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Arbuscular mycorrhizal fungi (AMF) are functional components of terrestrial ecosystems world-wide. These fungi in the phylum Glomeromycota (Schüssler et al., 2001) form symbiotic relationships with the majority of land plants. Among the benefits AMF confer to their plant hosts are enhanced mineral nutrition (Smith & Read, 1997) and greater root-pathogen resistance (Newsham et al., 1995). In exchange, the obligate biotrophic fungi receive carbon. These effects at the scale of the individual plant can influence processes at the scale of the ecosystem through their ability to aggregate soils (Rillig, 2004b).
Soil aggregation is a complex, hierarchical process mediated by both biotic and abiotic factors (Tisdall & Oades, 1982). Aggregation is essential to maintaining soil porosity, allowing gas exchange and water infiltration, and facilitating biogeochemical cycling (Diaz-Zorita et al., 2002). Soil structure is also crucial to the success of sustainable agriculture and erosion resistance. Over one-third of the world's arable land was damaged by erosion over the last 40 yr (Pimentel et al., 1995), and much of the focus of sustainable agriculture has shifted towards managing for well-aggregated soils.
Hyphae of AMF are considered to be primary soil aggregators for several reasons: the extraradical hyphae of AMF have a significant biomass in most soils (Rillig & Allen., 1999), as obligate biotrophs these fungi do not need to compete with saprobes for soil carbon and AMF hyphae are more resistant to fungivory than saprobic fungi (Klironomos & Kendrick, 1996). Arbuscular mycorrhizal fungi may stabilize soils up to 5 months after their host's death (Tisdall & Oades, 1980). A positive correlation between AMF hyphae and aggregate stabilization in natural systems is described by Miller & Jastrow (1990) and Jastrow et al. (1998). Rillig et al. (2002) described significant indirect effects of AMF hyphal length on water-stable aggregate (WSA) stabilization via the production of glomalin-related soil protein (GRSP) in a natural grassland system. Arbuscular mycorrhizal fungi showed similar results on the five plant hosts used, but as in the other grassland studies no AMF species involved were described (Rillig et al., 2002).
Little is known about the effects of different AMF taxa on aggregate stabilization. Schreiner et al., 1997) tested the WSA-forming ability of three AMF species on soybean (Glycine max). The authors found that Glomus mosseae stabilized aggregates in the 2–4 mm size class significantly more than Glomus etunicatum and Gigaspora rosea, but there were no differences between species in the 1–2 mm or 0.25–1 mm size classes. In natural grasslands, Miller and Jastrow (1992) found a correlation between spore densities of Gigaspora gigantea with %WSA, but not densities of G. etunicatum.
Plants with dense, fibrous root systems (such as grasses) assist aggregate formation (Oades, 1993; Amézketa, 1999). Similarly, hyphal characteristics may contribute to aggregation ability. An AMF with dense hyphal clusters may hold soil particles together better than diffuse hyphae, but this hypothesis has never been tested. These mechanisms of aggregation may be, like other AMF characters, species dependent and this has given rise to our hypothesis that AMF species exist that are particularly adept at soil aggregation. The idea of an aggregation ‘specialist’ is attractive to agriculture as well as to applications in ecosystem restoration. If a species of AMF promoted WSA stabilization independent of plant host or soil type, it could be used to inoculate crops or other soils with poor water aggregate stability.
Arbuscular mycorrhizal fungal effects can run the gamut from mutualist to parasite depending on its host plant. Klironomos (2003) illustrated that the effects of a single AMF on many hosts ranged from greatly stimulating shoot biomass to drastically reducing it. We propose that these types of interactions may also manifest in differential amounts of soil aggregation. If the effects of an AMF species vary widely from host to host, then the search for an applicable aggregation ‘specialist’ may be complicated. If a certain AMF interacts with specific hosts to strongly promote aggregation, AMF–host combinations could be customized to yield the highest aggregate formation.
Overall, little is known about the effects of species of AMF on soil aggregation in natural ecosystems or agroecosystems. We know of no study in which the ability to promote stable soil aggregation has been compared among several hosts and fungal species combinations. This is the focus of the present study. Importantly, many studies have used biological material that was not derived from the same soil and ecosystem. The existence of intraspecific variation in AMF with respect to their ecosystem origin casts doubts on the degree of ecological realism of such studies (Klironomos, 2003). This experiment takes advantage of co-occurring inhabitants of a long-term mycorrhizal research site to study possible effects of plant–fungi interactions on WSA formation. To help explain the mechanism of aggregate stabilization we measured root biomass and hyphal lengths. As these variables are thought to be major determinants of stabilization, correlation between these and aggregate percentage could predict which fungi–plant combinations are most suited to form WSAs.
We tested the following hypotheses:
Fungi of the family Gigasporaceae will be better soil aggregators independent of their plant host compared with non-Gigasporaceae members, as this family produces greater hyphal lengths and denser hyphal clusters (Hart & Reader, 2002
Plants in the Poaceae (grasses) will be better soil aggregators independent of their AMF partner, as grasses have more fibrous root biomass to ensnare soil particles (Amézketa, 1999
There will be three levels of aggregation based on the above assumptions. The combinations of grasses with Gigasporaceae fungi will yield the most WSAs, followed by the combinations of Gigasporaceae fungi and non-grasses and grasses and non-Gigasporaceae fungi, finally the lowest percentage of WSAs will be found in pots with the combination of non-grasses and non-Gigasporaceae fungi.
