PSF and grassland invasion by Bromus
Bromus tended to experience positive growth responses to soil inocula, especially those from Bromus -invaded plots (Table 1). This could appear as evidence for enemy release, whereby the invasiveBromus is escaping pathogens that drive negative PSF on native species (e.g., Klironomos, 2002). However, the lack of pathogenic OTUs specifically detrimental to Koeleria (Fig. 2) fails to support this hypothesis. Positive PSF developed by Bromus in these grasslands may rather arise from its capacity to establish beneficial interactions with non-symbiotic, nutrient-cycling microorganisms. This deserves further consideration in invasion ecology, with many study providing evidence for exotic species experiencing less negative PSF (Klironomos et al. , 2002; Reinhart et al. , 2003; Aldorfovaet al. , 2020), but few actually demonstrating the implications for soil-borne enemies in this pattern (e.g., Beckstead & Parker, 2003). Exotic species could be more versatile in developing positive interactions with foreign soil microbiota. Our results show (Fig. 2) that non-pathogenic and non-symbiotic microorganisms deserve more attention that they have received in prior research on PSF in the context of plant invasion (e.g., Vogelsang & Bever, 2009; Zhang et al., 2010).