Many studies indicate that increases in resource variability promote plant invasion. However, it remains unknown to what extent these effects might indirectly be mediated by other organisms. To test this, we grew eight alien species in pot-mesocosms with five different native communities under eight combinations of two nutrient-availability, two nutrient-fluctuation and two soil-microbe treatments. We found that when plants grew in sterilized soil, nutrient fluctuation promoted the dominance of alien plants under low nutrient availability, whereas its effect was minimal under high nutrient availability. However, the opposite pattern was found when plants grew in living soil. Analysis of the soil microbial community suggests that this might reflect that nutrient fluctuation strongly increased the soil fungal pathogen diversity under high nutrient availability, but slightly decreased it under low nutrient availability. Our findings indicate that besides its direct influence, environmental variability could also indirectly affect plant invasion via changes in soil-microbial communities.