Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn long-term experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ2H and δ18O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15–20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate tracer-aided hydrological models either at the plot to the catchment scale.

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Stable isotopes reveal evaporation dynamics at the soil-plant-atmosphere interface of the critical zone

10.5194/hess-2017-87 ◽

2017 ◽

Cited By ~ 3

Author(s):

Matthias Sprenger ◽

Doerthe Tetzlaff ◽

Chris Soulsby

Keyword(s):

Stable Isotopes ◽

Isotopic Composition ◽

Scots Pine ◽

Soil Depth ◽

Critical Zone ◽

Soil Evaporation ◽

Pore Waters ◽

Catchment Scale ◽

Scottish Highlands ◽

Northern Latitudes

Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn long-term experimental catchment in the Scottish Highlands; a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analysed for their isotopic composition (δ2H and δ18H) with the direct equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15–20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate tracer-aided hydrological models either at the plot to the catchment scale.

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Water Stable Isotopes in an Alpine Setting of the Northeastern Tibetan Plateau

Water ◽

10.3390/w11040770 ◽

2019 ◽

Vol 11 (4) ◽

pp. 770 ◽

Cited By ~ 2

Author(s):

Xue Qiu ◽

Mingjun Zhang ◽

Shengjie Wang ◽

Athanassios A. Argiriou ◽

Rong Chen ◽

...

Keyword(s):

Stable Isotopes ◽

Soil Water ◽

Water Cycle ◽

Soil Depth ◽

Soil Layer ◽

Water Source ◽

Hydrological Processes ◽

Deep Soil ◽

Monthly Basis ◽

Mountainous Regions

Hydrological processes produce effects on water resources in inland mountainous regions. To perform a comprehensive investigation of the important segments of the water cycle, using the Qilian Mountains as a case study, precipitation, soil, plant, river, and groundwater were collected during the plant growing season of 2016. All samples were collected on a monthly basis, except precipitation, which was collected on a per event basis. The results showed that: the “temperature effect” was apparent, which suggested a drier climate background; there were differences in the slope and intercept of the local meteoric water line, using different regression methods; and the δ18O of soil water varied greatly in the topsoil, tended to be similar in the deep soil, and became increasingly depleted as the soil depth increased. The responses of the soil water isotopes to precipitation pulses had different boundaries. The major water source for Caragana Fabr. in no-precipitation month was located in the 0–30 cm soil layer, but was different in months when precipitation occurred. Overall, the findings from the stable isotopes provide insights into hydrological processes and offer a platform to understand mountainous water cycle in arid areas.

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Measurements and APSIM modelling of soil C and N dynamics

Soil Research ◽

10.1071/sr19021 ◽

2020 ◽

Vol 58 (1) ◽

pp. 41

Author(s):

C. J. Smith ◽

B. C. T. Macdonald ◽

H. Xing ◽

O. T. Denmead ◽

E. Wang ◽

...

Keyword(s):

