Abstract
The spatial scale on which microbial communities respond to plant invasions may provide important clues as to the nature of potential invader–microbe interactions. Lespedeza cuneata (Dum. Cours.) G. Don is an invasive legume that may benefit from associations with mycorrhizal fungi; however, it has also been suggested that the plant is allelopathic and may alter the soil chemistry of invaded sites through secondary metabolites in its root exudates or litter. Thus, L. cuneata invasion may interact with soil microorganisms on a variety of scales. We investigated L. cuneata-related changes to soil bacterial and fungal communities at two spatial scales using multiple sites from across its invaded N. American range. Using whole-community DNA fingerprinting, we characterized microbial community variation at the scale of entire invaded sites and at the scale of individual plants. Based on permutational multivariate analysis of variance, soil bacterial communities in heavily invaded sites were significantly different from those of uninvaded sites, but bacteria did not show any evidence of responding at very local scales around individual plants. In contrast, soil fungi did not change significantly at the scale of entire sites, but there were significant differences between fungal communities of native versus exotic plants within particular sites. The differential scaling of bacterial and fungal responses indicates that L. cuneata interacts differently with soil bacteria and soil fungi, and these microorganisms may play very different roles in the invasion process of this plant.
Highlights
We investigate variation in soil bacterial and fungal community composition associated with L. cuneata invasions at two spatial scales: (1) in localized soil zones around individual plants (hereafter referred to as plant “neighborhoods” and (2) that of whole sites, including sites with sparse and those with dense populations of L. cuneata
BACTERIA RESPOND ON THE SITE-SCALE When considering sites from across the U.S invaded range of L. cuneata, we found that significant changes in the community composition of soil bacteria were correlated with the invasion level of L. cuneata at each site (Table 2; Figure 2)
If soil fungi are more likely than bacteria to respond on this neighborhood scale, we suggest that the kinds of pairwise plant soil interactions described by Bever (2003) may be more likely to be mediated by fungal networks extending from one plant to another instead of by localized alterations to the soil bacteria
Summary
A growing body of research indicates that soil microorganisms can both respond to and strongly impact plant communities, including those affected by invasive plants (Klironomos, 2002; Kourtev et al, 2003; Reinhart et al, 2003; Callaway et al, 2004; Reinhart and Callaway, 2004; van der Putten et al, 2007b; Engelkes et al, 2008); reviewed by (Wardle et al, 2004; Wolfe and Klironomos, 2005; Reinhart and Callaway, 2006; van der Putten et al, 2007a). Our understanding of the specific microbial mechanisms at play in plant community dynamics is limited because much previous work has considered microbial communities from a purely functional, “black box” perspective; that is, a particular microbial species pool either net positively or net negatively influences plant growth (examples: Reinhart et al, 2003; Agrawal et al, 2005; Zou et al, 2006). Microbes can interact directly with invaders on very fine scales through a number of symbiotic interactions with roots: e.g., mycorrhizas, mutualistic bacteria, or known pathogens (Klironomos, 2002, 2003; Fumanal et al, 2006; Reinhart and Callaway, 2006; van der Putten et al, 2007b; Pringle et al, 2009). The soil milieu is broader than the rhizosphere, and understanding the microbial ecology of some plant invasions may require observations at larger scales, where interactions may be more indirect
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