Soil faunal assemblage composition modifies root in-growth to plant litter patches

SummaryLocal-scale heterogeneity in the spatial distribution of soil nutrients promotes a suite of physiological and morphological responses in plants. These responses influence the competitive ability of plants within communities and potentially plant primary productivity. There is a growing appreciation then for the need to study factors that may modulate these plant responses to soil nutrient heterogeneity. Soil fauna are potentially one such modulating factor. For example, through impacts on organic matter decomposition and distribution they may directly modify nutrient patches and therefore the stimulus plants are responding to. In addition they may modify plant root mass and architecture through processes such as herbivory, potentially altering the outcome of a plant's response to a nutrient patch. Using grassland microcosms, consisting of a multi-species plant assemblage, multiple soil horizons and a speciose soil biota, we tested whether soil faunal assemblage composition might modulate plant responses to nutrient patches represented by litterbags. We show that root proliferation into a nutrient patch, a variable which is positively related to a plant's success under conditions of interspecific competition in a nutrient-limited environment, is reduced by the presence of mesofauna, and even more so by the presence of mesofauna together with macrofauna. Reductions in this proliferation response when mesofauna were present without macrofauna appeared to be a function of reduced root density. When macrofauna were included, reduced root density, and higher rates of litter patch disappearance, together contributed to the reduction in proliferation but additional mechanisms must also have played a role. Our results suggest that the effects and interactions generated by soil fauna need to be explicitly considered in analyses of how plants forage for nutrients in a patchy environment.