Signatures of the deep rhizosphere: Novel instrumentation and predictive models

Vegetation simultaneously drives transpiration, a significant component of the hydrological balance, and stimulates the breakdown of bedrock and formation of soil. Together these actions impact both the magnitude of streamflow and the chemical or solute load of the stream. Yet, it is still unknown under what conditions deeply rooted plants enhance or impede chemical weathering of rocks. An unestablished link at the heart of this gap in knowledge are the ways in which coupling between plant water demand, plant nutrient demand and recycling of these elements through the ecosystem manifest in the geochemical composition of watersheds and the rivers that drain them. Addressing this unknown is of utmost practical importance to water resource management, environmental stewardship, ecosystem resilience to disturbance (storms, fire, drought), and ultimately nutrient effluxes from watersheds. This presentation introduces two recent advancements in our collective capacity to deconvolve these vital linkages between ecology, hydrology and chemical weathering: novel observational tools and ecologically informed chemical weathering models. First, we present hydrologic and geochemical observations from within the deep root-zone gained from the successful deployment of a Vadose zone Monitoring System (VMS). The VMS allows for real-time moisture content monitoring as well as discrete sampling of water and reactive gases across partially saturated bedrock. This novel capability has now revealed that the mature, deeply rooted forest relies on water stored in bedrock above the water table during the extended dry season. The VMS has also shown CO 2 concentrations and production rates in the deep root zone comparable to what is typically observed in shallow soils. This deep CO 2 is radiocarbon modern and thus associated with recent photosynthetically fixed carbon. Water chemistry observations from these depths indicate that this CO 2 production in the deep root-zone enhances chemical weathering by increasing carbonic acid formation. Thus, the extension of water and carbon fluxes to depths meters below soils leads to a hotspot of chemical weathering in the deep root zone where meteoric water, carbonic acid weathering potential, and primary minerals all intersect. Second, we present the derivation and testing of a new ecologically informed reactive transport model (RTM) which directly simulates the uptake of both water and nutrients across the deep rhizosphere. A key aspect of our modeling framework is that, unlike transpired water, rock-derived nutrients taken up by plants are not lost to the atmosphere but rather recycled into litter and soils, creating a biological pool in the Critical Zone cycling of rock-derived nutrients. Our model leads to the hypothesis that the water and nutrient demands of ecosystems, coupled with the capacity to partially or fully recycle elements to the soil surface, regulate the observed rates and depth of chemical weathering reactions. These results reveal the capacity for plant water and nutrient demands to both enhance and impede mineral weathering reactions, to drive formation of secondary minerals, and to redistribute elements across the vertical weathering profile. Ultimately, our model allows us to demonstrate how plant water and nutrient requirements manifest in the export of water and solutes by streams. The development of this forward model in tandem with critical advancements in direct observation and sampling of the deep rhizosphere is now poised to provide a foundation upon which to improve our understanding of reactive transport processes in watersheds and Critical Zone systems, which in turn supports advancements in ecohydrology, global elemental budgets, watershed stewardship, and water quality resources.