Resource fluctuations are common in ecosystems and can generate resource pulses that exceed typical background levels. These pulses, particularly those involving nutrients, often facilitate exotic plant growth. While nutrient pulses may occur before invasion, during invasion, or both, the combined effects of historical (pre-invasion) and contemporary (during-invasion) pulses on exotic growth remain unexplored. We conducted a two-phase greenhouse experiment. In phase I, we grew eight native and eight exotic plant species individually under constant or pulsed nutrient treatments. In phase II, we grew eight exotic species individually with soil inocula created in phase I and exposed them to additional constant or pulsed nutrient treatments. We found that exotic plants exhibited stronger growth in soils conditioned by exotics than in those conditioned by natives, and in soils conditioned by nutrient pulses versus constant nutrients. Exotic growth was also enhanced by nutrient pulses compared with constant nutrients, which occurred in phase II. Importantly, the plant origin used to condition the soil and nutrient treatments in phase I, as well as the nutrient treatments in phase II, had significant interactive effects on exotic growth. Exotic plants produced maximal biomass when they were grown in soils conditioned by exotics under pulsed nutrients in phase I and then exposed to recurring nutrient pulses in phase II. Our findings expand the fluctuating resource hypothesis, demonstrating that exotic growth depends not only on contemporary resource fluctuations but also critically on the soil legacies of historical fluctuations. Furthermore, our results support the invasional meltdown hypothesis, which theorizes that exotic plants engineer self-reinforcing soil legacies that disproportionately enhance exotic performance during subsequent resource pulses. Given that real-world resource pulses vary in time, frequency and intensity, future work should quantify how these attributes modulate soil legacies to improve predictions of exotic performance under increasing resource variability.
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