Soil Temperature ◽

Soil Water ◽

Soil Depth ◽

Soil Evaporation ◽

15N Balance ◽

N Dynamics ◽

Soil C ◽

Ammonium N ◽

Balance Technique ◽

Using Data

Process-based models capture our understanding of key processes that interact to determine productivity and environmental outcomes. Combining measurements and modelling together help assess the consequences of these interactions, identify knowledge gaps and improve understanding of these processes. Here, we present a dataset (collected in a two-month fallow period) and list potential issues related to use of the APSIM model in predicting fluxes of soil water, heat, nitrogen (N) and carbon (C). Within the APSIM framework, two soil water modules (SoilWat and SWIM3) were used to predict soil evaporation and soil moisture content. SWIM3 tended to overestimate soil evaporation immediately after rainfall events, and SoilWat provided better predictions of evaporation. Our results highlight the need for testing the modules using data that includes wetting and drying cycles. Two soil temperature modules were also evaluated. Predictions of soil temperature were better for SoilTemp than the default module. APSIM configured with different combinations of soil water and temperature modules predicted nitrate dynamics well, but poorly predicted ammonium-N dynamics. The predicted ammonium-N pool empties several weeks after fertilisation, which was not observed, indicating that the processes of mineralisation and nitrification in APSIM require improvements. The fluxes of soil respiration and nitrous oxide, measured by chamber and micrometeorological methods, were roughly captured by APSIM. Discrepancies between the fluxes measured with chamber and micrometeorological techniques highlight difficulties in obtaining accurate measurements for evaluating performance of APSIM to predict gaseous fluxes. There was uncertainty associated with soil depth, which contributed to surface emissions. Our results showed that APSIM performance in simulating N2O fluxes should be considered in relation to data precision and uncertainty, especially the soil depths included in simulations. Finally, there was a major disconnection between the predicted N loss from denitrification (N2 + N2O) and that measured using the 15N balance technique.

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Macropore flow of old water revisited: where does the mixing occur at the hillslope scale?

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-9-4333-2012 ◽

2012 ◽

Vol 9 (4) ◽

pp. 4333-4380 ◽

Cited By ~ 3

Author(s):

J. Klaus ◽

E. Zehe ◽

M. Elsner ◽

C. Külls ◽

J. J. McDonnell

Keyword(s):

Stable Isotope ◽

Isotopic Composition ◽

Soil Water ◽

Irrigation Water ◽

Initial Conditions ◽

Soil Depth ◽

Lower Boundary ◽

Base Flow ◽

Macropore Flow ◽

Hillslope Scale

Abstract. The mechanisms allowing the rapid release of stored water to streams are poorly understood. Here we use a tile drained field site to combine naturally structured soils at the hillslope scale with the advantage of at least partly controlled lower boundary conditions. We performed a series of three irrigation experiments combining hydrometric measurements with stable isotope and bromide tracers to better understand macropore-matrix interactions and stored water release processes at the hillslope scale. Stable isotope concentrations were monitored in the irrigation water, the tile drain discharge and the soil water before and after the experiment. Bromide was measured at mainly every 5–15 min in the tile drain hydrograph. Different initial conditions for each experiment were used to examine how pre-event soil moisture conditions influenced flow and transport. Different amounts of irrigation water were necessary to increase tile drain discharge above the base flow level. Hydrograph separation based on bromide data revealed that irrigation water contributions to peak tile drain discharge were on the order of 20%. Oxygen-18 and deuterium data were consistent with the bromide data and showed that pre-event soil water contributed significantly to the tile drain event flow. However, the isotopic composition of soil water converged towards the isotopic composition of irrigation water through the course of the experiment. Mixing calculations revealed that by the end of the irrigation experiments 20% of the soil water in the entire profile was irrigation water. The isotopic data showed that the pre-event water in the tile drain was mobilized in 20–40 cm soil depth were the macropore-matrix interaction leads to an initiation of macropore flow after a moisture threshold is exceeded.

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Mechanisms of consistently disjunct soil water pools over (pore) space and time

Hydrology and Earth System Sciences ◽

10.5194/hess-23-2751-2019 ◽

2019 ◽

Vol 23 (6) ◽

pp. 2751-2762 ◽

Cited By ~ 11

Author(s):

Matthias Sprenger ◽

Pilar Llorens ◽

Carles Cayuela ◽

Francesc Gallart ◽

Jérôme Latron

Keyword(s):

