Composition Of Soil Water Research Articles

Illuminating isotopic offset between bulk soil water and xylem water under different soil water conditions

The temporal and spatial heterogeneity of soil water content affects the isotopic composition of bulk soil water and xylem water. However, there is a lack of systematic assessment of the impact of soil water conditions on the widely reported bulk soil–xylem water isotopic offset. For this purpose, we measured δ2H and δ18O values of xylem water of the shrub plant Caragana korshinskii and bulk soil water, for two consecutive years at two sites with contrasting water conditions. At Site 1, with moister conditions, xylem water showed a clear isotopic deviation towards more negative values relative to the bulk soil water values in both years (< 20% overlap), with a mean of 10.7‰ for the bulk soil–xylem water line-conditioned offset (Δlc-excess) and −9.1‰ for the soil water line conditioned excess (SW-excess). However, xylem water was strongly matched with immobile water in the dry year. At Site 2, with drier conditions, an isotopic offset occurred in the wet year, with a 20% overlap between xylem water and bulk soil water. Mean Δlc-excess and SW-excess were 13.7‰ and −11.8‰, respectively. Interestingly, the overlap reached 97% in the dry year, with the xylem water falling within the isotope space of bulk soil water. At Site 2, soil water content was positively correlated with Δlc-excess and negatively correlated with SW-excess. These results suggest that the isotopic signatures of xylem water and immobile water tend to overlap, supporting the “two water worlds” hypothesis. The bulk soil–xylem water isotopic offset is likely to be closely associated with the heterogeneity of soil water isotopes driven by soil water content. Our results elucidate changes in the isotopic signals of xylem water and bulk soil water under different water conditions and help better understand how plants take up water in a heterogeneous soil.

Seasonal isotopic cycles used to identify transit times and the young water fraction within the critical zone in a subtropical catchment in China

Transit times and the young water fraction (Fyw, the proportion of the water younger than a certain age threshold, τyw) can help characterize the hydrological behaviors within a catchment. Previous studies of water age have concentrated on individual catchment hydrological components, such as river water, groundwater, plant water or soil water, which hinders the dynamic linking of various hydrological components in the critical zone. In this study, the isotopic compositions of precipitation, soil water, xylem water, river water, and groundwater in a subtropical catchment in China were continuously monitored for two years. We found that the seasonal isotopic cycles of shallow soil water (0–60 cm), xylem water and river water had different attenuations and phase shifts relative to those of precipitation. There were no obvious seasonal isotopic cycles of deep soil water (60–130 cm) and groundwater. Additionally, the transit time distributions (TTDs) of soil water at the depth of 0–10 cm, xylem water and river water in the catchment varied between the wet year and normal year. With increasing depth, the mean transit times (MTTs) of soil water increased and the Fyw decreased. For xylem water and soil water, the MTTs and Fyw in the wet year were shorter and larger than those in the normal year, respectively. The river water consisted of a small proportion of fast flow components and a large proportion of slow flow components, with the young water accounting for 16% in the wet year and 21% in the normal year. Our results contribute to an improved understanding of water ages, flow paths and linkages of different hydrological components in the critical zone at the catchment scale.

A meta-analysis of understory plant removal impacts on soil properties in forest ecosystems

Understory plants have an important role in regulating various functions and processes in forest ecosystems. However, the overall consequences of understory plant losses on soil properties remain unclear. Here, we used a meta-analysis to determine overall trends in soil organic carbon, total nitrogen and microorganism following understory removal experiments in forest ecosystems of China. Sites with removed understories exhibited significant increases in soil temperature and decreases in soil organic carbon, total nitrogen, soil water content, pH, and microbial biomass. Fungi responded more quickly to plant removal than bacteria. Negative effects were increased with treatment duration and were stronger in broad-leaved forests than in coniferous forests. The responses of soil water, nutrient content, and microbial community composition to understory plant removal diminished as mean annual temperature and precipitation decreased. Our study showed that the effects of understory plant removal on soil properties varied among experimental durations, ecosystem types, and environmental factors (i.e., mean annual temperature, mean annual precipitation, and latitude). These results indicate that understory plants help in sustaining soil organic carbon, total nitrogen, and microbial activities in forest ecosystems. Our findings suggest that understory plants should be maintained to support the health of forest ecosystems.

