Plants for amelioration of subsoil constraints and hydrological control: the primer-plant concept

In this review, we propose the use of suitable plant species, termed primer-plants, for the primary purpose of preparing soil conditions for the benefit of following crops. Such plants may be used in the temperate agricultural belts of southern Australia, where dryland salinity is a major environmental and agricultural problem that threatens the viability of many crop production enterprises. It is recognised that growing plants that have deeper roots and use more water than the current shallow-rooted annual crops provide a long-term solution for managing the dryland salinity problem. Increased plant water-use is expected to mitigate the rising watertable that transfers salt to the root-zone of crop plants. On medium to heavy textured soils, common in this region, impermeability of the subsoil to roots and water movement is another major impediment to high water-use and productivity by plants, which may lead to other adverse hydrological events in the soil such as water-logging and excessive run-off. Plants that possess the ability to penetrate the dense subsoil and make it porous, in addition to having the capacity for using soil water at high rates, should be effective in combating dryland salinity. These plants normally should have thick roots that grow deep in the soil and are able to modify or withstand the adverse chemistry of the often-saturated subsoil, so that upon the death and decay of their roots, channels or biopores are created. These {biopores} have greater vertical and lateral continuity and last longer than porosity created through mechanical tillage. In this paper, we argue that potential exists for inclusion of short to medium-term phases of primer-plants in farming systems as a mimic of pre-existing perennial vegetation. We propose that ideal plants for combating dryland salinity should have high water-use and capacity to also improve soil structure and, possibly, nutrition. Examples are presented of soil amelioration that generally supports the viability of primer-plant concept, including the limited work undertaken in south-eastern Australia. We identified key knowledge-gaps, such as lack of well-defined agronomic packages for growing short-phases of Australian native species in mixtures, and our limited understanding of their root dynamics, which need to be addressed before effective implementation of the primer-plant concept.     

A review of the salt sensitivity of the Australian freshwater biota

In Victoria, Australia, both dryland salinity and salinity in irrigation regions are serious agricultural problems. One option to control the latter is to pump groundwater to maintain it below the surface. However, this leaves a saline wastewater for disposal, probably into local streams or wetlands. This review of the salt sensitivity of the biota of Australian streams and wetlands gives information of interest to those responsible for developing controls on these discharges. The review addresses the lethal and sub-lethal effects of salinity on microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation, invertebrates, fish, amphibians, reptiles, mammals, and birds. Data suggest that direct adverse biological effects are likely to occur in Australian river, stream and wetland ecosystems if salinity is increased to around 1 000 mg L⁻¹. The review highlights a general lack of data on the sensitivity of freshwater plants and animals to salinity increases.     

Seasonal water use characteristics of tall wheatgrass [Agropyron elongatum (Host) Beauv.] in a saline environment

It is essential to characterize the water use of plants that have potential for the stabilization of rising saline ground-water which could lead to increases in soil salinity. In this study, several techniques were used to determine the seasonal water use characteristics of the perennial grass Agropyron elongatum (tall wheatgrass) growing in a moderately saline, dryland environment with a fluctuating shallow groundwater table varying in electrical conductivity between 0 and 10 dS m^?1. Soil conditions were examined in terms of water potential measurements, plant water sources were identified using a stable isotope of water (deuterium, ²H) and evapotranspiration was estimated using hydrological and ventilated chamber methods. Seasonal changes in soil water potential were caused by salt accumulation and soil moisture leading to changes in plant water availability, particularly in the surface soil region over summer and autumn. Evapotranspiration in A. elongatum was high over summer during the peak period of growth (4 mm d^?1), with evidence of water use from groundwater and from specific regions of the soil profile. Evapotranspiration was low during the period that A. elongatum was partially senescent in autumn (< 0.5 mm d^?1) and the lowest leaf water potential of -3 MPa that was measured occurred during this period of moderate water stress. Intermediate levels of water use (1.5 mm d^?1) were measured during winter when the entire soil profile was available for water uptake. Based on physiological characteristics, including aspects of summer water use, root morphology and salt tolerance in A. elongatum, we conclude that this species is suitable for stabilizing the level of moderately saline groundwater in parts of southern Australia, which could reduce the potential for soil salinization.     

