Diel plant water use and competitive soil cation exchange interact to enhance NH and K availability in the rhizosphere.

Aims: Hydro-biogeochemical processes in the rhizosphere regulate nutrient and water availability, and thus ecosystem productivity. We hypothesized that two such processes often neglected in rhizosphere models - diel plant water use and competitive cation exchange - could interact to enhance availability of K and NH , both high-demand nutrients. Methods: A rhizosphere model with competitive cation exchange was used to investigate how diel plant water use (i.e., daytime transpiration coupled with no nighttime water use, with nighttime root water release, and with nighttime transpiration) affects competitive ion interactions and availability of K and NH . Results: Competitive cation exchange enabled low-demand cations that accumulate against roots (Ca, Mg, Na) to desorb NH and K from soil, generating non-monotonic dissolved concentration profiles (i.e. 'hotspots' 0.1-1 cm from the root). Cation accumulation and competitive desorption increased with net root water uptake. Daytime transpiration rate controlled diel variation in NH and K aqueous mass, nighttime water use controlled spatial locations of 'hotspots', and day-to-night differences in water use controlled diel differences in 'hotspot' concentrations. Conclusions: Diel plant water use and competitive cation exchange enhanced NH and K availability and influenced rhizosphere concentration dynamics. Demonstrated responses have implications for understanding rhizosphere nutrient cycling and plant nutrient uptake. [ABSTRACT FROM AUTHOR]