Technical note: Conservative storage of water vapour – practical in situ sampling of stable isotopes in tree stems

Abstract. Using water-stable isotopes to track plant water uptake or soil water processes has become an invaluable tool in ecohydrology and physiological ecology. Recent studies have shown that laser absorption spectroscopy can measure equilibrated water vapour well enough to support inference of liquid-stable isotope composition of plant or soil water, on-site and in real-time. However, current in situ systems require the presence of an instrument in the field. Here we tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition that does not require an instrument in the field. We developed a vapour storage vial system (VSVS) that relies on in situ sampling into crimp neck vials with a double-coated cap using a pump and a flow metre powered through a small battery and measuring the samples in a laboratory. All components are inexpensive and commercially available. We tested the system's ability to store the isotopic composition of its contents by sampling a range of water vapour of known isotopic compositions (from −95 ‰ to +1700 ‰ for δ2H) and measuring the isotopic composition after different storage periods. Samples for the field trial were taken in a boreal forest in northern Sweden. The isotopic composition was maintained to within 0.6 ‰ to 4.4 ‰ for δ2H and 0.6 ‰ to 0.8 ‰ for δ18O for natural-abundance samples. Although 2H-enriched samples showed greater uncertainty, they were sufficient to quantify label amounts. We detected a small change in the isotopic composition of the sample after a long storage period, but it was correctable by linear regression models. We observed the same trend for the samples obtained in the field trial for δ18O but observed higher variation in δ2H than in the lab trial. Our method combines the best of two worlds, sampling many trees in situ while measuring at high precision in the laboratory. This provides the ecohydrology community with a tool that is not only cost efficient but also easy to use.

Evidence of hydraulic redistribution and its water sharing mechanism in a karst field of Southwest China

• Large roots and xylem diameters were found to facilitate large HR. • HR water flux to neighbor trees depends on the volume of HR and their distance. • A comprehensive method for monitoring HR occurrence in field is proposed. • HR was initially confirmed in a karst field of Southwest China. Hydraulic redistribution ( HR ) might serve as a crucial drought-tolerance mechanism to address dry and harsh environments in mixed-species communities. However, it remains unclear just how the unique water supply capacity of karst ecosystems (higher potential water content in karst water bearing media and lower effective water content in rock stratum) impact HR patterns. Here, dominant evergreen trees and their neighboring species were selected to determine the occurrence of HR and its water sharing mechanism by employing three methods (soil water content ( SWC )/soil water potential ( Ψs ), deuterium labeling, and thermal technique methods) in a subtropical karst ecosystem with shallow, patchy, and skeletal soils. The results revealed that the water potential gradient (>-1.28 Mpa) between 0 and 20 cm and 20–40 cm soil depths provide favorable conditions for the occurrence of HR . A marked increase in the isotopic composition of soil water following a deuterium labeling experiment showed the distinct occurrence of HR between sampled species. In contrast, the stable isotopic composition of xylem water in sampled species implied that there was no uptake of water released from HR . The downward flow rate of sap largely explained the quantity and maintenance of HR . A weak increase in the isotopic composition of the xylem water in neighboring species following a deuterium labeling experiment indicated that water sharing between species was contingent on the distance between the sampled and neighboring species. The water sharing kinetics of HR between species was observed to depend on the distance between the trees, as well as the HR volume and sustainability. This HR was initially confirmed in a karst field of Southwest China. The water released through the sampled plant roots was not for their own use, but for that of neighboring species. Our results provide multiple research strategies and indicated that HR is a significant mechanism in facilitating species coexistence, which sheds light on the restoration of vegetation in karst areas.

Investigation on Water Transformation and Pore Structure of Cement-Stabilized Dredged Sediment Based on NMR Technology.

Cement-stabilized dredged sediment (CDS) when used as a new road construction material cannot only solve the problem of abandoned sediment disposal, but also effectively save natural soil resources. This study aimed to evaluate the strength and permeability of CDS and establish corresponding prediction models from the perspective of a stabilization mechanism. The soil–water composition and pore size distribution were investigated by the nuclear magnetic resonance (NMR) technique. The results demonstrated that more liquid pore water inside the CDS specimen transformed into combined water with cement hydration. The amount of combined water, which essentially characterized the hydration process of cement, presented a linear relationship with log (t). The cementation and filling action of hydrates resulted in the transformation of large pores into smaller ones, hence the optimal pore size decreased with an increasing curing period and cement content. The stress–strain curves and hydraulic conductivity were determined based on unconfined compression and flexible wall penetration tests, respectively. The unconfined compressive strength increased exponentially with the amount of combined water, and the functional correlations of hydraulic conductivity and micropore parameters were established. The reliability of the NMR technique as a new method to study the microscopic evolution mechanism of the strength and permeability of CDS was further verified by scanning electron microscopy and mercury intrusion porosimetry tests.