盐渍化灌区土壤盐分的时空变异特征及其与地下水埋深的关系

土壤盐分空间变异特征和地下水埋深状况是指导灌区合理用水和防治土壤盐碱化的重要依据。运用经典统计学和地质统计学方法,结合GIS技术,分析了河套灌区沙壕渠灌域0-20 cm、20-40 cm、40-60 cm土壤EC值的空间变异特征及地下水埋深对土壤盐分分布的影响。结果表明:沙壕渠灌域土壤盐分Cv值在不同灌溉时期和不同土壤深度均大于36%,表现为强变异特征;各灌水时期和不同土壤深度土壤EC值均表现为中等强度的空间自相关性,表层0-20 cm土壤空间自相关程度最高;秋浇前不同层次土壤EC值的空间分布在灌域内从南到北呈增大趋势,秋浇后土壤含盐量的高值区在西北部或东北部;土壤盐分受地下水埋深影响显著,灌域内地下水埋深南深北浅,土壤盐分随地下水埋深的增大而减小,二者之间满足指数关系。因此,应采取合理措施控制地下水埋深,防止区域土壤盐渍化加剧。     

Impact of land use and land cover change on groundwater recharge and quality in the southwestern US

Humans have exerted large‐scale changes on the terrestrial biosphere, primarily through agriculture; however, the impacts of such changes on the hydrologic cycle are poorly understood. The purpose of this study was to test the hypothesis that the conversion of natural rangeland ecosystems to agricultural ecosystems impacts the subsurface portion of the hydrologic cycle by changing groundwater recharge and flushing salts to underlying aquifers. The hypothesis was examined through point and areal studies investigating the effects of land use/land cover (LU/LC) changes on groundwater recharge and solute transport in the Amargosa Desert (AD) in Nevada and in the High Plains (HP) in Texas, US. Studies use the fact that matric (pore‐water‐pressure) potential and environmental‐tracer profiles in thick unsaturated zones archive past changes in recharging fluxes. Results show that recharge is related to LU/LC as follows: discharge through evapotranspiration (i.e., no recharge; upward fluxes <0.1 mm yr^?1) in natural rangeland ecosystems (low matric potentials; high chloride and nitrate concentrations); moderate‐to‐high recharge in irrigated agricultural ecosystems (high matric potentials; low‐to‐moderate chloride and nitrate concentrations) (AD recharge: ～130–640 mm yr^?1); and moderate recharge in nonirrigated (dryland) agricultural ecosystems (high matric potentials; low chloride and nitrate concentrations, and increasing groundwater levels) (HP recharge: ～9–32 mm yr^?1). Replacement of rangeland with agriculture changed flow directions from upward (discharge) to downward (recharge). Recent replacement of rangeland with irrigated ecosystems was documented through downward displacement of chloride and nitrate fronts. Thick unsaturated zones contain a reservoir of salts that are readily mobilized under increased recharge related to LU/LC changes, potentially degrading groundwater quality. Sustainable land use requires quantitative knowledge of the linkages between ecosystem change, recharge, and groundwater quality.     

植物对盐分空间不均匀分布的形态和生理响应研究进展

盐胁迫是干旱、半干旱地区以及灌溉土地主要的非生物胁迫,是影响农业生产的主要不利环境因子之一。随集约化灌溉农业的发展、水资源的缺乏、气候干旱带来的蒸发量的增加,土壤及地下水盐渍化程度不断增加。自然界中,土壤盐分在时空上呈不均匀分布。关于植物对均匀盐胁迫的响应研究报到较多,然而植物对不均匀盐胁迫的响应研究报道较少。分析了国内外植物适应不均匀盐胁迫的研究案例,从植物地上部分生长、地下部分生长、水分调节、光合作用以及离子调控等方面阐述植物适应盐分不均匀分布的生理机制,并提出展望。     

Groundwater use and salinization with grassland afforestation

Vegetation changes, particularly transitions between tree- and grass-dominated states, can alter ecosystem water balances and soluble salt fluxes. Here we outline a general predictive framework for understanding salinization of afforested grasslands based on biophysical, hydrologic, and edaphic factors. We tested this framework in 20 paired grassland and adjacent afforested plots across ten sites in the Argentine Pampas. Rapid salinization of groundwater and soils in afforested plots was associated with increased evapotranspiration and groundwater consumption by trees, with maximum salinization occurring on intermediately textured soils. Afforested plots (10–100 ha in size) showed 4–19-fold increases in groundwater salinity on silty upland soils but 50% of the days, and depressed the water table 38 cm on average compared to the adjacent grassland. Soil cores and vertical electrical soundings indicated that ≈6 kg m⁻² of salts accumulated close to the water table and suggested that salinization resulted from the exclusion of fresh groundwater solutes by tree roots. Groundwater use with afforestation in the Pampas and in other regions around the world can enhance primary production and provide a tool for flood control. However, our framework and experimental data also suggest that afforestation can compromise the quality of soils and water resources in predictable ways based on water use, climate, and soil texture.     

