Roots as agents of soil change over time and as mediators of humanity’s imprint on Earth’s critical zone

Abstract

Plant roots mediate the movement of materials such as nutrients and carbon (C) across Earth’s surface. Roots allocate fixed C to processes that govern nutrient uptake, and in doing so redistribute soil resources. However, plant C for nutrient exchange strategies are not well understood and are proceeding amidst climate and land use changes that alter the amount of available C and nutrients, as well as the depth of root exploration. I examine the role of rooting system economies in nutrient availability and C cycling, and the ways those economies are changing as a consequence of anthropogenic activity. In Chapter 1, I characterize rooting systems’ C for nutrient strategies across diverse forest ecosystems and demonstrate that, if left unperturbed, rooting systems increasingly tap into deeply distributed, mineral bound nutrient stocks over time to sustain forest nutrition. We even observed this effect in highly weathered soils where we would expect accumulating organic matter to play the primary role in ecosystem nutrition. Chapter 2 then characterizes the global extent of changes to rooting systems, demonstrating that human activity is inducing shallower rooting depths, on average, across the planet. Chapter 3 begins to address the biogeochemical consequences of shallower rooting systems in the Anthropocene by examining the C for nutrient economies in forests with and without extensive anthropogenic legacies at the Calhoun Critical Zone Observatory. There, I found that even after 80 years of forest regrowth, rooting systems developing after intense ecosystem disturbance remained truncated and thus reliant on shallow, organically-bound nutrient stocks, in contrast to unperturbed forests with deep roots that tapped into deep, mineral-bound nutrients. In Chapter 4, I examine the plasticity of rooting system C for nutrient strategies in the presence of distinct nutrient forms to better understand roots’ capacity to cope with these changing soil environments. I found that rooting systems were able to adapt their C for nutrient exchanges across varied depth distributions of mineral and organic nutrient pools, resulting in more C allocated at depths where the most potentially bioavailable nutrient forms reside. This suggests that plants’ nutrient economies may be sufficiently plastic to respond to the novel nutrient depth distributions emerging with anthropogenic environmental change. Together, these studies demonstrate that rooting systems and their depth distributions are ecosystem traits that have unanticipated, outsized importance in governing the movement of C and nutrients within Earth’s subsurface environments. In the context of the Anthropocene, these rooting depth-driven functions are subject to global perturbations that may promote wide-spread shifts in the ways nutrient and C cycles are structured from the ground up.