Stable Isotopes ◽

Stable Isotope ◽

Soil Water ◽

Soil Depth ◽

Stream Water ◽

Pore Space ◽

Bound Water ◽

Subsurface Water ◽

Data Set ◽

Soil Pores

Abstract. The storage and release of water in soils is critical for sustaining plant transpiration and groundwater recharge. However, how much subsurface mixing of water occurs, and how much of the water is available for plants or otherwise percolates to streams and the groundwater is not yet understood. Based on stable isotope (2H and 18O) data, some studies have found that water infiltrating into soils can bypass older pore water. However, the mechanisms leading to the separation of water routed to the streams and water held tightly in smaller pores are still unclear. Here, we address the current limitations of the understanding of subsurface mixing and their consequences regarding the application of stable isotopes in ecohydrological studies. We present an extensive data set, for which we sampled the isotopic composition of mobile and bulk soil water in parallel with groundwater at a fortnightly temporal resolution and stream water and rainfall at a much higher resolution in a Mediterranean long-term research catchment, in Vallcebre, Spain. The data reveal that the mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix well; however, they constitute two disjunct subsurface water pools with little exchange, despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentalization results from the rewetting of small soil pores by isotopically depleted winter/spring rain. Thus, stable isotopes, and, in turn, water residence times, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing the water uptake by plants or the process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.

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Macropore flow of old water revisited: experimental insights from a tile-drained hillslope

Hydrology and Earth System Sciences ◽

10.5194/hess-17-103-2013 ◽

2013 ◽

Vol 17 (1) ◽

pp. 103-118 ◽

Cited By ~ 79

Author(s):

J. Klaus ◽

E. Zehe ◽

M. Elsner ◽

C. Külls ◽

J. J. McDonnell

Keyword(s):

Stable Isotope ◽

Isotopic Composition ◽

Soil Water ◽

Irrigation Water ◽

Initial Conditions ◽

Soil Depth ◽

Lower Boundary ◽

Hydrograph Separation ◽

Macropore Flow ◽

Hillslope Scale

Abstract. The mechanisms allowing the rapid release of stored water to streams are poorly understood. Here we use a tile-drained field site to combine macroporous soils at the hillslope scale with the advantage of at least partly controlled lower boundary conditions. We performed a series of three irrigation experiments combining hydrometric measurements with stable isotope and bromide tracers to better understand macropore–matrix interactions and stored water release processes at the hillslope scale. Stable isotope concentrations were monitored in the irrigation water, the tile-drain discharge and the soil water before and after the experiment. Bromide was measured every 5–15 min in the tile-drain hydrograph. Different initial conditions for each experiment were used to examine how these influenced flow and transport. Different amounts of irrigation water were necessary to increase tile-drain discharge above the baseflow level. Hydrograph separation based on bromide data revealed that irrigation water contributions to peak tile-drain discharge were on the order of 20%. Oxygen-18 and deuterium data were consistent with the bromide data and showed that pre-event soil water contributed significantly to the tile-drain event flow. However, the isotopic composition of soil water converged towards the isotopic composition of irrigation water through the course of the experiment. Mixing calculations revealed that by the end of the irrigation experiments 20% of the soil water in the entire profile was irrigation water. The isotopic data showed that the pre-event water in the tile drain was mobilized in 20–40 cm soil depth where the macropore–matrix interaction leads to an initiation of macropore flow after a moisture threshold is exceeded.

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Insights into the isotopic mismatch between bulk soil water and <i>Salix matsudana</i> Koidz trunk water from root water stable isotope measurements

Hydrology and Earth System Sciences ◽

10.5194/hess-25-3975-2021 ◽

2021 ◽

Vol 25 (7) ◽

pp. 3975-3989

Author(s):

Ying Zhao ◽

Li Wang

Keyword(s):