排水沟蓄水条件下农田与排水沟水盐监测

为了查明盐碱地排水沟蓄水条件下农田与排水沟之间的水盐交换,基于两年现场观测试验数据,分析了农田与排水沟的水位响应以及电导率变化规律。结果发现:研究区排水沟蓄水条件下,相邻排水沟水位与农田地下水位变化基本一致,作物生长期较为强烈的蒸发蒸腾作用进一步降低了田间地下水位;排水沟水位变化可以直接或间接地影响周围农田的地下水位和水质状况,在无外界来水干扰的情况下,农田地下水和排水沟水样的电导率变化趋势一致,当排水沟受到外来淡水补给时,沟内浓缩的盐分得到稀释,电导率明显低于农田地下水。研究结果可为类似地区盐碱地治理和生态环境保护提供参考依据。     

再生(污)水灌溉生态风险与可持续利用

作为一个农业大国,水资源贫乏及地域分布不均匀造成了我国严重的农业用水危机。为缓解我国农业用水危机,污水灌溉及再生水灌溉已成为解决农业灌溉水源不足的一项重要措施。在总结污水灌溉及再生水灌溉生态风险的基础上,针对国内研究现状,分析了我国再生水灌溉利用的可行性。研究发现,再生水灌溉的污染风险远小于污水灌溉,且再生水灌溉还具有回用成本低、减少农作物生产成本等经济效益,以及减少污染物向水环境中排放、改善土壤质量等环境效益。与污水灌溉相比再生水在农业灌溉上具有较大的应用前景,应加大其推广与应用的力度。最后,根据国内外的研究现状,提出了一些再生水灌溉可持续管理措施及其安全利用的相关建议。     

Root dynamics of Melaleuca halmaturorum in response to fluctuating saline groundwater

Melaleuca halmaturorum is a salt and waterlogging tolerant tree and thus often occurs in saline areas fringing permanent wetlands and in ephemeral swamps. The dominance of this tree in natural groundwater discharge areas may result in M. halmaturorum transpiration making a major contribution to groundwater discharge. To quantify this the seasonal changes in tree water sources in response to fluctuating soil salinity and waterlogging were examined. This study was conducted in a natural system where seasonally fluctuating saline groundwater (64 dS m^–1; 0.3–1.2 m deep) allowed the patterns of M. halmaturorum root water uptake to be followed over a 15 month period. Tree water sources were examined using the naturally occurring stable isotopes of water, while new root growth was examined using a field root observation window and from soil cores. The presence of isotopic fractionation of ²H under conditions of soil salinity and waterlogging was tested in a glasshouse experiment. Measurements of soil and leaf water potential were also made to examine the possible water sources and limits to water uptake. No isotopic fractionation was found by tree roots under conditions of salinity and waterlogging. M. halmaturorum trees were active in taking up groundwater at most times and combined this with a shallower soil water source replenished by rainfall in winter. Water uptake was concentrated in the deeper parts of the soil profile when the groundwater was at its deepest and salt had accumulated in the surface soils, at the end of summer. When groundwater rose, at the end of winter, roots responded by extracting water from near the soil surface (0–0.1 m), at the new watertable. This pattern of water uptake in response to groundwater fluctuations and salt accumulation in the surface soil was also reflected in new root tip appearance at the root observation window. Fluctuations in leaf water potential fallowed fluctuations in surface soil (0.1 m depth) water potential at all times. In winter leaf water potential reflected the absolute values of the surface soil water potential but in summer it was between surface soil and groundwater water potentials. We conclude that M. halmaturorum used groundwater in summer and a combination of rainfall and groundwater from the surface soils in winter. The ability to take up water from saline substrates through the maintenance of low leaf water potential, combined with this ability to rapidly alter root water uptake in response to changes in soil water availability contributed to the survival of M. halmaturorum in this saline swamp.     