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval.

New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential. We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory. In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Inter-method deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities. We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential.

黄土丘陵区优势造林树种水分来源对季节性干旱的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 黄土丘陵区优势造林树种水分来源对季节性干旱的响应 DOI: 10.5846/stxb202106221657 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 山西省高等学校科技创新（2020L0141，2020L0146）；山西农业大学科技创新基金（博士科研启动）（2020BQ44）；山西省优秀博士来晋工作奖励（SXYBKY2020007） Responses of water source to seasonal drought of dominant afforestation tree species in the loess hilly region of China Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:黄土丘陵区处于季节性干旱生态脆弱地带，探明水分环境变化对区域优势造林树种水分来源的影响，对干旱区植物水分利用及其共生关系具有重要意义。以该地区广泛种植的沙棘（Hippophae rhamnoides）+油松（Pinus tabuliformis）、沙棘（H.rhamnoides）+刺槐（Robinia pseudoacacia）混交林（HrPt，HrRp）及沙棘（Hr）、油松（Pt）和刺槐（Rp）纯林为对象，测定其茎秆水、土壤水和降水稳定同位素组成，采用IsoSource模型量化水分来源，相似性比例指数（PS）定量分析共生植物间水分利用关系。结果表明：（1）旱季，同一树种纯林和混交林间吸水层位存在差异，纯林中主要利用0-100 cm土层土壤水，其中Hr和Rp对40-100 cm的利用率为46.8%和43.8%，Pt对0-40 cm为83.9%；混交林中则更倾向于利用40-200 cm，其中HrPt（沙棘）对100-200 cm为58.8%，HrPt（油松）对40-100 cm为47.4%；HrRp（沙棘）对100-200 cm为58.2%，HrRp（刺槐）对100-200 cm为65.3%。（2）湿润季，纯林和混交林中各树种主要利用0-100 cm土层土壤水，利用率均在70%以上。（3）旱季PS指数显著大于湿润季，且HrRp的PS指数最大，表明沙棘和刺槐之间存在水源竞争现象。综上，相比HrRp，HrPt中共生植物间主要通过水分利用空间上的差异应对季节性干旱，使其组成的生态系统更好地适应区域脆弱的生态环境。研究结果可为区域退化生态恢复与重建及干旱适应性措施体系构建等提供依据。 Abstract:The loess hilly region is located in an ecologically fragile area with seasonal drought. Clarifying the impact of changes in the water environment on the water source of regional dominant afforestation tree species is of great significance to the water use of plants and their symbiosis in arid areas. Therefore, two kinds of mixed forests (Hippophae rhamnoides + Pinus tabuliformis, HtPt, H. rhamnoides+Robinia pseudoacacia, HtRp), the pure forests of H. rhamnoides (Ht), P. tabuliformis (Pt), and R. pseudoacacia (Rp) were selected. The isotope composition of xylem water, soil water and precipitation were determined. The water use and the relationship among symbiotic plants were quantitatively analyzed by using the IsoSource model and the proportional similarity (PS) index. The results indicated that:(1) during the dry season, there were differences in water absorption layer between pure forest and mixed forest of the same tree species. The tree species in the pure forest mainly used soil water in the 0-100 cm. The utilization ratios of Hr and Rp to 40-100 cm were 46.8% and 43.8%, and Pt from 0-40 cm was 83.9%. While, in the mixed forests were more inclined to use 40-200 cm. Among them, the utilization rate of HrPt (H. rhamnoides) from 100-200 cm layer was 58.8%, HrPt (P. tabuliformis) from 40-100 cm was 47.4%. HrRp (H. rhamnoides) and HrRp (R. pseudoacacia) from 100-200 cm were 58.2% and 65.3%, respectively. (2) During the wet season, tree species in pure and mixed forests were mainly use soil water in the 0-100 cm, and the utilization rates was above 70%. (3) The PS index in the dry season was significantly greater than that in the wet season, and in the HrRp was the highest, indicating that there might be water competition between H. rhamnoides and R. pseudoacacia. In summary, compared with HrRp, symbiotic plants in HrPt mainly respond to seasonal droughts through spatial differences in water use, which made it much easier for this ecosystem to be adaptive to the fragile ecological environment in the region. The research results can provide a basis for the restoration and reconstruction of regional degraded ecology and the construction of drought-adaptive measures system. 参考文献 相似文献 引证文献

Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

AbstractRiparian trees located in seasonally dry environments may be reliant on groundwater supplies, but the prevalence and magnitude of groundwater uptake is often unclear. Using soil water matric potential and water stable isotopes, we examined the relative contributions of soil water and groundwater to the dry season water uptake of five riparian tree species along an intermittent river of tropical northern Australia. Because xylem water was depleted in deuterium relative to source water (average offset −14.0‰), we numerically removed this offset and assessed the effect of the correction on mixing model results. We also estimated the isotopic composition of unbound soil water (i.e., the portion of soil water not tightly bound to soil particles) from bulk soil water data by using an empirical formulation from the literature and tested whether considering unbound soil water as a source would affect our results. Despite the hot and dry surface environment, we found that soil moisture was available for trees at relatively shallow (~0.7–1.5 m) depths. When unbound soil water and corrected xylem water data were considered, most tree species used a combination of this soil moisture source and groundwater from the capillary fringe. However, not correcting for isotopic effects resulted in large underestimations of the groundwater contributions to tree water uptake. Our findings suggest that ignoring soil isotopic effects and deuterium depletion in xylem water may reduce the validity of source water partitioning assessments. Further research is needed on the likely causes for deuterium depletion in xylem water.

Technical note: Evaporating water is different from bulk soil water in &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O and has implications for evaporation calculation

Abstract. Soil evaporation is a key process in the water cycle and can be conveniently quantified using δ2H and δ18O in bulk surface soil water (BW). However, recent research shows that soil water in larger pores evaporates first and differs from water in smaller pores in δ2H and δ18O, which disqualifies the quantification of evaporation from BW δ2H and δ18O. We hypothesized that BW had different isotopic compositions from evaporating water (EW). Therefore, our objectives were to test this hypothesis first and then evaluate whether the isotopic difference alters the calculated evaporative water loss. We measured the isotopic composition of soil water during two continuous evaporation periods in a summer maize field. Period I had a duration of 32 d, following a natural precipitation event, and period II lasted 24 d, following an irrigation event with a 2H-enriched water. BW was obtained by cryogenically extracting water from samples of 0–5 cm soil taken every 3 d; EW was derived from condensation water collected every 2 d on a plastic film placed on the soil surface. The results showed that when event water was heavier than pre-event BW, δ2H of BW in period II decreased, with an increase in evaporation time, indicating heavy water evaporation. When event water was lighter than the pre-event BW, δ2H and δ18O of BW in period I and δ18O of BW in period II increased with increasing evaporation time, suggesting light water evaporation. Moreover, relative to BW, EW had significantly smaller δ2H and δ18O in period I and significantly smaller δ18O in period II (p&lt;0.05). These observations suggest that the evaporating water was close to the event water, both of which differed from the bulk soil water. Furthermore, the event water might be in larger pores from which evaporation takes precedence. The soil evaporative water losses derived from EW isotopes were compared with those from BW. With a small isotopic difference between EW and BW, the evaporative water losses in the soil did not differ significantly (p&gt;0.05). Our results have important implications for quantifying evaporation processes using water stable isotopes. Future studies are needed to investigate how soil water isotopes partition differently between pores in soils with different pore size distributions and how this might affect soil evaporation estimation.

Elemental composition of groundwater from peat soils in taiga landscapes of Western Siberia

A total of 112 samples of peat and 26 samples of soil water were collected from three key areas of Histosols within undisturbed taiga ecosystems of Western Siberia (Russia). Data on elemental composition of peat ash, soil water, soil extracts (three mobile fractions: weakly bound acid-soluble exchangeable compounds, bound with complexes, and specifically absorbed) were obtained by methods of inductive coupled plasma mass-spectrometry and X-Ray fluorescence spectrometry, respectively. The soil waters had pH values from 3.1 to 4.4 and electrical conductivity from 22 to 130 µS cm-1. Element concentrations in soil waters formed the following decreasing sequence: Ca, Si > K, Mg, Fe, Na, Al >> Mn, Sr, Zn > Ni, Cu > Pb, Co, As, which differs from the sequence of elements arranged according to the content in peat and three studied mobile fractions. The influence of the elemental composition of peat on the composition of soil waters studied is insignificant. From the northern to the southern key areas, there were significant (p-value < 0.05) decreases in concentrations of As, Ba, Cd, Co, K, Mo, Mn, Ni, Pb, Rb, Sr, Si and V in soil waters, with insignificant differences in Al, Ca, Cr, Cu, Fe, Mg, Na, Ti and Zn.

