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  1. Summary

    To distinguish among hypotheses on the importance of resource‐exchange ratios in outcomes of mutualisms, we measured resource (carbon (C), nitrogen (N), and phosphorus (P)) transfers and their ratios, betweenPinus taedaseedlings and two ectomycorrhizal (EM) fungal species,Rhizopogon roseolusandPisolithus arhizusin a laboratory experiment.

    We evaluated how ambient light affected those resource fluxes and ratios over three time periods (10, 20, and 30 wk) and the consequences for plant and fungal biomass accrual, in environmental chambers.

    Our results suggest that light availability is an important factor driving absolute fluxes of N, P, and C, but not exchange ratios, although its effects vary among EM fungal species. Declines in N : C and P : C exchange ratios over time, as soil nutrient availability likely declined, were consistent with predictions of biological market models. Absolute transfer of P was an important predictor of both plant and fungal biomass, consistent with the excess resource‐exchange hypothesis, and N transfer to plants was positively associated with fungal biomass.

    Altogether, light effects on resource fluxes indicated mixed support for various theoretical frameworks, while results on biomass accrual better supported the excess resource‐exchange hypothesis, although among‐species variability is in need of further characterization.

     
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  3. Abstract

    To understand how diverse mutualisms coevolve and how species adapt to complex environments, a description of the underlying genetic basis of the traits involved must be provided. For example, in diverse coevolving mutualisms, such as the interaction of host plants with a suite of symbiotic mycorrhizal fungi, a key question is whether host plants can coevolve independently with multiple species of symbionts, which depends on whether those interactions are governed independently by separate genes or pleiotropically by shared genes. To provide insight into this question, we employed an association mapping approach in a clonally replicated field experiment of loblolly pine (Pinus taedaL.) to identify genetic components of host traits governing ectomycorrhizal (EM) symbioses (mycorrhizal traits). The relative abundances of different EM fungi as well as the total number of root tips per cm root colonized by EM fungi were analyzed as separate mycorrhizal traits of loblolly pine. Single‐nucleotide polymorphisms (SNPs) within candidate genes of loblolly pine were associated with loblolly pine mycorrhizal traits, mapped to the loblolly pine genome, and their putative protein function obtained when available. The results support the hypothesis that ectomycorrhiza formation is governed by host genes of large effect that apparently have independent influences on host interactions with different symbiont species.

     
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  4. Abstract

    Most plants engage in symbioses with mycorrhizal fungi in soils and net consequences for plants vary widely from mutualism to parasitism. However, we lack a synthetic understanding of the evolutionary and ecological forces driving such variation for this or any other nutritional symbiosis. We used meta-analysis across 646 combinations of plants and fungi to show that evolutionary history explains substantially more variation in plant responses to mycorrhizal fungi than the ecological factors included in this study, such as nutrient fertilization and additional microbes. Evolutionary history also has a different influence on outcomes of ectomycorrhizal versus arbuscular mycorrhizal symbioses; the former are best explained by the multiple evolutionary origins of ectomycorrhizal lifestyle in plants, while the latter are best explained by recent diversification in plants; both are also explained by evolution of specificity between plants and fungi. These results provide the foundation for a synthetic framework to predict the outcomes of nutritional mutualisms.

     
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  5. Abstract

    Understanding soil systems is critical because they form the structural and nutritional foundation for plants and thus every terrestrial habitat and agricultural system. In this paper, we encourage increased use of mathematical models to drive forward understanding of interactions in soil ecological systems. We discuss several distinctive features of soil ecosystems and empirical studies of them. We explore some perceptions that have previously deterred more extensive use of models in soil ecology and some advances that have already been made using models to elucidate soil ecological interactions. We provide examples where mathematical models have been used to test the plausibility of hypothesized mechanisms, to explore systems where experimental manipulations are currently impossible, or to determine the most important variables to measure in experimental and natural systems. To aid in the development of theory in this field, we present a table describing major soil ecology topics, the theory previously used, and providing key terms for theoretical approaches that could potentially address them. We then provide examples from the table that may either contribute to important incremental developments in soil science or potentially revolutionize our understanding of plant–soil systems. We challenge scientists and mathematicians to push theoretical explorations in soil systems further and highlight three major areas for the development of mathematical models in soil ecology: theory spanning scales and ecological hierarchies, processes, and evolution.

     
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