Diversity and salt tolerance of native Acacia rhizobia isolated from saline and non‐saline soils

Re‐establishing native vegetation in stressed soils is of considerable importance in many parts of the world, leading to significant interest in using plant–soil symbiont interactions to increase the cost‐effectiveness of large‐scale restoration. However, effective use of soil microbes in revegetation requires knowledge of how microbe communities vary along environmental stress gradients, as well as how such variation relates to symbiont effectiveness. In Australia, shrubby legumes dominate many ecosystems where dryland salinity is a major issue, and improving plant establishment in saline soils is a priority of regional management agencies. In this study, strains of rhizobial bacteria were isolated from a range of Acacia spp. growing in saline and non‐saline soils. Replicates of each strain were grown under several salinity levels in liquid culture and characterized for growth and salt tolerance. Genetic characterization of rhizobia showed considerable variation among strains, with salt tolerance and growth generally higher in rhizobial populations derived from more saline soils. These strains showed markedly different genetic profiles and generic affiliations to those from more temperate soils, suggesting community differentiation in relation to salt stress. The identification of novel genomic species from saline soils suggests that the diversity of rhizobia associated with Australian Acacia spp. is significantly greater than previously described. Overall, the ability of some symbiotically effective strains to tolerate high salinity is promising with regard to improving host plant re‐establishment in these soils.     

Uptake of saline groundwater by plants: An analytical model for semi-arid and arid areas

An analytical model, based on unsaturated zone water and solute balances, was developed to describe the uptake of saline groundwater by plants in dry regions. It was assumed that: i. initially, the profile had low water and salt contents to some depth; ii. both water and solutes move upwards from the water table by piston flow due only to plant water extraction; iii. the uptake of water concentrates solutes in the soil solution until some threshold salinity is reached, above which plants can no longer extract water due to osmotic effects; iv. uptake of the groundwater does not affect the water table level; and v. uptake of groundwater is only limited by transmission of groundwater through the soil. Model predictions were compared with measurements of groundwater uptake made over 15 months at five sites in aEucalyptus forest in a semi-arid area, using independently measured model parameters. Depth and salinity of groundwater, and soil type varied greatly between sites. Predicted groundwater uptake rates were close to measured values, generally being within ∼ 0.1 mm day^-1. Sensitivity analysis showed that groundwater depth and salinity were the main controls on uptake of groundwater, while soil properties appeared to have a lesser effect. The model showed that uptake of groundwater would result in complete salinisation of the soil profile within 4 to 30 yr at the sites studied, unless salts were leached from the soil by rainfall or flood waters. However, a relatively small amount of annual leaching may be sufficient to allow groundwater uptake to continue. Thus groundwaters, even when saline, may be important sources of water to plants in arid and semi-arid areas.     

Water budgets and nutrients in a native Banksia woodland and an adjacent Medicago sativa pasture

In southern Australia, the spread of dryland salinity can be traced to increased leakage of water through the root zone to the groundwater, associated with clearing of perennial vegetation and its replacement with annual crops and pastures. Agricultural activity, through fertilizer addition and subsequent nutrient export, has also changed the nutrient status of the soils, often causing increases in soil acidity. In this trial, an area of native vegetation on a deep sandy soil in south-western Australia (dominated by Banksia prionotes Lindley), and an adjacent introduced perennial pasture (Medicago sativa L.), were compared in terms of their water balance and nutrient fluxes for the period between August 1998 and March 2001. Initially, the Banksia woodland vegetation maintained a drier soil profile below 2 m than the establishing lucerne pasture, and leakage beyond 4 m in 1999 was 80 mm under the Banksia woodland and 180 mm under the lucerne. However, in 2000, lucerne's rate of water use during winter was faster than any other vegetation observed on this soil type, possibly due to direct groundwater extraction, and it dried the soil to the same level as the Banksia woodland vegetation. Nutrient (nitrate, phosphate, sulphate, potassium, calcium, magnesium and sodium) fluxes under both systems were generally low, reflecting the inherently low fertility of the soil type. However, sodium and nitrate appeared to accumulate in soil at a depth of 4 m under the Banksia woodland (particularly between the Banksia canopies), but not under the lucerne, possibly due to a history of leaching under the lucerne. Whilst both vegetation types effectively controlled excess water leakage, the differences in nutrient cycling and production levels suggests that some aspects of native perennial vegetation function may not be suitable for incorporation into agricultural systems.     

