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

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.