Search

Partitioning soil water balance (SWB) is an effective approach for deciphering the impacts of vegetation change on soil hydrological processes. Growing apple trees on the Loess Plateau, China, leads to a substantial deep soil water deficit, posing a serious threat to the sustainable development of apple production. However, the impact of deep-rooted apple trees on SWB remains poorly understood. In this study, we conducted a "Paired Plot" experiment to achieve this objective by decoupling SWB components using water stable isotopes, tritium, and soil water contents from deep soil cores (up to 25 m) under apple orchards with a stand age gradient of 8–23 years. The results showed that deep soil water storage under apple orchards was notably reduced compared to nearby farmland, showing a stand age-related pattern of deep soil water deficit (R2 = 0.91). By analyzing the changing patterns of SWB components, we found that the main factor driving this deficit is the water uptake process controlled by the deep root system. This process is triggered by the increased transpiration demand of apple trees and short-term water scarcity. These findings have implications for understanding soil water dynamics, sustainable agroforestry management, and soil water resources' protection in this region and other similar water-limited areas. [ABSTRACT FROM AUTHOR]

In water‐limited ecosystems, soil water regulates root water uptake (RWU) strategies. However, RWU responses to soil water changes under different species are not well‐understood. We assessed RWU responses of three revegetation species [shrub (Hippophae rhamnoides Linn.), coniferous forest [Platycladus orientalis (L.) Franco], and broad‐leaved forest (Robinia pseudoacacia L.) during the dry (May to June) and rainy (July to August) seasons in 2020 on the Loess Plateau using stable isotope methods. We sampled soil and xylem for each species at approximately weekly intervals and used the MixSIAR model to quantify RWU contribution with stable water isotopes. The results indicated that soil water in the shallow (0–40 cm) and middle (40–200 cm) soil layers fluctuated more strongly than the deep soil layer (200–300 cm) due to precipitation and evapotranspiration. Before precipitation in the dry season, most of the RWU for H. rhamnoides and R. pseudoacacia (97% and 98%) came from the middle layer under limited soil water. After precipitation in the dry season, the three species had similar RWU responses to soil water changes. After precipitation in the rainy season, the RWU change of H. rhamnoides and R. pseudoacacia with deep soil drying was more sensitive to soil water change than P. orientalis with sufficient deep water on August 3 and 11, while, the RWU of H. rhamnoides was more sensitive to soil water change than R. pseudoacacia on August 11 and 19. Thus, by switching its water‐use strategy, H. rhamnoides adapted better to the soil water environment than P. orientalis and R. pseudoacacia. This finding will help in selecting the optimal revegetation species for water use in a changing climate environment. [ABSTRACT FROM AUTHOR]

Temperature proxies for paleoclimate reconstruction have been made typically via ice cores, tree rings, stalagmites, and lake sediments. While extremely useful, these proxies can be limited spatially. Here we sampled a 98 m "soil core" from Loess Plateau of China and examined the relationship between pore water isotopic values and hydroclimate history. We extracted soil pore water for δ18O, δ2H, and 3H and measured chloride concentration. The 3H‐peak at 6 m and chloride mass balance were used to turn depth into calendar year. A 1000 year span was revealed. δ18O and δ2H values between 14–50 m were anomalously low—bracketing well the Little Ice Age period from 1420 to 1870. The identification was consistent with other standard proxies in the region and showed the same temporal dynamics of temperature anomalies. Our study shows the potential of stable isotopes of soil water for paleoclimate reconstruction in deep soils. Plain Language Summary: While paleoclimate reconstructions have been made with ice cores, tree rings, lake sediments and other proxies, few if any studies have explored the potential use of soil cores from the unsaturated zone to tease out paleoclimate information. Here, we report the soil water stable isotope data for an unusually deep, unsaturated zone soil core from the Loess Plateau of China. We found that waters extracted from the soil have relatively low δ2H and δ18O values in between the depths of 14–50 m. Waters at these depths likely derive from precipitation from the Little Ice Age period that occurred in that region of China from 1420 to 1870. Beyond simple time period identification, our isotope signal also captured the temperature dynamics during the Little Ice Age. We determined this by analyzing pore waters at 20 cm intervals. Tritium (3H, a radioactive tracer) and a salt mass balance method enabled us to covert core depth increments into calendar year. Our study shows the potential of soil water extractions from the deep critical zone for meaningful interpretation of paleoclimate conditions. Key Points: A 98 m "soil core" from the Loess Plateau of China recorded clear paleoclimate informationδ2H and δ18O values of extracted soil water between 14 and 50 m were anomalously low relative to the rest of the coreThe 14–50 m soil pore water bracketed the Little Ice Age period from the years 1420 to 1870 [ABSTRACT FROM AUTHOR]

