Interactions between nutrient levels and fluctuations shape soil fungal legacies and mediate non‐native plant growth

Abstract Resource variability has multiple dimensions, and its complexity is intensifying under global change. While elevated resource levels and temporal fluctuations often promote non‐native plant growth, less is understood about how these factors interact to influence non‐native plants via changes in soil microbial communities, which are key responders to resource change and regulators of plant performance. We conducted a two‐phase greenhouse experiment to test the relative effects of soil fungal legacies created by different historical nutrient regimes on subsequent non‐native plant growth. In phase I, we conditioned soils with six 4‐species assemblages of native plants under factorial combinations of nutrient level (low or high) and fluctuation regime (constant or pulsed). We quantified the resulting shifts in soil fungal community structure, focussing on the composition and relative abundance of arbuscular mycorrhizal fungi (AMF) and putative pathogens. In phase II, we grew 10 non‐native plant species individually with soil inocula from phase I and harvested their biomass at the end of this phase. We found that nutrient level and fluctuation interacted to generate distinct soil fungal legacies that altered non‐native plant growth at the end of phase II. Under low nutrients, pulsed additions produced fungal legacies that significantly suppressed non‐native biomass compared with constant additions, whereas under high nutrients, fungal legacy effects did not differ between fluctuation regimes. This growth response was positively correlated with AMF relative abundance and richness from phase I, which was lower in the low‐pulsed treatment than in other treatments. In contrast, non‐native biomass was unrelated to the relative abundance or richness of putative fungal pathogens. Synthesis . Our findings demonstrate that resource variability is likely to influence non‐native plants indirectly via soil fungal legacies, with effects depending on the interactions between resource level and temporal fluctuation. By showing that low‐nutrient pulses create inhibitory soil fungal legacies, this study offers a possible explanation for inconsistent effects of resource fluctuations on non‐native performance and underscores the need to incorporate historical resource conditions predictions of invasion success under global change.