Increasing soil protist diversity alters tomato plant biomass in a stress-dependent manner

Biodiversity and ecosystem functioning (BEF) often correlate positively with BEF studies focusing mostly on plant diversity manipulations. Plant performance is directly and indirectly impacted by soil organisms, but the role of increasing soil biodiversity on plant performance has mainly been tested in an uncontrolled way or with low biodiversity levels. An additional knowledge gap exists on the effect of (interactive) global change drivers - such as drought - on the soil BEF (sBEF) relationship. We here tested sBEF relationships by manipulating microbiome predatory protist diversity (0-30 species) in ambient controls and under abiotic (drought) and biotic stresses (nematode addition dominated by plant parasites). We then used plant (Solanum lycopersicum) biomass as a response in an 8-week greenhouse experiment. We show that the increasing biodiversity effect on plant biomass ranged from positive (up to 23% with biotic stress), to neutral (ambient conditions and with both stresses co-occurring), to negative (up to 39% with abiotic stress). Together, sBEF relationships were context-dependent and often contradicted generally reported positive (s)BEF relationships. Therefore, we propose that positive sBEF claims likely are not the norm and should be evaluated in a context-dependent manner. To better elucidate sBEF relationships, more manipulative studies should be performed under different conditions such as global change drivers and with a range of organismal groups.