This study aimed to improve runoff simulations and explore deep soil hydrological processes for a watershed in the center of the Loess Plateau (LP), China. This watershed, the Wuding River Basin (WRB), has very complex topography, with soil depths ranging from 0 to 197 m. The hydrological model used for our simulations was Community Land Model version 5 (CLM5) developed by the National Center for Atmospheric Research. Actual soil depths and river channels were incorporated into CLM5 to realistically represent the physical features of the WRB. Through sensitivity tests, CLM5 with 150 soil layers with the observed variable soil depths produced the most reasonable results and was adopted for this study. Our results showed that CLM5 with actual soil depths significantly suppressed unrealistic variations of the simulated subsurface runoff when compared to the default simulations. In addition, when compared with the default version with 20 soil layers, CLM5 with 150 soil layers slightly improved runoff simulations but generated simulations with much smoother vertical water flows that were consistent with the uniform distribution of soil textures in our study watershed. The runoff simulations were further improved by the addition of river channels to CLM5, where the seasonal variability of the simulated runoff was reasonably captured. Moreover, the magnitude of the simulated runoff remarkably decreased with increased soil evaporation by lowering the soil water content threshold, which triggers surface resistance. The lowered threshold was consistent with the loess soil, which has a high sand component. Such soils often generate stronger soil evaporation than soils dominated by clay. Finally, with the above changes in CLM5, the simulated total runoff matched very closely with observations. When compared with those for the default runoff simulations, the correlation coefficient, root mean square error, and Nash–Sutcliffe coefficient for the improved simulations changed dramatically from 0.02, 10.37 mm, and - 12.34 to 0.62, 1.8 mm, and 0.61. The results in this study provide strong physical insight for further investigation of hydrological processes in complex terrain with deep soils. [ABSTRACT FROM AUTHOR]

The source and hydrochemical makeup of a stream reflects the connectivity between rainfall, groundwater, the stream, and is reflected to water quantity and quality of the catchment. However, in a semi‐arid, thick, loess covered catchment, temporal variation of stream source and event associated behaviours are lesser known. Thus, the isotopic and chemical hydrographs in a widely distributed, deep loess, semi‐arid catchment of the northern Chinese Loess Plateau were characterized to determine the source and hydrochemical behaviours of the stream during intra‐rainfall events. Rainfall and streamflow were sampled during six hydrologic events coupled with measurements of stream baseflow and groundwater. The deuterium isotope (2H), major ions (Cl−, SO42−, NO3−, Ca2+, K+, Mg2+, and Na+) were evaluated in water samples obtained during rainfall events. Temporal variation of 2H and Cl− measured in the groundwater and stream baseflow prior to rainfall was similar; however, the isotope compositions of the streamflow fluctuated significantly and responded quickly to rainfall events, likely due to an infiltration excess, overland dominated surface runoff during torrential rainfall events. Time source separation using 2H demonstrated greater than 72% on average, the stream composition was event water during torrential rainfall events, with the proportion increasing with rainfall intensity. Solutes concentrations in the stream had loglinear relationships with stream discharge, with an outling anomaly with an example of an intra‐rainfall event on Oct. 24, 2015. Stream Cl− behaved nonconservative during rainfall events, temporal variation of Cl− indicated a flush and washout at the onset of small rainfall events, a dilution but still high concentration pattern in high discharge and old water dominated in regression flow period. This study indicates rainfall intensity affects runoff responses in a semi‐arid catchment, and the stored water in the thick, loess covered areas was less connected with stream runoff. Solute transport may threaten water quality in the area, requiring further analysis of the performance of the eco‐restoration project. [ABSTRACT FROM AUTHOR]