Effects of plastic mulch on soil water migration in arid oasis farmland: Evidence of stable isotopes

In arid areas, where water resources are extremely scarce, the consumption of agricultural water resources is generally higher than 80% of the total water resources. Reasonable irrigation practices and effective water conservation methods are critical to the efficient utilization of water resources. In order to analyze the rationality of existing irrigation mode and evaluate the effect of plastic mulch on irrigation water migration, this research monitored the hydrogen and oxygen stable isotopic composition of soil water in Minqin oasis farmland located in the arid area of Northwest China. We used the soil water balance equation and the Craig-Gordon model to quantify the infiltration and evaporation of soil water. The result shows: (1) After irrigations in the corn growing season, the average irrigation water of 29.9 ± 4.2% (29.0 ± 6.7 mm), 29.6 ± 2.9% (28.4 ± 2.7 mm) and 30.3 ± 4.3% (29.0 ± 3.5 mm) penetrated into the soil layer of 0–10 cm, 10–50 cm, and 50–100 cm respectively. The field drainage under plastic film mulching was higher than that of no film mulching. (2) Plastic film mulching is the main reason for the difference in SWlc-excess of the surface layer. The evaporative loss of 0–5 cm soil water in non-mulched soil was generally higher than that of plastic-mulched soil. During the whole corn growing season, plastic film mulching effectively reduces evaporation losses of surface soil water and improves water use efficiency. The results of the reserch are very useful for understanding the hydrological process of agricultural lands in arid oases, improving agricultural irrigation water utilization rates, and optimizing water conservation models.

Insights into the isotopic mismatch between bulk soil water and &amp;lt;i&amp;gt;Salix matsudana&amp;lt;/i&amp;gt; Koidz trunk water from root water stable isotope measurements

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.

Flood‐Induced Recharge of Matrix Water in a Vertic Forest Soil

AbstractVertisols shrink and swell with changes in soil moisture, influencing hydraulic properties. Vertisols are often in floodplains, yet the importance of flooding as a source of soil moisture remains poorly understood. We used blue dye and deuterated water as tracers to determine the role of the macropore network in matrix recharge under artificial flood durations of 3 and 31 days in large soil monoliths extracted from a forested soil. Gravimetric soil moisture content increased by 47% in the first three days, then increased only 3.5% from day 3–31. Post‐flood moisture content was greatest in the organic‐rich, top 10 cm and was lower at 10–75 cm where organic matter was less. Deuterium concentration revealed that soil moisture in the top 10 cm was quickly dominated by artificial flood water, but at depth remained &lt;80% floodwater even after 31 days. Pervasive dye staining of ped surfaces in the top 4 cm indicated connectivity to flood waters but staining at depth was less and highly variable. The isotopic composition of soil water at depth continued to shift toward flood water despite no differences in dye staining between days 3 and 31. Results indicate flooding initially but incompletely recharges matrix water via macropores and suggest the importance of flooding as a source of matrix recharge in vertic floodplain soils may depend more on flood frequency than duration. Isotopic composition of matrix water in vertic soils depends on both advective and diffusional processes, with diffusion becoming more dominant as porosity decreases.

Reviews and syntheses: Gaining insights into evapotranspiration partitioning with novel isotopic monitoring methods

