In this commentary, we summarize and build upon discussions that emerged during the workshop "Isotopebased studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to provide society and policymakers with the scientific background to manage water resources sustainably, especially considering the ever-increasing worldwide concern about water scarcity. Stable isotopes of hydrogen and oxygen in water have proven to be a powerful tool for tracking water fluxes in the critical zone. However, both mechanistic complexities (e.g. mixing and fractionation processes, heterogeneity of natural systems) and methodological issues (e.g. lack of standard protocols to sample specific compartments, such as soil water and xylem water) limit the application of stable water isotopes in critical-zone science. In this commentary, we examine some Published by Copernicus Publications on behalf of the European Geosciences Union. 6400 D. Penna et al.: Tracing terrestrial ecosystem water fluxes using stable isotopes of the opportunities and critical challenges of isotope-based ecohydrological applications and outline new perspectives focused on interdisciplinary research opportunities for this important tool in water and environmental science.

Recent studies using water isotopes have shown that trees and streams appear to return distinct water pools to the hydrosphere. Cryogenically extracted plant and soil water isotopic signatures diverge from the meteoric water lines, suggesting that plants would preferentially use bound soil water, while mobile soil water that infiltrates the soil recharges groundwater and feeds streamflow all plots on meteoric water lines. These findings have been described under the “two water worlds” (TWW) hypothesis. In spite of growing evidence for the TWW hypothesis, several questions remain unsolved within the scope of this framework. Here, we address the TWW as a null hypothesis and further assess the following: (a) the theoretical biophysical feasibility for two distinct water pools to exist, (b) plant and soil processes that could explain the different isotopic composition between the two water pools, and (c) methodological issues that could explain the divergent isotopic signatures. Moreover, we propose a way forward under the framework of the TWW hypothesis, proposing alternative perspectives and explanations, experiments to further test them, and methodological advances that could help illuminate this quest. We further highlight the need to improve our sampling resolution of plants and soils across time and space. We ultimately propose a set of key priorities for future research to improve our understanding of the ecohydrological processes controlling water flows through the soil–plant‐atmosphere continuum.

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Understanding the legacy of persistent organic pollutants requires studying the transition from primary to secondary source control.

Westudied tree water uptake patterns, tested for complementary water use among species and analysed controlling factors in a tropical tree diversity experiment. The water uptake depth of five species was investigated across seasons and diversity levels using the natural abundance of water isotopes (δ2H, δ18O) and modelling. Three distinct water acquisition strategies were found for trees growing inmonocultures during the dry season: strong reliance (>70%) on soil water fromthe upper layer (0–30 cm) (Cedrela odorata), uptake from the upper and deeper layers (>30cm) in equal proportions (Hura crepitans, Anacardium excelsum and Luehea seemannii) and water uptake predominately from deeper layers (Tabebuia rosea). Seasonal shifts in water uptake were most pronounced for T. rosea. The water uptake pattern of a given species was independent of the diversity level underlining the importance of species identity and species characteristics in spatial and temporal tree water use. Statistics did not show a significant effect of diversity on source water fractions, but we did see some evidence for complementary water resource utilization in mixed species plots, especially in the dry season. Our results also demonstrated that the depth of soil water uptake was related to leaf phenology and tree transpiration rates. A higher proportion of water obtained from deeper soil layers was associated with a high percentage foliage cover in the dry season, which explained the higher transpiration rates

In the next few decades, climate of the Amazon basin is expected to change, as a result of deforestation and rising temperatures, which may lead to feedback mechanisms in carbon (C) cycling that are presently unknown. Here, we report how a throughfall exclusion (TFE) experiment affected soil carbon dioxide (CO 2 ) production in a deeply weathered sandy Oxisol of Caxiuanã (Eastern Amazon). Over the course of 2 years, we measured soil CO 2 efflux and soil CO 2 concentrations, soil temperature and moisture in pits down to 3 m depth. Over a period of 2 years, TFE reduced on average soil CO 2 efflux from 4.3 AE 0.1 lmol CO 2 m À2 s À1 (control) to 3.2 AE 0.1 lmol CO 2 m À2 s À1 (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO 2 production was higher in the TFE plot (28%) compared with the control plot (17%), and it did not differ between years. We distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO 2 efflux in the TFE plot was related to a reduction of soil and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO 2 production was dominated by a water stress-induced decrease in total root respiration. Our results contrast to results of a drought experiment on clay Oxisols, which may be related to differences in soil water retention characteristics and depth of rooting zone. These results show that large differences exist in drought sensitivity among Amazonian forest ecosystems, which primarily seem to be affected by the combined effects of texture (affecting water holding capacity) and depth of rooting zone.

Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux, i.e., the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation, and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, first, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil, and vegetation types at global scale and second, to identify potential determinants of the site-to-site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil, and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R 2 = 0.67-0.99) in DOC concentrations in mineral European forest soils correlates with NH 4 + , C/N, Al, and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.

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