Diel plant water use and competitive soil cation exchange interact to enhance NH4 + and K+ availability in the rhizosphere

Abstract

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 exchangecould interact to enhance availability of K + and NH , Na + ) to desorb NH 4 + 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 4 + 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 4 + and K + availability and influenced rhizosphere concentration dynamics. Demonstrated responses have implications for understanding rhizosphere nutrient cycling and plant nutrient uptake.

To Fix or Not To Fix: Controls on Free-Living Nitrogen Fixation in the Rhizosphere

“…Rhizosphere oxygen concentration also likely follows a diel pattern that could partially control the activity of rhizosphere-associated microbes (42). Modeling efforts suggest that diel changes in water flow through the rhizosphere (43) would have a big impact on oxygen concentration. Active root and microbial growth in the rhizosphere may also create oxygen depletion zones within the rhizosphere (44).…”

An efficient three-dimensional rhizosphere modeling capability to study the effect of root system architecture on soil water and reactive transport

“…The root uptake of nutrient is assumed to be an active uptake process as opposed to a passive process where solutes are simply taken up with the porewater by the roots. The active uptake rate is represented by a Michaelis-Menten expression (Somma et al 1998;Wu et al 2007;Espeleta et al 2017) in this model as shown in the following equation:…”

“…Using a simple 1D model given prescribed water potential at soil-root boundary, Espeleta et al (2017) found that competitive cation exchange enabled NH 4 + and K + from soil to be desorbed from soil exchange sites and made available for plant use when low-demand cations such as Ca 2+ , Mg 2+ , and Na + accumulated near roots during water uptake. To simulate competitive ion exchange in 3D, cation exchange reactions (Table 1) and root nutrient uptake were included in our model.…”

“…The longitudinal and transverse dispersivities are 0.1 cm and 0.01 cm, respectively. The other input parameters are taken from Espeleta et al (2017) and shown in Table 2 for completeness. Cl − is a conservative tracer (assumed to not be taken up by the root and not be reactive) in this simulation, included for charge balance.…”

“…Three-dimensional (3D) models can better resolve the spatial distribution of water and nutrient resulting from interactions between individual roots and the local soil hydrobiogeochemistry (Garrigues et al 2006;Manoli et al 2014). On the other hand, simulations of nutrient dynamics are mainly limited to one and two dimensions (Espeleta et al 2017;Mollier et al 2008;Nietfeld and Prenzel 2015), or 3D non-reactive nutrient uptake, explicitly taking into account the geometry of the plant root system (Clausnitzer and Hopmans 1994;Dunbabin et al 2004;Dunbabin et al 2006;Leitner et al 2010c;Simunek and Hopmans 2009;Tournier et al 2015). The model by Gérard et al (2017) was the first to couple a root system architecture model with a multicomponent reactive transport model using a macroscopic approach.…”