Tree species richness and identity influence the abundance of excysted soil ciliates in young forest plantations

Soil protists play crucial roles in nutrient cycling, organic matter decomposition, and regulating belowground food webs, but relatively little is known about the effects of tree diversity on this microscopic group. We used the International Diversity Experiment Network with Trees (IDENT) experiment in Sault Ste. Marie, Canada to address the effects of tree species richness and functional diversity, and high and low growing season soil moisture treatments on soil ciliate abundance. In total, we found 2,285 ciliates from 181 plots representing 22 tree communities and an unplanted control plot. Soil ciliate abundance varied significantly with species identity of tree communities, with non-additive net mixing effects on abundance observed for two of 16 mixed communities. Ciliate abundance was also significantly and negatively influenced by soil temperature on the day of sampling. We found a significant, albeit weak, positive relationship between tree species richness and total ciliate abundance that was driven primarily by Paramecium spp. Functional tree diversity had a similar relationship with ciliate abundance that was only marginally significant, while soil moisture treatment had no effect on abundance. These findings suggest that higher tree species richness in young plantations can have a positive influence on the abundance of soil ciliates, though the relationship is context-dependent, differing slightly with species identity of trees. Our study contributes to our current knowledge of tree diversity effects on soil ciliate abundance and highlights the need for further research to fully understand these complex relationships and their implications for forest ecosystem functioning. • Tree species richness has a significant but weak positive effect on active ciliate abundance. • Functional tree diversity shows only marginal influence on ciliate abundance. • White birch–dominated communities support consistently higher soil ciliate abundances. • Long-term soil moisture manipulation does not affect excysted ciliate abundance. • Soil temperature at sampling negatively influences active ciliate abundance.