Soil fertility, heterogeneity, and microbes: towards an integrated understanding of grassland structure and dynamics

Objective: To highlight the need and the potential for an integrated understanding of three key soil-based drivers of plant community structure and dynamics – soil fertility, soil heterogeneity, and microbes. Location: European and North American grasslands. Methods: Review and discussion of conceptual models and empirical literature, including examples of observational and manipulative studies from both natural and restored grassland communities. Results and Conclusions: In general, the results of empirical studies on soil fertility, soil heterogeneity, and soil microbes in grassland communities do not support expectations of common conceptual models. Ecological theory assumes a unimodal relationship between soil fertility and plant community diversity, yet empirical relationships from grassland communities are variable, the mechanisms underlying these variable patterns are not yet well understood, and there is mixed success at manipulating soil fertility to facilitate restorations. While theory predicts that increased soil heterogeneity will lead to increased plant community diversity, results of experimental manipulations of soil heterogeneity often show the opposite. Of two major conceptual models proposed for how microbes structure plant communities, there is little support for the hypothesis of microbially mediated niche partitioning. Plant-microbe feedbacks do have significant empirical support to date and there is increasing application of positive feedback dynamics in restoration, yet field tests of feedback dynamics remain limited. We suggest that an understanding of interactions between these soil drivers may help to resolve discrepancies between conceptual models and empirical results, improving our understanding of grasslands and our ability to restore them.