Plant diversity drives soil microbial biomass carbon in grasslands irrespective of global environmental change factors Running head: Soil microbial biomass in grasslands

1 Soil microbial biomass is a key determinant of carbon dynamics in the soil. Several studies have 2 shown that soil microbial biomass significantly increases with plant species diversity, but it 3 remains unclear if plant species diversity can also stabilize soil microbial biomass in a changing 4 environment. This question is particularly relevant as many global environmental change (GEC) 5 factors, such as drought and nutrient enrichment, have been shown to reduce soil microbial 6 biomass. Experiments with orthogonal manipulations of plant diversity and GEC factors can 7 provide insights whether plant diversity can attenuate such detrimental effects on soil microbial 8 biomass. Here, we present the analysis of 12 different studies with 14 unique orthogonal plant 9 diversity × GEC manipulations in grasslands, where plant diversity and at least one GEC factor 10 (elevated CO2, nutrient enrichment, drought, earthworm presence, or warming) were 11 manipulated. Our results show that higher plant diversity significantly enhances soil microbial 12 biomass with the strongest effects in long-term field experiments. In contrast, GEC factors had 13 inconsistent effects with only drought having a significant negative effect. Importantly, we report 14 consistent non-significant effects for all 14 interactions between plant diversity and GEC factors, 15 which indicates a limited potential of plant diversity to attenuate the effects of GEC factors on 16 soil microbial biomass. We highlight that plant diversity is a major determinant of soil microbial 17 biomass in experimental grasslands that can influence soil carbon dynamics irrespective of GEC. 18