1. 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. 2. 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. 3. 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. 4. 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. 5. 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.

In this article we discuss the possible significance of biological processes, and of fungi in particular, in weathering of minerals. We consider biological activity to be a significant driver of mineral weathering in forest ecosystems. In these environments fungi play key roles in organic matter decomposition, uptake, transfer and cycling of organic and inorganic nutrients, biogenic mineral formation, as well as transformation and accumulation of metals. The ability of lichens, mutualistic symbioses between fungi and photobionts such as algae or cyanobacteria, to weather minerals is well documented. The role of mycor-rhizal fungi forming symbioses with forest trees is less well understood, but the mineral horizons of boreal forests are intensively colonised by mycorrhizal mycelia which transfer protons and organic metabolites derived from plant photosynthates to mineral surfaces, resulting in mineral dissolution and mobilisation and redistribution of anionic nutrients and metal cations. The mycorrhizal mycelia, in turn provide efficient systems for the uptake and direct transport of mobilised essential nutrients to their host plants which are large sinks. Since almost all (99.99 %) non-suberised lateral plant roots involved in nutrient uptake are covered by ectomycorrhizal fungi, most of this exchange of metabolites must take place through the plant–fungus interface. This idea is still consistent with a linear relationship between soil mineral surface area and weathering rate since the mycelia that emanate from the tree roots will have a larger area of contact with minerals if the mineral surface area is higher. Although empirical models based on bulk soil solution chemistry may fit field data, we argue that biological processes make an important contribution to mineral weathering and that a more detailed mechanistic understanding of these must be developed in order to predict responses to environmental changes and anthropo-genic impact.se (R. Finlay). j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / f b r f u n g a l b i o l o g y r e v i e w s 2 3 (2 0 0 9) 1 0 1 – 1 0 6