The deep-time history of soils and plants

For a large community of ecosystem ecologists, plant-soil interactions have moved center stage in the understanding of both plant and soil ecology. Frequently, plants are deemed to be the main drivers of these interactions. However, soils existed before the evolution of land plants (i.e., embryophytes), and terrestrial communities were already highly diverse around 400 million years ago (Ma), reflecting an earlier origin of the organisms that populated them. But which organisms were contemporaneous at the origin of soil and in what type of environment they lived is not well established. Here, we trace the development of soil and its biotic components from the beginnings, about one billion years ago (Ga), by using the fossil record and geochemical data. We show that the first terrestrial ecosystems were probably situated near the ocean, where organisms were already adapted to high oxygen pressures and where plants were not yet present. Drawing on this, we discuss the need to rethink the primacy of plants in modern terrestrial ecosystems, considering that persistence of the soil system over deep time could help explain the enormous current biodiversity found in soil. We advocate for a shift in perspective by embracing a vision of terrestrial ecosystems with interdependent and equally important above- and below-ground system compartments. Finally, we highlight the importance of 'bending the curve' on the increasing rate of soil degradation and loss of soil health, and we discuss the outlook for the future of soils and their interactions with plants.