Effects of three-dimensional soil heterogeneity on ecosystem functioning explained by root exploration across and resource transfer among soil patches

Ivan Nijs and Hans J De Boeck, Department of Biology, University of Antwerp, Wilrijk, Belgium, Yongjie Liu, State Key Laboratory of Grassland Agro-ecosystems, Lanzhou University, Lanzhou, China

Background/Question/Methods
Heterogeneity is an inherent property of soils, which is increasingly recognized as a regulator of plant community structure and ecosystem functioning. Yet, studying the role of soil heterogeneity is daunting because the spatial variation of soil features can be high and is characterized by multiple dimensions. Experimental studies have tried to disentangle configurational heterogeneity (grain size of soil patches) from qualitative heterogeneity (difference in the properties of the patches), but were mostly limited to two dimensions. Here we present findings of a series of recent experiments on synthesized mesocosms where different types of substrate were alternated in three dimensions and the grain size was varied from near zero to 48 cm. Species-rich seed mixtures were sown to study the impact of heterogeneity at community scale, and various response variables were recorded, among which root growth, species diversity, and resistance to drought extremes by placing the mesocosms under a rainout shelter. We hypothesized that the 3-D patterns of root exploration would be central to explaining the results, with smaller grain sizes offering greater opportunity for the roots of plants growing on resource-poor patches to venture into neighboring (adjacent or below) species-rich patches.
Results/Conclusions
We first demonstrated that the greater proximity to resource-rich patches that is associated with higher heterogeneity (i.e. smaller patch size) indeed allows plants on resource-poor patches to become more productive, at a lower relative investment in roots. The same phenomenon was observed in a second experiment focusing on the influence of soil heterogeneity on species diversity, although the emerging unimodal heterogeneity-diversity curve could not be entirely explained by these productivity changes. In the experiment with imposed drought, plants on resource-poor patches were again more productive when the patch size was smaller, suggesting the same improved resource transfer in their favor. However, this led to faster soil water depletion, and ensuing greater senescence and drought-induced heat stress. We conclude that root exploration responses and transfer of resources among patches are a principle common driver behind the wide variety of ecological effects of soil heterogeneity, though probably not the only driver. Finally, during our mesocosm studies the very high levels of soil heterogeneity (i.e. the alternation between resource-rich and resource-poor patches at very short distance) remained enigmatic as they may actually be experienced as homogeneous by plants, raising questions about diverging physical and organism-perceived heterogeneity.