Contrasting water relations of three coastal tree species with different exposure to salinity

This field study examined the ecophysiological responses of three tree species to salinity in the Austin Bay Nature Reserve, adjacent to the Peel-Harvey Estuary in Western Australia (115°46' E 32°37' S). The area is at increased risk of flooding with saline water during storm surges due to the construction of a channel between the estuary and Indian Ocean in 1994. Banksia attenuata R.Br. occurs on small sandy ridges adjacent to a seasonal wetland, while Melaleuca cuticularis Labill. and Casuarina obesa Miq. occur in a seasonally flooded wetland. Landscape position determined exposure to salinity, with M. cuticularis and C. obesa experiencing high soil and groundwater salinity during summer (electrical conductivity, EC, up to 70 dS m⁻¹) while B. attenuata was not exposed to soil or groundwater with EC greater than 20 dS m⁻¹. B. attenuata had relatively stable leaf water status throughout the year and did not osmotically adjust as root-zone salinity increased. By contrast, M. cuticularis and C. obesa had large variation in stem water potential and exhibited osmotic adjustment during summer. Whereas the sap flow rates of M. cuticularis and C. obesa remained high throughout the year, sap flow of B. attenuata decreased during summer which may have limited uptake of salt. The three species also exhibited differences in traits associated with tissue-level salt tolerance, as M. cuticularis and C. obesa produced compatible organic solutes (methyl proline in M. cuticularis and proline in C. obesa ), whereas B. attenuata did not. The distributions of these species within the Austin Bay Nature Reserve are determined in part by their tolerance to salinity, which will influence their responses to hydrological disturbance.     

Dryland Salinity and Ecosystem Distress Syndrome: Human Health Implications

Clearing of native vegetation for agriculture has left 1.047 million hectares of southwest Western Australia affected by dryland salinity, and this area may expand up to a further 1.7–3.4 million hectares if trends continue. Ecosystems in saline-affected regions display many of the classic characteristics of Ecosystem Distress Syndrome, one outcome of which has not yet been investigated in relation to dryland salinity: adverse human health implications. This article seeks to review existing information and identify potential adverse human health effects. Three key potential impacts on human health resulting from dryland salinity are identified: wind-borne dust and respiratory health; altered ecology of the mosquito-borne disease Ross River virus; and mental health consequences of salinity-induced environmental degradation. Given the predicted increase in extent and severity of dryland salinity over coming decades, adverse outcomes of salinity are likely to be further exacerbated, including those related to human health. There is a clear need to investigate the issues discussed in this review and also to identify other potential adverse health effects of dryland salinity. Investigations must be multidisciplinary to sufficiently examine the broad scope of these issues. The relationship between human health and salinity may also be relevant beyond Australia in other countries where secondary soil salinization is occurring.     

World salinization with emphasis on Australia

Salinization is the accumulation of water-soluble salts in the soil solum or regolith to a level that impacts on agricultural production, environmental health, and economic welfare. Salt-affected soils occur in more than 100 countries of the world with a variety of extents, nature, and properties. No climatic zone in the world is free from salinization, although the general perception is focused on arid and semi-arid regions. Salinization is a complex process involving the movement of salts and water in soils during seasonal cycles and interactions with groundwater. While rainfall, aeolian deposits, mineral weathering, and stored salts are the sources of salts, surface and groundwaters can redistribute the accumulated salts and may also provide additional sources. Sodium salts dominate in many saline soils of the world, but salts of other cations such as calcium, magnesium, and iron are also found in specific locations. Different types of salinization with a prevalence of sodium salts affect about 30% of the land area in Australia. While more attention is given to groundwater-associated salinity and irrigation salinity, which affects about 16% of the agricultural area, recent investigations suggest that 67% of the agricultural area has a potential for "transient salinity", a type of non-groundwater-associated salinity. Agricultural soils in Australia, being predominantly sodic, accumulate salts under seasonal fluctuations and have multiple subsoil constraints such as alkalinity, acidity, sodicity, and toxic ions. This paper examines soil processes that dictate the exact edaphic environment upon which root functions depend and can help in research on plant improvement.     

The potential impact of dryland salinity on the threatened flora and fauna of New South Wales

Summary We used digital map overlays in a geographical information system (GIS) to quantify the potential impact of dryland salinity on the threatened flora and fauna of New South Wales (NSW). Geographical areas of conservation priority were identified based on richness of threatened species with distribution records overlapping dryland salinity. Two alternative schemes – Interim Biogeographical Regionalization for Australia (regions) and catchment boundaries (catchments) – were used to subdivide NSW. Sydney Basin, North Coast and South-western Slopes regions – and Hunter, Sydney, Macquarie, Murrumbidgee and Lachlan catchments – were identified as priority areas with more than 10 salinity-overlap species present. Five threatened plant species were identified as priority species due to more than half of their known distributions overlapping areas of dryland salinity. Threatened animal species of most concern had 10–50% of their records overlapping areas of dryland salinity. Our findings demonstrate that landscape exposure to dryland salinity should be used in conjunction with total richness of threatened species for prioritizing conservation of geographical areas with respect to the potential impact of dryland salinity on threatened species.     

