Do specific root zones shape plant net N uptake? Modeling insights from Root-CyNAPS

Abstract Net nitrogen (N) uptake results from active and water-mediated N inflows, partly offset by N exudation. These processes exhibit significant variations along root axes, suggesting the existence of specific zones of higher N exchanges. However, the precise location, drivers, and significance of such active root zones in root N budget remain unclear. Here, we identified and characterized active root zones using Root-CyNAPS, a new functional-structural plant model that simulates the space-time variations in net N uptake, based on the interactions between nitrogen, carbon, and water flows, and root anatomy in each segment of a 3D root system architecture. Our simulations on wheat for various plant ages and external nitrate concentrations revealed two zones of preferential net N uptake: one near the root apices and the other one coinciding with the lateral root initiation zone, both characterized by a net N uptake activity about 10 times higher than adjacent segments. Higher water uptake and N exudation rates were also located next to apices. The contribution of most active root zones (defined here by the top 10% of root length) varied between 20% and 80\% of plant net N uptake, depending on root environment and age. Our simulations also showed that water-mediated N uptake could represent up to two thirds of gross N uptake, while N losses could offset more than half of it. Root-CyNAPS offers coupling opportunities with other functional-structural models to simulate nitrogen, carbon, and water multiscale interactions across the soil-plant-atmosphere continuum.