Céline Richard-Molard

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.

Background and Aims: Rhizodeposition, i.e. the release of organic matters by roots, constitutes a significant fraction of the plant carbon (C) budget and plays a key role in plant-soil interactions. However, its spatial and temporal dynamics remain poorly understood. Methods: We developed RhizoDep, a new functional-structural root model that simulates 3D root growth, respiration, and rhizodeposition based on C balance and root morphology at the individual root segment level. Results: Our model successfully reproduced the dynamics of belowground C flows observed in a previous pulse-labelling field experiment on spring wheat. Our simulations revealed that root C exudation largely dominated over mucilage secretion and cap cells sloughing in terms of C release. The spatial distribution of exudation rate along the roots was driven by the preferential unloading of sugars to support root elongation and emergence, and was modulated by the formation of apoplastic barriers. Furthermore, our results demonstrated that, for a given C allocation flow to roots, variations in root hairs or lateral root number had minimal effects on rhizodeposition, whereas changes in root tissue density had a significant impact. Conclusion: RhizoDep offers a new opportunity to explore the dynamics of C exchange at the plant-soil interface and to identify traits and environmental conditions that favor rhizodeposition.

Rhizodeposition, i.e. the release of organic matters by roots, constitutes a significant fraction of the plant carbon (C) budget and plays a key role in soil-plant interactions. However, its spatial and temporal dynamics remain poorly understood. We developed RhizoDep, a new functional-structural root model that simulates 3D root growth, respiration, and rhizodeposition based on C balance and root morphology at the individual root segment level. Our model successfully reproduced the dynamics of belowground C flows observed in a previous pulse-labelling field experiment on spring wheat. Our simulations revealed that root C exudation largely dominated over mucilage secretion and cap cells sloughing in terms of C release. The spatial distribution of exudation rate along the roots was driven by the preferential unloading of sugars to support root elongation and emergence, and was modulated by the formation of apoplastic barriers. Furthermore, our results demonstrated that, for a given C allocation flow to roots, variations in root hairs or lateral root number had minimal effects on rhizodeposition, whereas changes in root tissue density had a significant impact. RhizoDep offers a new opportunity to explore the dynamics of C exchange at the plant-soil interface and to identify traits and environmental conditions that favor rhizodeposition. Highlight: Using the new model RhizoDep , we simulated distinct spatial and temporal patterns of rhizodeposition along the roots of spring wheat over its complete growth cycle, and identified their main drivers. Graphical

Intercropping management strategy involves the use of legumes alongside a commercial crop to achieve various benefits, such as improved soil nutrient circulation, water retention, and pest control. However, research has shown that there is a lack of understanding of the long-term benefits of legumes within cropping systems and their specific interactions with the soil. In our experimental setup, we conducted four treatments using soil columns that were 24 cm in diameter and 1 m in length. We grew two winter rapeseeds as a monocrop, an intercrop of one rapeseed and one faba-bean, and two faba-beans as a monocrop in each soil column. We killed the faba-bean during the winter frost and left it as green mulch in the rapeseed intercrop. The final treatment was bare soil control columns. We measured soil nitrogen, the biomass, and nitrogen content of living plants and plant litter in September, October, November, and June. Additionally, we studied the decomposition of rapeseed and faba-bean residues in the soil in a laboratory incubation experiment to measure carbon and nitrogen mineralization. The majority of mineral nitrogen leaching occurred during late autumn at the beginning of the growing season (September to October) in all treatments. The soil mineral nitrogen contents over the growing season for the rapeseed-faba-bean intercrop system were similar to the rapeseed monocrop system, but lower than in the faba-bean monocrop system. The nitrogen balance in the columns for each treatment revealed that bare soil lost the most nitrogen over time due to leaching and lack of plants to uptake mineral nitrogen and immobilize it as biomass. The second one which lost the most nitrogen by leaching is The faba-bean monocrop. In this treatment, the plant did not use soil nitrogen in the lower portions of the soil column and this portion of the nitrogen in the soil may have been lost by leaching. The nitrogen was better conserved in the soil column and in the plants in the treatments with rapeseed intercrop and monocrop. Soil nitrogen was removed by the plant more efficiently leading to less leaching. The nitrogen content and biomass of one rapeseed plant in the intercrop was nearly double the one rapeseed plant in monocrop. Indeed, the total biomass and nitrogen content of the two rapeseeds in monocrop was equivalent to the single rapeseed in the intercrop. Conversely, rapeseed mulch had less nitrogen in intercrops than in the monocrop system. Lastly, the incubation of crop residues initially immobilized soil mineral nitrogen. The faba-bean mulch started releasing more mineral nitrogen than the bare soil after day 70. The release of mineral nitrogen of rapeseed and rapeseed-faba-bean mulch mixture exceeded the nitrogen of bare soil after day 90. Overall, it is clear that intercropping with legumes can have positive effects on soil nitrogen and plant growth, but more research is needed to fully understand the long-term benefits and interactions between legumes, the soil, and commercial crops.