The rhizosphere has a controlling role in the flow of water and nutrients from soil to plant roots; however, its hydraulic properties are not well understood. As roots grow, they change the pore size distribution of the surrounding soil. Roots release polymeric substances such as mucilage into their rhizosphere. Microorganisms living in the rhizosphere feed on these organic materials and release other polymeric substances into the rhizosphere. The presence of these organic materials might affect the water retention properties and the hydraulic conductivity of the rhizosphere soil during drying and rewetting. We used neutron radiography to monitor the dynamics of water distribution in the rhizosphere of lupin (Lupinus albus L.) plants during a period of drying and rewetting. The rhizosphere was shown to have a higher water content than the bulk soil during the drying period but a lower one during the subsequent rewetting. We evaluated the wettability of the bulk soil and the rhizosphere soil by measuring the contact angle of water in the soil. We found significantly higher contact angles for the rhizosphere soil than the bulk soil after drying, which indicates slight water repellency in the rhizosphere. This explains the lower soil water content in the rhizosphere than the bulk soil after rewetting. Our results suggest that the water holding capacity of the rhizosphere is dynamic and might shift toward higher or lower values than those of the surrounding bulk soil, not affected by roots, depending on the history of drying and rewetting cycles.

The significance of soil water‐repellent properties has been discussed with respect to water dynamics and distribution; however, there are indications of their importance also for C stabilization processes. Water‐repellent aggregates, for example, have been shown to protect soil organic matter (SOM) due to their stability against water slaking. Soil wettability can act as a key factor for SOM decomposition as it controls the microbial availability of water and nutrients. The main objective of this study was therefore to investigate the impact of wettability on C release by linking the wetting properties in terms of the contact angle to soil respiration parameters. For this, the wetting properties of two topsoil samples (an Orthic Luvisol and a Dystric Cambisol) were altered by the addition of particles that were hydrophobized by treatment with dichlorodimethylsilane (DCDMS). Additionally, aggregates were created to assess whether artificial aggregation also can contribute to SOM protection. Environmental scanning electron microscopy revealed a locally confined distribution of water for the DCDMS‐treated material, compared with untreated soil where water was uniformly distributed. Measurements indicated an increasing contact angle with increasing amount of DCDMS‐treated particles in the mixtures. With increasing contact angle, C release decreased, suggesting that wettability‐induced changes in water distribution can significantly affect the decomposition of SOM. Respiration from artificial aggregates, however, was not reduced compared with the corresponding homogeneous material. We conclude that wettability is an important factor for SOM decomposition as it governs the spatial distribution and availability of water necessary for microbial activity.

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Core Ideas Matric potential controls the equilibrium fractionation factor between soil water and water vapor. Surface chemistry determined by X‐ray spectroscopy affects the equilibrium fractionation factor. A conceptual isotope retention characteristic approach is presented. Soil water stable isotopes are widely used for geo‐ and ecohydrological applications. However, the signature of the soil water isotopic composition in the environment depends on various factors. While recent work has shown matric potential effects on equilibrium fractionation, little work has examined other soil parameters concerning soil water energy status like the surface wettability, usually quantified in terms of contact angle. We simultaneously explored the role of matric potential, contact angle, and soil surface chemistry effects on the equilibrium fractionation factor during soil water evaporation. We present a simple laboratory experiment with four different soils of various textures. Subsamples of each texture class were treated with dichlorodimethylsilane to modify surface wetting properties. Additionally, we tested two natural soil samples to explore wettability effects. Samples were dried at temperatures between 40 and 550°C to produce chemically modified surface properties. All samples were spiked with water of known isotopic composition at different water contents. The isotopic signature was determined using the vapor‐bag equilibration method. The matric potential of each sample was measured with a soil water potential meter, the contact angle was determined with the sessile drop method, and the surface chemistry by X‐ray photoelectron spectroscopy. In addition to temperature and soil matric potential, the elemental composition has apparently some control on the equilibrium fractionation factor. Based on findings, we introduce a new soil water isotope retention characteristic approach to summarize how these factors (matric potential, contact angle, and soil surface chemistry) each control the equilibrium fractionation factor for 18O/16O and 2H/H. Corresponding retention curve approach parameters are promising to be applied in the future to predict soil water fractionation effects under natural and non‐stationary conditions.

Many soils worldwide show water repellency to some degree. Soil water repellency (SWR) is known to alter hydraulic processes. Par cularly in water-repellent soils a decreased water infi ltra on rate can be observed. In this case soil hydraulic proper es, like the hydraulic conduc vity, not only are a result of the soil's pore system but also depend on the physicochemical proper es of the pore surfaces (water repellency). Ethanol as a completely we ng liquid is not infl uenced by the soil's water repellency. In this study we introduce the concept of intrinsic soil hydraulic proper es, that is, the hydraulic proper es that are only dependent on the porous system and independent of its surface proper es. We used the concept of intrinsic permeability, originally developed for saturated condi ons. The eff ect of diff erent liquid surface tensions of water and ethanol, important under unsaturated condi ons, was incorporated using a correc on factor for the matric poten al. Reten on and saturated and unsaturated liquid conduc vity of water and ethanol were systema cally measured in sand and glass-bead porous media with diff erent we abilites. Results showed no diff erence between the intrinsic hydraulic proper es (measured with ethanol) and the hydraulic proper es (measured with water) in fully we able porous media. In waterrepellent porous media, intrinsic hydraulic proper es deviate from measured hydraulic proper es. Measurements further showed that the infl uence of soil water repellency on hydraulic conduc vity and reten on of water can be predicted as a func on of the macroscopic contact angle (CA) in these model substrates. In summary, at least for well-defi ned substrates such as sands, we suggest measuring the hydraulic conduc vity and intrinsic liquid reten on of the pore system with ethanol as a standard procedure.Abbrevia ons: CA, contact angle; CPS, capillary pressure-satura on; MEK, methyl ethyl ketone; SWR, soil water repellency.Vadose Zone J. 9:445-450