The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.

Abstract. Plant root water uptake (RWU) has been documented for the past five decades from water stable isotopic analysis. By comparing the (hydrogen or oxygen) stable isotopic compositions of plant xylem water to those of potential contributive water sources (e.g., water from different soil layers, groundwater, water from recent precipitation or from a nearby stream), studies were able to determine the relative contributions of these water sources to RWU. In this paper, the different methods used for locating/quantifying relative contributions of water sources to RWU (i.e., graphical inference, statistical (e.g., Bayesian) multi-source linear mixing models) are reviewed with emphasis on their respective advantages and drawbacks. The graphical and statistical methods are tested against a physically based analytical RWU model during a series of virtual experiments differing in the depth of the groundwater table, the soil surface water status, and the plant transpiration rate value. The benchmarking of these methods illustrates the limitations of the graphical and statistical methods while it underlines the performance of one Bayesian mixing model. The simplest two-end-member mixing model is also successfully tested when all possible sources in the soil can be identified to define the two end-members and compute their isotopic compositions. Finally, the authors call for a development of approaches coupling physically based RWU models with controlled condition experimental setups.

[1] In soils, the isotopic composition of water ( 2 H and 18 O) provides qualitative (e.g., location of the evaporation front) and quantitative (e.g., evaporation flux and root water uptake depths) information. However, the main disadvantage of the isotope methodology is that contrary to other soil state variables that can be monitored over long time periods Citation: Rothfuss, Y., H. Vereecken, and N. Br€ uggemann (2013), Monitoring water stable isotopic composition in soils using gas-permeable tubing and infrared laser absorption spectroscopy, Water Resour.

International audienceIn this study, we performed a partitioning of evapotranspiration (ET) under fully controlled conditions (climatic chamber) along growth of a tall fescue cover (Festuca arundinacea) into soil evaporation (Ev) and plant transpiration (Tr) by measuring their stable oxygen isotopic compositions (δET, δEv and δTr). We showed that it was possible, under the chamber's particular conditions, to realize the partition without (1) making the hypothesis of steady state transpiration usually done in the field, nor (2) calculating δEv as a function of air relative humidity, soil water and atmospheric vapour isotopic compositions. The contribution of Ev to total ET decreased over the experiment from 100% (bare soil) to 94% [16 days after the seeding (DAS), 83% (28 DAS), 70% (36 DAS) and 5% (43 DAS)]. Soil isotopic profiles calculated using a typical exponential-type expression and measured Ev flux were compared with the bare soil steady state measured profiles. Agreement between modelled and measured values was sensitive to soil tortuosity and kinetic fractionation values. Another significant result was highly enriched isotopic values estimated for soil water at the evaporation front [the surface under our experimental conditions (δsurf)]. The theoretical estimate of δsurf was about 1-6‰ enriched as compared to the values measured in the top 1 cm of the soil, raising important implications for the determination of δEv as a function of δsurf under field conditions. Our work also points out uncertainties related to the determination of partition values and isotopic composition measured in the field, a point that is often ignored in many papers on isotopic biogeochemistry applied to geochemical fluxes, although it can be important. Copyright © 2010 John Wiley & Sons, Ltd