The role of locally adapted mycorrhizas and rhizobacteria in plant–soil feedback systems

Abstract

Summary The plant–soil feedback (PSF) framework has become an important theory in plant ecology, yet many ecological and evolutionary factors that influence PSFs have yet to be fully considered. Here, we discuss the importance of local adaptation among plants and root‐associated fungi and bacteria. Furthermore, we show how inclusion of the optimal resource allocation (OA) model can help predict the direction and outcome of PSFs under environmental change. Plants and associated soil microbes have co‐evolved for millennia, generating adaptations to each other and to their local environment. This local co‐adaptation is likely generated by a suite of multidirectional exchanges of goods and services among plants, fungi and bacteria, and the constant changes in above‐ground–below‐ground interactions. Resource limitation may be a driver of local adaptation among organisms involved in nutritional symbioses. The OA model states that when an essential resource is limited, natural selection will favour taxa that forage optimally by adjusting their biomass and energy allocation such that productivity is equally limited by all resources. Co‐adaptation will therefore respond to the local limiting resource conditions through taxa‐specific resource transfer interactions. The OA model can help predict the outcomes of PSFs across a range of resource gradients and environmental changes such as increasing drought or atmospheric nitrogen deposition. Positive feedback is predicted in systems where resource exchange among plants and associated soil microbes can ameliorate resource limitation, or in systems where microbes provide another important service such as pathogen defence. Feedback strength is expected to diminish as resources become less limiting. Negative feedback is predicted when resources are in luxury supply and populations of opportunistic plant pathogens increase relative to commensal or mutualist microbes. Future, field‐based studies that integrate naturally co‐occurring systems of plants, microbes and their local soil are needed to further test the hypothesis that resource availability is an effective predictor of the direction and magnitude of PSFs. A more mechanistic understanding of PSFs will help land managers and farmers to manipulate plant–microbial soil interactions to respond to environmental change and to effectively harness beneficial symbioses for plant nutrition and pathogen control.