Abstract. Disentangling ecosystem evapotranspiration (ET) into evaporation (E) and transpiration (T) is of high relevance for a wide range of applications, from land surface modelling to policymaking. Identifying and analysing the determinants of the ratio of T to ET (T/ET) for various land covers and uses, especially in view of climate change with an increased frequency of extreme events (e.g. heatwaves and floods), is prerequisite for forecasting the hydroclimate of the future and tackling present issues, such as agricultural and irrigation practices. One partitioning method consists of determining the water stable isotopic compositions of ET, E, and T (δET, δE, and δE, respectively) from the water retrieved from the atmosphere, the soil, and the plant vascular tissues. The present work emphasizes the challenges this particular method faces (e.g. the spatial and temporal representativeness of the T/ET estimates, the limitations of the models used, and the sensitivities to their driving parameters) and the progress that needs to be made in light of the recent methodological developments. As our review is intended for a broader audience beyond the isotopic ecohydrological and micrometeorological communities, it also attempts to provide a thorough review of the ensemble of techniques used for determining δET, δE, and δE and solving the partitioning equation for T/ET. From the current state of research, we conclude that the most promising way forward to ET partitioning and capturing the subdaily dynamics of T/ET is by making use of non-destructive online monitoring techniques of the stable isotopic composition of soil and xylem water. Effort should continue towards the application of the eddy covariance technique for high-frequency determination of δET at the field scale as well as the concomitant determination of δET, δE, and δE at high vertical resolution with field-deployable lift systems.

Effects of precipitation on water use characteristics of Caragana intermedia plantations with different stand ages in alpine sandy land

To clarify the changes of water sources for Caragana intermedia plantations at different ages (4, 9, 17 and 31 years) in response to rainfall in the Gonghe Basin of Qinghai Province, China, we used the stable isotope technique to identify δ2H and δ18O compositions of soil water, xylem water, groundwater, and rain water before and after rainfalls. The proportions of different water sources were calculated by the Iso-Source model. The results showed that the δ2H and δ18O compositions of the shallow soil layer (0-40 cm) of all plantations responded significantly to the precipitation. The isotopic values of plant xylem water, soil water, and groundwater of each plantation were spotted on the lower right of the local meteoric water line (LMWL) either before or after rainfall, with lower intercepts and slopes than LMWL and the global meteoric water line (GMWL). The isotopic compositions of xylem water and soil water of C. intermedia plantations were closer to LMWL after rainfall. The 4- and 9-year-old C. intermedia plantations mainly used shallow soil water, the 17-year-old plantation mainly used middle layer soil water (40-90 cm), and the 31-year-old plantation primarily use deep soil water before rainfall. After rainfall, the shallow soil layer became sources of water absorption for all plantations. The utilization proportions of groundwater for all plantations were only 1.8%-11.9%. In general, water sources of different aged C. intermedia plantations showed similar responses to rainfall, by primarily absorbing the shallow soil water supplied by rainfall and reducing the use of groundwater.

Fractal and multifractal analysis of soil particle-size distribution and correlation with soil hydrological properties in active layer of Qinghai–Tibet Plateau, China

The soil hydrological properties (HPs) in active layers regulate soil water migration and affect the evolution of permafrost and biogeochemical cycles. There is relatively little understanding of the changes in soil HPs as a result of soil particle-size distribution (PSD) in the permafrost regions of the Qinghai–Tibet Plateau. In this study, different land-cover types, namely alpine wet meadow (AWM), extremely degraded wet meadow (EDWM), alpine meadow (AM), degraded alpine meadow (DAM), alpine steppe (AS), and alpine desert (AD), were used to determine the relationships between soil PSD and HPs. Soil HPs were determined using borehole nuclear magnetic resonance, and soil PSD (0–2 mm) was analyzed using fractal and multifractal theory. The results showed that vegetation degradation caused coarse soil particle accumulation in the root zone, reflected by the soil single-fractal dimension (Dv: 2.12–2.50). The Dv values showed significant positive correlations with soil clay and silt content as well as a negative correlation with sand content. Results from the generalized fractal and singularity spectra suggested an even distribution of soil particles in the root zone of the AWM, AM, and AD sites. Vegetation degradation increased the soil PSD capacity dimension (D0), information dimension (D1), correlation dimension, D1/D0 ratio, and Hölder exponent of order zero. This was indicative of a narrow soil PSD range and non-uniform soil texture in the root zone. Although capillary water dominated the total soil water content in the alpine soils, soil water composition was also dependent on the vegetation type and soil stratigraphic distribution. Soil particle composition and multifractal dimensions were found to be effective parameters for forecasting soil HPs in the non-root zone of alpine soils. However, the presence of vegetation weakens the relationship between soil PSD and HPs. This study contributes to the accurate evaluation of hydrological and biogeochemical cycles in permafrost regions.

