Contrasting growth responses of early- and late-successional native grasses to AMF-rich whole soil inocula under greenhouse conditions

Arbuscular mycorrhizal fungi (AMF) form symbioses in terrestrial ecosystems, yet plant responses to AMF-rich soil inocula are variable and do not necessarily reflect root colonization. We investigated how plant successional strategy and AMF-rich soil inocula interact to drive plant performance and root colonization structures. In a greenhouse experiment, four early- and late-successional native grasses were grown with five trap-cultured AMF-rich whole soil inocula. We assessed responses in biomass and height, expressed as log response ratios relative to sterile controls, alongside intraradical hyphal and arbuscular colonization. Biomass responses were generally negative and depended on the interaction between inoculum treatment and successional group. Late-successional species exhibited less negative growth responses than early-successional species. The probability of detecting hyphal colonization differed among inoculum treatments, whereas arbuscular colonization depended on the interaction between inoculum and successional group. However, neither arbuscules nor hyphal colonization predicted biomass responses, showing that greater physical colonization does not necessarily translate into greater plant growth. These findings show that structural colonization alone is insufficient to predict the growth effects of AMF-rich whole soil inocula. For grassland restoration, our results highlight that evaluating soil inocula based solely on physical colonization is insufficient; successful management requires assessing their functional effects on target native grasses.