Groundwater rise,soil salinization and the decline of Casuarina in southeastern Australia during the late Quaternary

Abstract The decline in interglacial importance of Casuarina over the late Quaternary across southeastern Australia is documented. Three previously proposed causes for the decline (change in fire regime, change to a wetter climate and competitive exclusion by eucalypts) are shown to be inadequate for explaining the majority of cases. Re-examination of the evidence shows the decline at most sites occurred synchronously with a rise in groundwater or soil salinization, or both. From a review of biological literature, it is established that Casuarina stricta, the main species considered to have been affected by the decline, is likely to be disadvantaged by high water tables and saline soils. A link is demonstrated between groundwater salinity and the nodulation status of Casuarina in Victoria. It is concluded that the late Quaternary Casuarina decline was caused by a combination of rising groundwater levels and soil salinization. Soil salinization and groundwater level must therefore be considered as major factors determining vegetation patterns in southeastern Australia through the Quaternary up to the present day.     

Variation in nutrient availability and plant species diversity across forb and graminoid zones of a Northern New England high salt marsh

Plant zonation patterns across New England salt marshes have been investigated for years, but how nutrient availability differs between zones has received little attention. We investigated how N availability, P availability, and plant N status varied across Juncus gerardii, Spartina patens, and mixed forb zones of a Northern New England high salt marsh. We also investigated relationships between several edaphic factors and community production and diversity across the high marsh. P availability, soil salinity, and soil moisture were higher in the mixed forb zone than in the two graminoid zones. NH⁺ ₄-N availability was highest in the J. gerardii zone, but NO^– ₃-N availability and mid season net N mineralization rates did not vary among zones. Plant tissue N concentrations were highest in the mixed forb zone and lowest in the S. patens zone, reflecting plant physiologies more so than soil N availability. Community production was highest in the J. gerardii zone and was positively correlated with N availability and negatively correlated with soil moisture. Plant species diversity was highest in the mixed forb zone and was positively correlated with P availability and soil salinity. Thus, nutrient availability, plant N status, and plant species diversity varied across zones of this high marsh. Further investigation is needed to ascertain if soil nutrient availability influences or is a result of the production and diversity differences that exist between vegetation zones of New England high salt marshes.     

Water sources of Populus euphratica and Tamarix ramosissima in Ejina Delta,the lower reaches of the Heihe River,China

Aims We aim to evaluate the water sources of typical riparian arbor species (Populus euphratica) and shrub species (Tamarix ramosissima), and analyze the spatial and temporal dynamics of plant water source in Ejina Delta, the lower reaches of the Heihe River, China.Methods Eight sampling sites were selected in the riparian zones along the East River and West River in Ejina. The plant xylem water, soil moisture, rainwater, stream water and groundwater were taken and pretreated during the growing season in 2015-2016, and the stable oxygen isotope ratio (δ¹⁸O) for each water sample was measured. The δ¹⁸O of plant xylem water and soil moisture were compared to estimate the dominant depth of root water uptake, and the linear-mixed model called “IsoSource” were applied to determine plant water sources and quantify their proportions.Important findings This study indicated that the main recharge sources for P. euphratica and T. ramosissima were stream water and groundwater. The contributions of rain water to them was negligible due to the limited amount and the shallow infiltration depth of local rainfall. As affected by groundwater level fluctuation, soil physical properties, as well as lateral and vertical recharge of stream water on soil moisture, the dominant depth of root water uptake spatially varied. However, the relative contributions of stream water or groundwater to plant water sources did not change significantly across space. Populus euphratica used more stream water (68%), while T. ramosissima used more groundwater (65%). Plant water sources were sensitive to environmental flow controls. The contributions of stream water to the water sources of the two species went up to 84% and 48% for P. euphratica and T. ramosissima respectively during the discharge period, but dropped to 63% and 30% during the non-discharge period. On the other hand, the contributions of groundwater decreased to 16% and 52% during the discharge period, but increased to 37% and 70% during non-discharge period. It is noteworthy that the high similarity of δ¹⁸O between stream water and groundwater due to extensive water exchange in the riparian zone made increase the uncertain in quantifying plant water sources.