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We showed that crown roots have a different capacity to transport water compared with seminal roots. Acknowledging such differences between root types is crucial to understand optimal root traits.

Abstract. In this paper, we present and analyze a novel global database of soil infiltration measurements, the Soil Water Infiltration Global (SWIG) database. In total, 5023 infiltration curves were collected across all continents in the SWIG database. These data were either provided and quality checked by the scientists who performed the experiments or they were digitized from published articles. Data from 54 different countries were included in the database with major contributions from Iran, China, and the USA. In addition to its extensive geographical coverage, the collected infiltration curves cover research from 1976 to late 2017. Basic information on measurement location and method, soil properties, and land use was gathered along with the infiltration data, making the database valuable for the development of pedotransfer functions (PTFs) for estimating soil hydraulic properties, for the evaluation of infiltration measurement methods, and for developing and validating infiltration models. Soil textural information (clay, silt, and sand content) is available for 3842 out of 5023 infiltration measurements (∼ 76%) covering nearly all soil USDA textural classes except for the sandy clay and silt classes. Information on land use is available for 76 % of the experimental sites with agricultural land use as the dominant type (∼ 40%). We are convinced that the SWIG database will allow for a better parameterization of the infiltration process in land surface models and for testing infiltration models. All collected data and related soil characteristics are provided online in *.xlsx and *.csv formats for reference, and we add a disclaimer that the database is for public domain use only and can be copied freely by referencing it. Supplementary data are available at https://doi.org/10.1594/PANGAEA.885492 (Rahmati et al., 2018). Data quality assessment is strongly advised prior to any use of this database. Finally, we would like to encourage scientists to extend and update the SWIG database by uploading new data to it.

This study tested a method to quantify and locate hydraulic lift (HL, defined as the passive upward water flow from wetter to dryer soil zones through the plant root system) by combining an experiment using the stable water isotope 1 H 2 18 O as a tracer with a soil-plant water flow model. Our methodology consisted in (i) establishing the initial conditions for HL in a large rhizobox planted with Italian ryegrass (Lolium multiflorum Lam.), (ii) labeling water in the deepest soil layer with an 18 O-enriched solution, (iii) monitoring the water O isotopic composition in soil layers to find out changes in the upper layers that would reflect redistribution of 18 O-enriched water from the bottom layers by the roots, and (iv) comparing the observed soil water O isotopic composition to simulation results of a three-dimensional model of water flow and isotope transport in the soil-root system. Our main findings were that (i) the depth and strength of the observed changes in soil water O isotopic composition could be well reproduced with a modeling approach (RMSE = 0.2‰, i.e., equivalent to the precision of the isotopic measurements), (ii) the corresponding water volume involved in HL was estimated to account for 19% of the plant transpiration of the following day, i.e., 0.45 mm of water, and was in agreement with the observed soil water content changes, and (iii) the magnitude of the simulated HL was sensitive to both plant and soil hydraulic properties.Abbreviations: DaS, days after seeding; HL, hydraulic lift; RLD, root length density; RWU, root water uptake.Hydraulic lift (HL) is defined as the upward water flow from wetter to drier soil layers through passive root transport (Richards and Caldwell, 1987) and has been reported for numerous plant species (Neumann and Cardon, 2012). In principle, HL takes place when the root water potential is higher in upper soil layers than the soil water potential containing these roots, while in deeper layers the soil water potential is higher than the root water potential contained in this layer. Hydraulic lift is a particular case of hydraulic redistribution, which may happen in any soil direction, e.g., from shallower to deeper layers or in the horizontal direction (Burgess et al., 1998).The quantitative and qualitative implications of HL on the plant water balance, nutrient mobilization, and competition for space and nutrients between plants are still unclear among the scientific community (Burgess, 2011). From a hydrological perspective, the absolute volume of hydraulically lifted water has been debated; it has been reported to vary by nearly two orders of magnitude depending on the species and environmental conditions (representing between 2 and 143% of the plant transpiration, with a mean of 30%) (Neumann and Cardon, 2012). Discriminating between soil and root hydraulic redistribution is also very challenging (Katul and Siqueira, 2010). This leads to divergence among researchers on the importance of HL: in some studies, it was estimated to be a negligible term of the water bu...

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Abstract. Stable isotopologues of water are widely used to derive relative root water uptake (RWU) profiles and average RWU depth in lignified plants. Uniform isotope composition of plant xylem water (δxyl) along the stem length of woody plants is a central assumption of the isotope tracing approach which has never been properly evaluated. Here we evaluate whether strong variation in δxyl within woody plants exists using empirical field observations from French Guiana, northwestern China, and Germany. In addition, supported by a mechanistic plant hydraulic model, we test hypotheses on how variation in δxyl can develop through the effects of diurnal variation in RWU, sap flux density, diffusion, and various other soil and plant parameters on the δxyl of woody plants. The hydrogen and oxygen isotope composition of plant xylem water shows strong temporal (i.e., sub-daily) and spatial (i.e., along the stem) variation ranging up to 25.2 ‰ and 6.8 ‰ for δ2H and δ18O, respectively, greatly exceeding the measurement error range in all evaluated datasets. Model explorations predict that significant δxyl variation could arise from diurnal RWU fluctuations and vertical soil water heterogeneity. Moreover, significant differences in δxyl emerge between individuals that differ only in sap flux densities or are monitored at different times or heights. This work shows a complex pattern of δxyl transport in the soil–root–xylem system which can be related to the dynamics of RWU by plants. These dynamics complicate the assessment of RWU when using stable water isotopologues but also open new opportunities to study drought responses to environmental drivers. We propose including the monitoring of sap flow and soil matric potential for more robust estimates of average RWU depth and expansion of attainable insights in plant drought strategies and responses.