MONITORING OF ATMOSPHERIC AND SOIL WATER COMPOSITION IN FOREST ECOSYSTEMS: MILESTONES AND OUTLOOK

The article provides an overview of Russian and foreign studies on assessment of the composition of atmospheric fallout and soil waters in forest ecosystems. The conclusion was made that little attention is given to the transformation of precipitation chemical composition by forest ecosystems, taking into account the influence of the species composition of the stand and the mosaic structure of the biogeocenosis (areas below the crowns, between the crowns, and open sites). European studies usually look at long-term development of the composition of atmospheric fallout and lysimetric waters, detecting long-term trends in composition changes of atmospheric and soil waters and identifying the factors driving these changes. In Russia, no such long-term (lasting for more than 10 years) continuous observations on the effect of man-made pollution on the composition and properties of atmospheric and soil waters were carried out. This task is very relevant for Russia and, especially, for its industrial regions.

Soil Water Dynamics Under Different Land Uses in Loess Hilly Region in China by Stable Isotopic Tracing

Exploring soil water dynamics under different land use types is important for water resource management and vegetation restoration in the Loess Plateau. In this study, we investigated the hydrogen and oxygen isotopic compositions of soil water from four different land use types to explore the mechanism of soil water movement and transformation and analyse the influence of land use. The results show that the range of stable isotopes (δD and δ18O) in soil water was smaller than that in precipitation. Values for δD and δ18O in soil water showed relatively similar temporal variation, heavy isotopes were enriched in the soil water in July and depleted in October. Stable isotope values in shallow (&lt;100 cm depth) soil water and deep (&gt;200 cm depth) soil water were low. The δD and δ18O values in woodlands decreased gradually with increasing depth. Across the four land use types, the maximum variation in δD and δ18O was in the shallow depth of the soil profile. Groundwater was recharged mainly from precipitation and then from soil water. The ratio of groundwater recharge by soil water under different land use types followed this rank order: woodland (35.70%) &gt; grassland (31.14%) &gt; shrubland (29.47%) &gt; cropland (29.18%). Matrix flow and preferential flow coexisted during infiltration, and the occurrence of preferential flow was related to the land use type. The main reason for the variation in isotopic composition in soil water is the difference in soil evaporation, which is influenced by different vegetation cover. Owing to the difference in soil evaporation and fractionation, precipitation on cropland, shrubland, and grassland can recharge more soil water than on woodland.

Seasonal changes in the water-use strategies of three herbaceous species in a native desert steppe of Ningxia, China

Frequent periods of drought conditions are known to limit plant performance, primary production, and ecosystem stability in arid and semi-arid desert steppe environments. Plants often avoid competition by shifting their water use seasonally, which affects the water-use patterns of dominant species as well as the composition and structure of plant communities. However, the water-use strategies of dominant herbaceous species, which grow under natural field conditions in the desert steppe region of Ningxia Hui Autonomous Region, China, are poorly known. Here, we explored the possible sources of water uptake and water-use efficiency (WUE) of three dominant herbaceous plant species (Stipa breviflora, Agropyron mongolicum, and Glycyrrhiza uralensis) in a native desert steppe in the semi-arid area of Ningxia through an analysis of multiple parameters, including (1) the stable isotopic oxygen and hydrogen (δ18O and δ2H) compositions of precipitation, soil water, and stem water, (2) the carbon isotope (13C) composition of leaves, and (3) the soil water contents, based on field sampling across varying water conditions from June to September, 2017. Frequent small precipitation events replenished shallow soil water, whereas large events only percolated down to the deep soil layers. Changes in soil water availability affected the water-use patterns of plants. Generally, during light precipitation periods, the deep root system of G. uralensis accessed deeper (>80 cm) soil water, whereas S. breviflora and A. mongolicum, which only have shallow roots, primarily absorbed water from the shallow and middle soil layers. As precipitation increased, all three plant species primarily obtained water from the shallow soil layers. Variation in soil water uptake between the dry and wet seasons enabled plants to make better use of existing satoil water. In addition, the δ13C values of G. uralensis and S. breviflora were higher than those of A. mongolicum. The δ13C values of the three plant species were significantly negatively correlated with soil water content. Therefore, G. uralensis and S. breviflora maintained a higher WUE through their conservative and water-saving strategies across the entire growing season. In contrast, A. mongolicum, with a relatively low WUE in the wet season but a high WUE in the dry season, exhibited a more flexible water-use strategy. The different water-use strategies of these dominant plant species demonstrated the mechanisms by which plant communities can respond to drought.