Isotopic Composition ◽

Soil Water ◽

Water Uptake ◽

Soil Depth ◽

Experimental Period ◽

Root Water Uptake ◽

Bulk Soil ◽

Mobile Water ◽

Isotopic Compositions ◽

Salix Matsudana

Abstract. Increasing numbers of field studies have detected isotopic mismatches between plant trunk water and its potential sources. However, the cause of these isotopic offsets is not clear, and it is uncertain whether they occur during root water uptake or during water transmission from root to trunk. Thus, we measured the specific isotopic composition (δ2H and δ18O) of each component (e.g. bulk soil water, mobile water, groundwater, trunk water and root water of Salix matsudana Koidz trees) in the soil–root–trunk continuum with a resolution of about 3 days. We report three main findings. First, we detected a clear separation between the isotopic compositions of mobile water and bulk soil water, but the distinction between mobile water and bulk soil water gradually decreased with increasing soil depth. Second, root water composition deviated from bulk soil water isotopic composition but overlapped with the composition derived for less mobile water. The maximum differences in δ2H and δ18O between bulk soil water and root water were −8.6 ‰ and −1.8 ‰, respectively. Third, trunk water was only isotopically similar to root water at 100–160 cm depths, and it remained stable during the experimental period, suggesting that the trees consistently used the stable deep water source. In conclusion, the isotopic offset between bulk soil water and trunk water of S. matsudana reflected an isotopic mismatch between root water and bulk soil water associated with the heterogeneity of the soil water. Our results illuminate relationships between the isotopic compositions of soil waters of various mobilities, root water and trunk water that may be useful for advancing our understanding of root water uptake and transport.

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Storage, mixing, and fluxes of water in the critical zone across northern environments inferred by stable isotopes of soil water

Hydrological Processes ◽

10.1002/hyp.13135 ◽

2018 ◽

Vol 32 (12) ◽

pp. 1720-1737 ◽

Cited By ~ 21

Author(s):

Matthias Sprenger ◽

Doerthe Tetzlaff ◽

Jim Buttle ◽

Sean K. Carey ◽

James P. McNamara ◽

...

Keyword(s):

Stable Isotopes ◽

Soil Water ◽

Critical Zone

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Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

Water ◽

10.3390/w12123520 ◽

2020 ◽

Vol 12 (12) ◽

pp. 3520

Author(s):

Huimei Pu ◽

Weifeng Song ◽

Jinkui Wu

Keyword(s):

Stable Isotopes ◽

Soil Water ◽

Water Conservation ◽

Rainy Season ◽

Preferential Flow ◽

Water Movement ◽

Soil Depth ◽

Dry Season ◽

Water Infiltration ◽

Soil Water Movement

Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ18O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ18O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

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Estimation of soil water evaporative loss after tillage operation using the stable isotope technique

International Agrophysics ◽

10.2478/v10247-012-0093-8 ◽

2013 ◽

Vol 27 (3) ◽

pp. 257-264 ◽

Cited By ~ 8

Author(s):

M.A. Busari ◽

F.K. Salako ◽

C. Tuniz ◽

G.M. Zuppi ◽

B. Stenni ◽

...

Keyword(s):

Stable Isotopes ◽

Soil Water ◽

Conservation Tillage ◽

Soil Depth ◽

Vapour Phase ◽

Conventional Tillage ◽

Water Evaporation ◽

Zero Tillage ◽

Tillage Practices ◽

Evaporative Loss

Abstract Application of stable isotopes in soil studies has improved quantitative evaluation of evaporation and other hydrological processes in soil. This study was carried out to determine the effect of tillage on evaporative loss of water from the soil. Zero tillage and conventional tillage were compared. Suction tubes were installed for soil water collection at the depths 0.15, 0.50, and 1.0 m by pumping soil water with a peristaltic pump. Soil water evaporation was estimated using stable isotopes of water. The mean isotopic composition of the soil water at 0.15 m soil depth were -1.15‰ (δ18O) and -0.75‰ (δD) and were highly enriched compared with the isotopic compositions of the site precipitation. Soil water stable isotopes (δ18O and δD) were more enriched near the surface under zero tillage while they were less negative down the profile under zero tillage. This suggests an occurrence of more evaporation and infiltration under conventional then zero tillage, respectively, because evaporative fractionation contributes to escape of lighter isotopes from liquid into the vapour phase leading to enrichment in heavy isotopes in the liquid phase. The annual evaporation estimated using the vapour diffusion equation ranges from 46-70 and 54-84 mm year-1 under zero and conventional tillage, respectively, indicating more evaporation under conventional tillage compared with zero tillage. Therefore, to reduce soil water loss, adoption of conservation tillage practices such as zero tillage is encouraged.

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