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Other studies have demonstrated an interaction between AMF and host plant species with respect to host and/or fungal growth and hence net primary productivity (Adjoud et al., 1996; Bever et al., 1996; Eom et al., 2000; Klironomos, 2003). Here, we significantly extend these findings by showing that AMF–host species combinations also differentially control the percentage of water-stable soil aggregates, and thus another major ecosystem state variable (i.e. soil structure). Although the functional significance of changes in %WSA in the range observed here is unknown, this indicates the potential for strong effects under circumstances in which aggregate stability is low (e.g. highly disturbed soil).
Previous studies established that %WSA varied between fungi associated with a single host (Schreiner et al., 1997). However, ours is the first study to use multiple co-occurring plant and fungal species combinations. Klironomos (2003) showed that exotic AMF species have far different effects on their plant host than co-occurring species, and vice versa. Previous pot experiments on aggregation have used soils or fungi that are exotic to the symbionts, adding further complications (Schreiner et al., 1997; Andrade et al., 1998; Bearden & Petersen, 2000).
This glasshouse study contrasts with other field studies showing positive correlation between %WSA and hyphal lengths/root biomass (Jastrow et al., 1998; Rillig et al., 2002). Negative correlations have, however, been observed (Schreiner et al., 1997). A possible explanation for the decrease in %WSA1−2 mm with grasses may be a result of our experiment's duration and the extremely high root biomass of B. inermis. The plants were grown in pots for a year and some B. inermis were pot-bound at harvest time. Such a high density of roots could have inhibited aggregate formation. The differences between the %WSA of grasses and non-grasses disappeared when the Bromus treatment combinations yielding extremely high root biomass were removed from the analysis. While root biomass is positively correlated with %WSA in field studies, glasshouse pot experiments must consider the deleterious effects of high root densities on %WSA formation.
In our study, in contrast to our initial hypothesis, the AMF family with greater overall hyphal lengths (Gigasporaceae) produced significantly lower %WSA. While members of the Gigasporaceae generally have more abundant and denser hyphal growth (Hart & Reader, 2002), the species used in our study yielded lower percentages of WSA than members of the Glomaceae and Acaulosporaceae. Although S. calospora (Gigasporaceae) hyphal lengths were greater, Jakobsen et al. (1992) states they do not spread as far from the root as A. laevis (Acaulosporaceae), which could explain this difference. Soil aggregate formation may depend more on hyphal spread from the host root than on hyphal length alone. Hyphae that forage farther from the host root could form more %WSAs because a higher proportion of runner hyphae (Friese & Allen, 1991) could ‘string’ together more soil particles.
Our hypothesis that combinations of grasses and Gigasporaceae fungi would stabilize more aggregates than non-grass and non-Gigasporaceae combinations was not supported since the grass–Gigaspora combination had the lowest mean %WSA. This again suggests that other mechanisms mediated by the symbiont's interaction may dictate WSA stabilization rather than root biomass and total hyphal length. Given that these obvious mechanisms may not function as strongly as first thought in WSA stabilization, we must consider that other aspects of extraradical hyphae and root development could be determined by host interaction and affect %WSA1−2 mm.
The AMF hyphae, like plant roots, can vary widely in their branching patterns. More highly branched hyphae or roots may be more effective in binding soil particles. Moreover, AMF species can differentially affect root branching (Norman et al., 1995). Future studies should consider measurement of both root and hyphal branching.
Glomalin-related soil protein is strongly positively correlated with %WSA1−2 mm (Wright & Upadhyaya, 1998; Wright & Anderson, 2000; Rillig et al., 2001; Rillig, 2004a). Production of GRSP per fungal mycelium biomass may vary as a function of AMF species (Wright et al., 1996), although the AMF species used for that study did not come from the same ecosystem. However, the same pattern appears to hold up for AMF from the same ecosystem (C. Rosier and M. C. Rillig, unpubl. data). Certain hosts might differentially stimulate GRSP production in their AMF symbionts, resulting in increased WSA formation. In this experiment we could not test for this mechanism because background levels of GRSP in the soils used were high and fluxes of GRSP are generally small (M. C. Rillig et al. unpubl. obs.). Further, beyond a level of WSA of c. 80% (using the WSA measurement technique we used here), the relationship between glomalin concentration and water-stability plateaux (Wright & Upadhyaya, 1998).
We observed negative correlation between hyphal lengths and root biomass in some cases (i.e. Bromus with Scutellospora). Only gigasporacean fungi caused the decreased root biomass, and it is likely a result of their extraradical growth. Member of the Gigasporaceae generally have greater soil hyphal biomass than the other AMF families (Hart & Reader, 2002), which requires a greater carbon supply from the host.
No evidence of an AMF aggregation ‘specialist’ was apparent in this study; even so, species less affected by their host, which simultaneously provide overall high WSA (i.e. S. calospora) may be better candidates for applications in restoration. These could confer the benefit of higher WSA stabilization to a broad range of hosts in the field. While we do not suggest that field inoculation should only be carried out with one fungal species, our data lend support to the idea of using a cocktail of AMF species, a component of which could be an AMF isolate that is specifically included for promoting soil stabilization.
In situations where a specific host plant is the target, such as in production agroecosystems or in certain restoration and revegetation applications, our data strongly suggest that AMF inoculum could be specifically tailored to maximize aggregate formation. Alternatively, in restoration situations where the host plant is a variable, it is clear that host plant choice can codetermine soil stabilization together with AMF inoculum identity. We conclude that soil aggregation is a function of both the fungi and its host, ranging from a poor interaction to strongly positive, much like other AMF–host exchanges.