Abstract
AimsHydro-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 NH4+, both high-demand nutrients.MethodsA 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 NH4+.ResultsCompetitive cation exchange enabled low-demand cations that accumulate against roots (Ca2+, Mg2+, Na+) to desorb NH4+ 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 NH4+ 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.ConclusionsDiel plant water use and competitive cation exchange enhanced NH4+ and K+ availability and influenced rhizosphere concentration dynamics. Demonstrated responses have implications for understanding rhizosphere nutrient cycling and plant nutrient uptake.
Highlights
IntroductionPlant roots and their associated soil environment (i.e., the rhizosphere) represent the belowground portion or ‘hidden half’ of ecosystems (Waisel et al 1991)
Plant roots and their associated soil environment represent the belowground portion or ‘hidden half’ of ecosystems (Waisel et al 1991)
Instead of simulating diel plant water use, a steady root-ward water flux is frequently imposed; instead of simulating competitive cation exchange, sorption interactions between dissolved nutrients and soil are often represented with a linear sorption isotherm or buffer coefficient, which implies that partitioning of a given ion between the aqueous and solid phase is independent of and unaffected by concentrations and partitioning behavior of other ions (e.g., Claassen et al 1986; Barber 1995; Tinker and Nye 2000; Nowack et al 2006; Lin and Kelly 2010)
Summary
Plant roots and their associated soil environment (i.e., the rhizosphere) represent the belowground portion or ‘hidden half’ of ecosystems (Waisel et al 1991). Instead of simulating diel plant water use (i.e., transpiration during the day and no water use during the night), a steady root-ward water flux is frequently imposed; instead of simulating competitive cation exchange, sorption interactions between dissolved nutrients and soil are often represented with a linear sorption isotherm or buffer coefficient, which implies that partitioning of a given ion between the aqueous and solid phase is independent of and unaffected by concentrations and partitioning behavior of other ions (e.g., Claassen et al 1986; Barber 1995; Tinker and Nye 2000; Nowack et al 2006; Lin and Kelly 2010). We hypothesized that these two often-neglected processes — diel plant water use and competitive cation exchange — could interact to alter nutrient availability and nutrient concentration patterns in the rhizosphere, with implications for understanding rhizosphere nutrient cycling and plant nutrient uptake
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