Soil heterogeneity and community composition jointly influence grassland biomass

Question: Does the spatial pattern of nutrient supply modify community biomass responses to changes in both species composition and richness? Location: Duke University Phytotron (Durham, North Carolina, USA). Methods: We conducted a microcosm experiment to evaluate individual plant and whole community responses to species richness, species composition and soil nutrient heterogeneity. The experiment consisted of seven levels of species composition (all possible combinations of Lolium perenne, Poa pratensis and Plantago lanceolata) crossed with three levels of soil nutrient distribution (homogeneous, heterogeneous-up, and heterogeneous-down, where up and down indicates the location of a nutrient patch in either the upper or the lower half of the soil column, respectively). Results: Communities containing Plantago and Lolium responded to nutrient heterogeneity by increasing above- and below-ground biomass. Nutrient heterogeneity also increased size inequalities among individuals of these species. Significant species composition x nutrient heterogeneity interactions on community biomass and individual size inequality were observed when nutrient patches were located in the upper 10 cm of the soil columns. However, root proliferation in nutrient patches was equivalent regardless of the vertical placement of the patch. Conclusions: Our results suggest that nutrient heterogeneity may interact with plant species composition to determine community biomass, and that small-scale vertical differences in the location of nutrient patches affect individual and community responses to this heterogeneity.