Arsenate (As V) is the predominant form of arsenic in soils under aerobic conditions and competes with the major plant nutrient phosphorus (P) in the form of phosphate (PV) not only for sorption sites on mineral surfaces in soil but also for root membrane transporters. Plants have evolved several mechanisms for the mobilization of PV in soils in response to P deficiency, such as the release of organic anions and protons. The aim of the present study was to test whether these mechanisms result in a simultaneous mobilization of arsenate and what would be the consequences for As transfer from soil to plant. The compartment system approach with Zea mays as model crop was chosen as an experimental setup. The system is equipped with micro suction cups and allowed us to investigate processes occurring in the vicinity of roots. As a case study, an artificial quartz substrate with well defined soil physical properties was fertilized, spiked with As V, and amended with increasing amounts of goethite (0, 1, and 4 g kg(-1) in treatments G-0, G-1, and G-4, respectively). The addition of goethite alleviated the As V-induced growth reduction and reduced As V transfer from the substrate to the plant but induced P deficiency at the same time. When low amounts of goethite (1 g kg(-1)) were added, plants mobilized PV but not As V, which might be related to differences in surface complexation reported for PV and As V. No mobilization of PV or As V was observed with the addition of higher amounts of goethite, probably because of decreasing competition between organic anions, PV, and As V for binding sites.

The transfer of nutrients and contaminants from bulk soil to roots and into plants depends on many plant and soil processes. The RhizoMath approach for modeling co‐occurring processes in the rhizosphere, including speciation in the soil solution, is based on coupling the mathematical package MATLAB with the geochemical code PHREEQC. In addition to the built‐in initialization module that performs calibration against experimental data, RhizoMath's greatest advantage is that different geochemical models (with and without charge balance) and geometries (planar and radial) are already included. Moreover, due to its graphical user interface, the tool can be applied without changing the source code or a complex input file. The model was verified using a benchmark and experimental data: (i) the initialization module was successfully applied to describe concentrations measured in soil solution samples; (ii) the theoretical problem “diffusion of K toward a single root” was used to demonstrate that the performance of applied numerical methods is comparable to other approaches; and (iii) for compartment system experiments involving more complex speciation, RhizoMath was able to describe the observed effects of citrate exudates on the simultaneous transport of arsenate and phosphate that compete for surface binding sites with each other and with other oxyanions such as citrate.

Solute transport from the bulk soil to the root surface is, apart from changes in soil moisture and plant nutrient uptake, a prerequisite for changes in soil osmotic potential (W o ). According to the convection-diffusion equation, solute transport depends on a number of parameters (soil moisturerelease curve, hydraulic conductivity, tortuosity factor) which are functions of soil texture. It was thus hypothesized that soil texture should have an effect on the formation of W o gradients between bulk soil and the root surface. The knowledge about such gradients is important to evaluate water availability in the soil-plant-atmosphere continuum (SPAC). A linear compartment system with maize grown under controlled conditions in two texture treatments (T1, pure sand; T2, 80% sand, 20% silt) under low and high initial application of salts (S1, S2) was used to measure the development of W o gradients between bulk soil and the root surface by microscale soil-solution sampling and TDR sensors. The differences in soil texture had a strong impact on the formation of W o gradients between bulk soil and the root surface at high and low initial salt application rate. At high initial salt application, a maximum osmotic-potential gradient (DW o ) of -340 kPa was observed for the texture treatment T2 compared to DW o of -180 in T1. The steeper gradients in osmotic potential in treatment T2 compared to T1 corresponded to higher cumulative water consumption in this treatment which can partly be explained by higher soil hydraulic conductivity in the range of soil matric potentials covered during the duration of the experiments. Differences between texture treatments in W o at the root surface did not result in differences in plant-water relations measured as gas-exchange parameters (transpiration rate, water-use efficiency) and leaf osmotic potential. If soil osmotic and matric potential are regarded as additive in calculating the driving force for water movement from the soil into the root, the observed differences in water flux between treatments cannot be explained.