Is more less? A comprehensive experimental test of soil depth effects on grassland diversity

Soil depth may affect plant diversity in two apparently opposing manners: on the one hand, deeper soil may increase the available space for below-ground niche partitioning, on the other hand, soil depth may have a negative effect on plant diversity by increasing productivity and the rate of competitive exclusion. Due to the scarcity of experimental studies that actively manipulate soil depth, the conditions under which each mechanism dominates are still unclear. Here, we studied the interactive effects of soil depth with common land use practices, namely, nutrient addition, trampling by grazers and mowing on plant diversity in grasslands. We manipulated these factors in a full-factorial manner in grassland mesocosms. Soil depth had a strong positive effect on species richness under mowing, suggesting increased space for niche differentiation in deeper soils. In unmown plots, deep soils harboured a similar diversity of species as shallow soils, and our findings suggest that this effect on diversity is due to larger biomass and lower light availability on deep soils. Fertilization and trampling had no effect on diversity. Overall, our findings indicate that soil depth effects on grassland communities strongly interact with common land use practices. We therefore advocate the inclusion of soil depth effects in manipulative experiments and management plans.