Plant and soil communities are linked via the plant and influence each other
In terrestrial ecosystems, aboveground and soil communities are inseparably linked via plants (Wardle et al., 2004). Such aboveground-belowground linkages determine plant diversity effects on ecosystem functioning (Eisenhauer, 2012). Plant species often harbor unique rhizosphere communities and even influence the surrounding community composition of root-associated organisms through species-specific and context-dependent organic matter inputs (Bezemer et al., 2010; van der Putten et al., 2013). Similarly, each plant species has a specific herbivore community which can affect soil communities via herbivory, either directly via frass or indirectly via induced responses (Bardgett & Wardle, 2010). Aboveground herbivory can, for instance, positively affect soil microbial activity by inducing the release of carbon into the rhizosphere, and change arbuscular mycorrhizal colonization by reducing the carbon allocation to roots (Gehring & Whitham, 1994; Hamilton & Frank, 2001). In turn, soil biota, especially root parasites, pathogens, and herbivores as well as mutualistic symbionts can influence plant community structure and functioning via soil feedback effects (Van Der Heijden et al., 2008; van der Putten et al., 2013; Wardle et al., 2004). Root parasites, pathogens, and herbivores generally induce a negative plant-soil feedback by directly removing or damaging root tissues and thus reducing root uptake capabilities. Mutualistic organisms, on the other hand, induce a positive soil feedback effect on plant growth by improving soil nutrient uptake (Bardgett & van der Putten, 2014; Wardle et al., 2004), and protection against antagonists (Latz et al., 2012). The magnitude and direction of those plant-soil feedback effects, however, is not equal for all plant species and community contexts (Cortois et al., 2016).