Quantifying the contribution of mass flow to nitrogen acquisition by an individual plant root.

Summary: The classic model of nitrogen (N) flux into roots is as a Michaelis–Menten (MM) function of soil‐N concentration at root surfaces. Furthermore, soil‐N transport processes that determine soil‐N concentration at root surfaces are seen as a bottleneck for plant nutrition. Yet, neither the MM relationship nor soil‐N transport mechanisms are represented in current terrestrial biosphere models. Processes governing N supply to roots – diffusion, mass flow, N immobilization by soil microbes – are incorporated in a model of root‐N uptake. We highlight a seldom considered interaction between these processes: nutrient traverses the rhizosphere more quickly in the presence of mass flow, reducing the probability of its immobilization before reaching the root surface. Root‐N uptake is sensitive to the rate of mass flow for widely spaced roots with high N uptake capacity, but not for closely spaced roots or roots with low uptake capacity. The results point to a benefit of root switching from high‐ to low‐affinity N transport systems in the presence of mass flow. Simulations indicate a strong impact of soil water uptake on N delivery to widely spaced roots through transpirationally driven mass flow. Furthermore, a given rate of N uptake per unit soil volume may be achieved by lower root biomass in the presence of mass flow. [ABSTRACT FROM AUTHOR]