The diurnal exchange of carbon dioxide, water vapor and heat between a vegetated land surface and the atmosphere is driven primarily by short-term climate fluctuations and the availability of moisture in the plant root zone. Over seasonal to inter-annual timescales, the coupling between above-ground and below-ground processes plays a more important role in ecosystem productivity through the regulation of the cycling of nutrients critical for vegetation functioning. Nitrogen plays a crucial role in controlling photosynthetic processes, acting as a potential limitation on carbon dioxide uptake and thus long-term biomass storage. Modifications to the carbon dioxide uptake of vegetation in turn 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. Nitrogen availability for plant processes is controlled by 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.
This talk will provide details of a new plant-root-soil model synthesis capable of resolving many of the complex biogeochemical interactions and feedbacks between the canopy, soil and root subsystems. The model canopy is partitioned into several layers, allowing for resolution of the shortwave and longwave radiation distributions that drive photosynthesis, stomatal conductance and leaf energy balance in each layer, along with the canopy microclimate. The above-ground component of the model is coupled to a multi-layer soil-root model that computes soil moisture and heat transport, root water uptake, and the passive redistribution of moisture across soil potential gradients by the root system (ie. hydraulic redistribution). Carbon and nitrogen transformations in each layer of the soil system are modulated by microbial activity, and act to provide nutrient constraints on the photosynthetic capacity of the canopy.
In conjunction with data collected at the Bondville, IL Ameriflux and SoyFACE (Savoy, IL) study sites, the model is applied to diagnose the impacts of elevated CO2 on observed modifications on canopy energy partitioning. Results from an examination of the potential role of hydraulic redistribution on subsurface carbon and nitrogen transformations and plant nitrogen availability are also presented.
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