BELOW-GROUND NUTRIENT TRANSFER IN TREES: PROXY DEVELOPMENT AND IMPLICATIONS FOR THE EVOLUTION AND PALEOECOLOGY OF MYCORRHIZAL SYMBIOSES

Nitrogen (N) is one of the most limiting nutrients for plant growth in modern terrestrial ecosystems. In order to overcome this, terrestrial plants establish a collection of strategies, such as mutualistic symbioses with soil fungi and bacteria, to gain access to N. The most widespread mutualistic symbiosis between plants and fungi are mycorrhizal associations. Ectomycorrhizal (ECM) associations are among the most efficient in terms of nutrient uptake for plants by allowing for the greater exploratory capacity of fungal hyphae and production of mycelial networks between neighboring plants. The role of ECM symbiosis in the ecology of fossil forests, however, is poorly understood despite clear modern evidence of the significance of these associations to overcome nutrient limitation under elevated atmospheric CO2. In this talk we present the results of a geochemical proxy, developed in modern forests, to assess below-ground nutrient transfer in ECM trees. We hypothesize that modern trees with ECM symbioses will have unique N isotope compositions and N to phosphorous ratios in comparison to non-ECM trees in the same forest. Moreover, that these chemical indexes have the potential to elucidate cross transfer of nutrients between ECM trees in the same forest via below-ground mycelial networks. We test this hypothesis through application of 15N labeled ammonium to soil near one ECM tree, and subsequent tracing of the labels in nearby ECM and non-ECM trees to evaluate the fidelity of this proxy relative to control trees of the same species. The proxy development is designed to be applicable to analysis of fossil wood and paleosols, and we highlight recommendations for applications of this approach to fossil forests using N and P geochemistry and N isotope geochemistry. We further suggest that this proxy may provide insight into the evolutionary history of (ecto-)mycorrhizal relationships with ancient plants that lack microbial body fossil evidence using geochemical approaches to infer that such mutualistic relationships were established in certain groups of fossil plants.