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Upscaling salt accumulation and passive solute uptake from single root to root system scale

Boek - Dissertatie

Producing enough food for an increasing world population is one of the major challenges of the 21st century. Improving management practices that optimize the usage of water, nutrients and other agricultural resources is vital to increase crop production and productivity while maintaining a healthy environment. The usage of computational models has been integrated in the decision making of agricultural management practices such as in irrigation scheduling or pesticide application. However, most models that are applied at the plant or field scale lack an appropriate representation of processes that occur at the scale of single roots. This thesis aimed at investigating differences between conditions at the soil-root interface and the bulk soil, and the impact on the transport of two substances: sodium and pesticides. On the one hand, Na+ is a major contributor to soil salinity, which is a significant abiotic stress limiting crop production, especially in arid and semiarid areas. On the other hand, the study of pesticide transport and uptake by plants is crucial to managing their usage and prevent leaching to groundwater bodies. For these purposes, we used the coupled 3D R-SWMS-ParTrace models, which link flow and transport in the soil profile and root zone with flow and transport processes towards single root segments and with flow and transport in the entire root system.First, Na+ accumulation around single roots and its impact on root water uptake was modeled. We assumed a full exclusion scenario where the root prevents the transport of Na+ into the xylem and Na+ transported towards the root accumulate at the root surface. This accumulation leads to an increase in the osmotic potential at the soil-root interface, which, in turn, affects the water potential gradient between the root and the soil that may result in reduced transpiration. Simulation results from single root scenarios showed increasing the root water uptake per unit root length led to larger Na+ accumulation and earlier onset of osmotic stress. Macroscopic parameters obtained by fitting a macroscopic stress function to the simulated data were found to be dependent on the water uptake per unit root length, indicating that macroscopic parameters are not constant throughout a growing season as it is usually assumed. Finally, differences in bulk and soil-root interface osmotic potentials depended not only on transpiration rate, but also on the transpiration rate per unit of root length. Thus, models that translate bulk concentrations to concentrations at the soil-root interface need to incorporate root length density information.In a next step, salt accumulation around roots as a function of salinity and transpiration demand was studied in an experimental setup. Tomato plants were grown in 2D rhizoslides for two weeks. At the end of the experimental period, Na+ concentration was measured at three distances from the root surface. The R-SWMS-ParTrace models were parameterized according to the experimental setup and used further to investigate Na+concentration gradients. Both experimental and simulated results showed larger Na+ concentration at the root surface than in the bulk soil. Under high transpiration, the model simulated larger Na+ concentrations at the root surface than the measurements. This could indicate that roots took up Na+ and thus questioned the model's assumption of full exclusion. Moreover, the model predicted redistribution fluxes of low salinity water to areas with high salinity, resulting in a reduction in Na+ concentration around roots. This indicates that upscaled models need to account for heterogeneous distribution of Na+ and its impact on compensation processes.Finally, the transport and passive uptake of pesticides by roots was investigated. The R-SWMS-ParTrace models were further developed to account for diffusive transport of organic neutral compounds across root membranes driven by concentration gradients between the soil and the root, and a diffusive permeability dependent on root and solute properties. The model was parameterized according to a FOCUS scenario used in the registration of pesticides in the EU for a pesticide application at the soil surface and compared to results from the 1D reference PEARL model. Simulation results with a single root and advective uptake, whose hydraulic properties were adjusted to match root water uptake distributions estimated by PEARL, led to similar pesticide uptake as PEARL, which validated our model implementation. Simulations with diffusive uptake predicted 3 times larger uptake of pesticide than the advective scenario as a result of diluting effects caused by vertical concentration gradients of pesticide in the soil. Finally, when simulating uptake with a 3D maize root system architecture, differences in root water uptake distribution, resulting from the root system architecture and root hydraulic properties, led to 1.25 times larger pesticide uptake as compared to the single root scenario. This study illustrated the impact of accounting for heterogeneous solute distribution as well as mechanistic descriptions of solute uptake on the fate of pesticides in soils.
Jaar van publicatie:2022
Toegankelijkheid:Embargoed