A simple method to vary soil heterogeneity in three dimensions in experimental mesocosms

Soil heterogeneity affects terrestrial plant communities both directly and indirectly. In nature, the exploration of the role of heterogeneity is made difficult because any co-varying factors (nutrients, soil depth, etc.) render it problematic to clearly link cause and effect. Attributing changes specifically to heterogeneity is facilitated if heterogeneity is varied in a controlled manner and other possible confounding factors are kept constant. The experiments conducted in such a way have up till now only considered heterogeneity in two dimensions, horizontally or vertically. In this methodological study, we present a novel technique that enables researchers to vary both qualitative and configurational heterogeneity in three dimensions by building up the soil cell by cell in experimental mesocosms. We illustrate the technique with an experiment where we test the effect of cell size (i.e. configurational heterogeneity) on the performance of grassland species that vary in nutrient preference (high N and low N species). Cell size did not affect aboveground biomass but modified species richness, both at the mesocosm and the patch scale, with most species being found when cells were small yet distinct (cell size 12 cm). High N species had significantly greater aboveground biomass and species richness than low N species, both on nutrient rich and nutrient poor cells. Remarkably, those differences disappeared when plants grew on the mesocosms with cell size close to zero. By allowing greater complexity in the design of experimental mesocosms, the 3-D approach can improve understanding of the interplay between soil heterogeneity and plant and ecosystem functioning.