DECIPHERING THE FOSSIL RECORD OF THE RHIZOSPHERE: PLANT-MICROBIAL RELATIONSHIPS INFERRED VIA PALEOBOTANICAL AND GEOCHEMICAL METHODS

The acquisition of reduced nitrogen (N) is essential for plant life, and plants have developed numerous strategies and symbioses with soil microorganisms to acquire this form of N. The evolutionary history of plant interactions with soil microorganisms, however, lacks evidence from the fossil record confirming mutualistic and/or parasitic relationships. Mutualistic relationships of particular importance in ecology and agriculture are plant symbiosis with N-fixing bacteria. While the evolution of N-fixing host plants is relatively well understood, the history of losses or gains of these symbioses as well as the precursor plants of the N-fixing clade remain poorly understood. Here we use modern plants in the N-fixing clade of Rosids to develop a geochemical method to assess the presence of symbiotic relationships with N-fixing soil bacteria via δ¹⁵N values of tree rings. Application of this method to Oligocene tree rings is the first analysis of N isotopes at high spatial resolution in fossil wood, confirming the symbiosis of certain arborescent legumes with N-fixing soil bacteria. The results also confirm actinorhizal symbiosis for some Oligocene non-leguminous trees. The results provide specific time points and generic level identification of trees in the N-fixing clade that have maintained or lost the ability to form symbioses with N-fixing soil bacteria. This study provides the platform to screen fossil wood broadly for potential N-fixing strategies, perhaps even in plants that previously engaged in N-fixing symbioses not present in modern ecosystems and vice versa. Moreover, this approach, as applied to paleoecology and concurrent with studies of fossil mycorrhizae, can lead to greater understanding of the ecologic significance of plant symbioses in time intervals of high atmospheric CO₂ and N-limited paleoecosystems.