Impact of soil texture on temporal and spatial development of osmotic-potential gradients between bulk soil and rhizosphere

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 (). According to the convection-diffusion equation, solute transport depends on a number of parameters (soil moisture-release 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 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 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 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 () of -340 kPa was observed for the texture treatment T2 compared to 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 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 difference