biotic soil community. Because C. fasciculata is a nodulating legume species ( Foote & Jackobs 1966) and microwaving of soil may affect some prokaryotes ( Ferriss 1984), this may not be surprising. A height reduction for A. gerardii in its own untreated soil compared with microwaved soil, together with a similar trend for tiller number, provide evidence for the opposite relationship in this species. Thus, for A. gerardii the effects of potential soil mutualists, such as mycorrhizae, may be outweighed by the effects of pathogenic soil organisms. Although this may be somewhat surprising given reports of the positive effects of mycorrhizae on A. gerardii growth on the Konza Prairie in central Kansas ( Hetrick et al. 1988 ; Hartnett et al. 1994 ), this grass does not seem to be nearly as dependent on mycorrhizae in other, more nutrient-rich prairies ( Anderson et al. 1994 ). Because A. gerardii height is greater on the Kansas Ecological Reserve study site compared with heights attained on the Konza prairie and the soil horizon is deeper, the relationship between A. gerardii and its mycorrhizal symbionts may be similar to that found in more eastern prairies ( Anderson et al. 1994 ).
Andropogon gerardii growth, specifically its below-ground biomass and tillering, is inhibited by C. fasciculata soil. The lack of this inhibition when the same soil is microwaved suggests that it is due to some biotic component of the legume’s soil. Species are known to differ in their response to the biotic community associated with any one soil type ( Martin et al. 1956 ; van der Putten et al. 1993 ; Bever 1994; Bever et al. 1996 ). Differential responses to mycorrhizal communities ( Hartnett et al. 1993 ; Sanders 1993; Bever et al. 1996 ) and bacterial communities ( Westover et al. 1997 ) in natural systems may be commonplace. In all of the few systems examined, the biotic soil community associated with a plant species suppresses that plant’s growth more than the biotic community associated with other plant species ( van der Putten et al. 1993 ; van der Putten & Peters 1997; Bever 1994). In models assuming equal competitive ability between species, negative feedback between plant species and their soil communities can lead to maintenance of species diversity ( Bever et al. 1997 ). However, if the competitive abilities of plant species are very different, as is likely between a tillering perennial such as A. gerardii and a weedy annual like C. fasciculata, negative feedback may not be necessary to explain co-existence. A competitively dominant species may not exclude a less competitive neighbour if that neighbouring species is associated with pathogens, or predators, that preferentially attack the competitive species. This is analogous to Paine’s landmark study ( Paine 1974) of a predator maintaining competing prey species in the intertidal zone and other studies of plant fungal pathogens that have demonstrated the role of disease in maintaining competing plant species in a community ( Burdon et al. 1984 ; Paul & Ayres 1990). Competition trials between the perennial grass and annual legume would be necessary to assess whether competition does indeed exist between these species and whether it is asymmetric.
The effects of the biotic community may normally be directly or indirectly mediated by the associated plant species, but we found evidence that the growth of A. gerardii is inhibited by non-microwaved C-type soil even if no C. fasciculata is present. Soil pathogens are probable candidates for causing this response, and we therefore tried to pinpoint those responsible.
The role of mycorrhizae, fungal pathogens and other members of the biotic soil community
The fungi isolated from A. gerardii in the different soil types were taken from within roots after surface sterilization and not from the soil itself, and were therefore likely to be pathogenic rather than saprophytic. Those fungi that were unique to the soil in which A. gerardii did worst (C-NM) did indeed have a negative effect on tillering, below-ground biomass and, initially, on above-ground growth, suggesting that they, rather than another component of the soil community, caused the observed inhibition of growth. However, soil invertebrates, which were eliminated by the microwaving treatment, and potential allelopathic chemicals associated with C. fasciculata, may also have contributed to the results of the first experiment.
It is interesting that the negative effects of C-NM soil on A. gerardii were severe in the first experiment, particularly in terms of above-ground growth and flowering, whereas by the end of the second experiment these measures had been enhanced by the fungal collection derived from this soil. This could be due to regular fertilization applied during the second experiment. Mycorrhizae should not have impacted the second experiment, as only specific fungal cultures were added to a sterile potting medium. AM fungi can alter allocation patterns (Streitwolf-Engel et al. 1997), and this may have contributed to the discrepancies in above-ground biomass results for the first and second experiments.
Microwaving soil did appear to reduce fungal inoculum: there was very little colonization of A. gerardii roots by AM fungi in microwaved A soil whereas, in untreated A soil, colonization was comparable to rates found previously for warm season grasses ( Hetrick et al. 1990 ). The low colonization observed in both treated and untreated C. fasciculata soil could be due either to absence of AM propagules or low AM colonization, even in untreated soil, due to the action of pathogenic organisms. A protective role for AM fungi, as suggested by Newsham et al. (1995) , cannot be discounted for A. gerardii. However, the absence of AM cannot by itself explain the poor growth of A. gerardii in C-NM soil because growth was not inhibited in microwaved soil, which contained few AM propagules.
An alternative explanation for the better growth of A. gerardii in its own soil compared to that of the legume (at least when untreated) is differences in mycorrhizal community rather than the pathogenic soil community between the two species due to specificity of plant–mycorrhizal associations ( Sanders 1993; Wetzel & van der Valk 1996). However, isolations from A. gerardii roots clearly showed differences between the fungal pathogen communities from the two types of soils. Soil origin (C vs. A soil) was also the strongest predictor of fungal classification in the randomization test (MRPP) used, which would not be the case if the potential pathogenic communities had been homogeneous.
The Trichoderma spp. often found in A soil were initially thought to act as potential mutualists to A. gerardii owing to the known mycoparasitic characteristics of these fungi ( Agrios 1988; Carlile & Watkinson 1994). Although the addition of these fungi did seem to ameliorate the initial decrease in growth and tillering experienced by plants exposed to the fungal mixture derived from C-NM soil alone, the significant reduction in below-ground biomass occurred regardless of Trichoderma presence. Thus, any protection may be short-term during the plant/tiller establishment phase, not long-term protection in the allocation of resources to perennating structures over time. However, the activity of Trichoderma could be dependent on pH and soil nutrients, which were not measured in this study.
The baiting techniques used to assess the presence of Oomycete fungi in the soil yielded few species. This suggests that populations of these fungi within A. gerardii roots were not significant, although one of the fungi unique to C-NM soil was a potentially pathogenic Oomycete. It should be noted that not all potential fungal pathogens would have grown on the general fungal media used, and other components of the biotic soil community, such as invertebrates, nematodes, protozoa and bacteria, were beyond the scope of this study.
The fact that C. fasciculata plants are associated with a biotic soil community that inhibits A. gerardii may be important for the persistence of an annual species with perennial neighbours. Although the in situ physical characteristics of the soils associated with A. gerardii in more intact prairie and C. fasciculata in more disturbed areas may have been very different, when microwaved they supported similar biomasses of A. gerardii (in terms of height). Untreated C. fasciculata soil inhibited tillering of A. gerardii and so germinating seedlings of this annual may not be threatened by encroaching tillers of A. gerardii. The hypothesis that the association of C. fasciculata with pathogenic organisms that primarily affect A. gerardii increases the likelihood of the annual persisting in the community needs to be tested by competition studies between the species.
This study, as well as previous studies on negative feedback, point to the fine-scale below-ground heterogeneity possible within plant communities. The heterogeneity is driven in part by the presence of the particular plant species themselves, and the interactions between species that result are probably complex and species-specific. Just as previous research has found that plant-specific herbivores ( Hulme 1996) could underpin the presence or absence of a species in the community, we need heightened awareness of the potential role of microscopic ‘predators’ that are ubiquitous both above-and below-ground.