Coupling Canopy-Root-Soil Processes to Subsurface Nitrogen Dynamics: Impacts of Root Moisture Uptake and Hydraulic Redistribution

Darren Drewry, University of Illinois at Urbana-Champaign

Ecosystem productivity over seasonal to inter-annual timescales is, in part, regulated by biogeochemical cycling of nutrients critical for vegetation functioning. Nitrogen plays a crucial role in controlling photosynthetic processes, acting as a limiting factor on carbon dioxide (CO₂) uptake and thus long-term biomass storage in many ecosystems. Modifications to vegetation CO₂ uptake can impact water and energy exchange with the atmosphere through the coupling of carbon, water and heat fluxes by stomatal dynamics, resulting in a modified subsurface moisture regime. Plant nitrogen availability is a function of subsurface moisture and heat transport, as organic matter decomposition and nitrogen mineralization are dependent on the moisture and temperature states of the soil. The impact of root functioning, through soil moisture uptake and hydraulic redistribution, on biogeochemical cycling and land-atmosphere exchange remains an open question.

Here I will present a model framework coupling above-ground vegetation processes with below-ground soil transport mechanisms and root moisture and nutrient uptake. The above-ground vegetation component consists of a multi-layer canopy that resolves radiation attenuation and leaf energy balance at each vertical level. The energy balance is coupled to photosynthetic carbon dioxide uptake, based on a Farquhar-based photosynthesis approach, through the sensitivity of stomatal conductance to CO₂ uptake. The below-ground moisture dynamics are based on a novel mechanistic coupling of soil moisture transport, by Richard's equation, with root moisture uptake and transport. This formulation allows for the passive redistribution of soil moisture by the root system, driven by vertical gradients in soil moisture potential (ie. hydraulic redistribution). These equations are solved simultaneously with subsurface temperature and nutrient transport and mineral N uptake relationships. The complete model couples the above-ground and sub-surface vegetation and soil systems, linked through biogeochemical, energy and water flows. The model is applied to a semi-arid ecosystem in the western United States. A particular focus is placed on the impacts of root moisture uptake and hydraulic redistribution on sub-surface carbon and nitrogen transformations and nutrient uptake.