Droughts affect plant communities, but their impacts may be mediated by soil biota. Soil communities may ameliorate drought stress, and droughts may leave legacies of altered soil communities that may affect future plant growth. However, it is not yet understood which groups of soil biota in particular affect plant performance under drought, nor which groups contribute to drought-legacy effects on future plant growth. We hypothesized that increasing soil-community complexity ameliorates drought stress and that drought-legacy effects are species-specific and soil-community-dependent. To test these hypotheses, we performed a two-phase experiment with six grassland species. In the first phase, we examined plant performance under drought and ambient conditions, in soils inoculated with a sterilized inoculum, or increasingly complex soil communities created by wet-sieving through 20-, 40-, and 200-µm mesh sieves. In the second phase, we examined drought-legacy effects on conspecific plant performance. We separately analysed plant performance in both phases, and integrated data from both phases using structural equation models. Drought effects on first-phase root biomass depended on soil inoculum, and this interaction differed among plant species, while effects on shoot biomass differed among species, but did not depend on inoculum. Only one species experienced drought-stress-ameliorating effects of soil biota. Drought-legacy effects on plant performance were positive, but depended on soil inoculum in case of root biomass, and on soil inoculum and species identity in case of shoot biomass. Drought-legacy effects were often mediated by first-phase biomass. In some species this effect was independent of inoculum, suggesting an abiotic legacy effect. In others, low first-phase biomass corresponded with high second-phase performance in presence of the most complex soil community. We conclude that drought-legacy effects on plant performance were soil-community-dependent but positive, suggesting that plants establishing after drought may benefit from increased nutrient availability and more positive impacts of soil biota.

Plant performance data from a two-phase experiment. In the first phase, 6 grassland plant species were exposed to a drought treatment and different soil communities, created using wet-sieving with different mesh sizes. In the second phase, the same plant species were grown in soil conditioned by conspecifics, and the legacy effects of first-phase treatments (drought and inoculum) were examined. In both phases, root, shoot and total biomass were determined.