Copyright of Hydrological Sciences Journal/Journal des Sciences Hydrologiques is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Numerous studies have been conducted to understand the mechanism of groundwater recharge for water resources management. But the roles of wide-spread village ponds on the Chinese Loess Plateau on groundwater recharge remains uncertain. Here, to evaluate the groundwater recharge under village ponds, we measured environmental tracers (2H, 18O, Cl-) in rainwater, groundwater, and pore waters of the deep unsaturated zone under three land-cover types (cropland, a village pond, and two apple orchards with stand ages of 10 and 27 years). The results show that shallow groundwater is recharged by both diffusive recharge and focused recharge. Specifically, diffusive recharge occurs beneath farmlands, grasslands, orchards and similar land-cover types, but diminishes in old orchards due to deep root water uptake. The diffusive recharge beneath crop field had an infiltration velocity of 10.3–12.5 cm yr−1; the resulting recharge rate was 30–33 mm yr−1, accounting for 5–6% of the annual precipitation. However, focused recharge occurs mainly beneath village ponds with low-lying terrain, and had an infiltration velocity of 26 cm d−1 during the rainy season; the resulting mean recharge rate was 134 mm yr−1 in the catchment of the pond, accounting for 23.1% of the annual precipitation. Given that the water table is generally more than 30 m below the soil surface, it takes hundreds of years for rainwater to reach the water table via the diffusive recharge but only days to months via the focused recharge. Diffusive recharge contributed 55–68% to the total recharge while focused recharge contributed 32–45%. However, the conversion of croplands to apple orchards and the decrease in the number of village ponds are occurring, which diminishes diffusive recharge, and reduces focused recharge, respectively. These findings are critical to understanding hydrological processes and protecting groundwater resources. We should preserve, reclaim, and construct village ponds to ensure groundwater recharge, in addition to optimizing areal ratio of large farmland to apple orchards in the region for the sustainable utilization of groundwater resources. • Diffusive recharge and focused recharge coexist as processes of groundwater recharge in the Loess Tableland. • Diffusive recharge occurs in farmland and natural area and has a slow, steady velocity (10.3–12.5 cm yr−1). • Focused recharge occurs beneath village ponds and has a relatively rapid velocity (26 cm d−1) during the rainy season. • Diffusive recharge contributed 55–68% of the total recharge while focused recharge contributed only 32–45%. [ABSTRACT FROM AUTHOR]

Land-use change could substantially alter the soil water balance and hydrological cycles; however, little is known on the changes in deep soil water following a cycle of afforestation and deforestation. The purpose of this study was to quantify the soil water deficit in an apple orchard and subsequent replenishment of deep soil water after the orchard was felled. Soil water changes were quantified using the "space-for-time" method through a paired plot design. The results showed that the water storage in deep soil (>3 m in depth) began to decrease when the apple tree reached about 10 years of age. The cumulative deficit of deep soil water storage in the 3–18 m soil depth could reach about 1200 mm; however, deep soil water was so depleted that apple trees can no longer adsorb water from the deep soil when apple trees are older (>22 years old). After the apple orchard was converted to cropland, precipitation replenished the desiccated deep soil to a depth of about 7 m in the first two years, but thereafter, both water recovery amount and the advance rate of the wetting front were slowed down. After 15–16 years of recovery, soil water storage increased by 512–646 mm, accounting for 42.7–53.8% of the total cumulative soil water deficit caused by the apple orchard. However, it will take more than 26 years for soil water to be replenished to the level of the original cropland prior to planting apple trees. The considerable water deficit after afforestation and subsequent long water recovery time following deforestation extend our understanding of the effect of deep-rooted trees on water balance at the decade scale. [ABSTRACT FROM AUTHOR]

Piston and preferential water flow are viewed as the two dominant water transport mechanisms regulating terrestrial water and solute cycles. However, it is difficult to accurately separate the two water flow patterns because preferential flow is not easy to capture directly in field environments. In this study, we take advantage of the afforestation induced desiccated deep soil, and directly quantify piston and preferential water flow using chloride ions (Cl−) and soil water profiles, in four deforested apple orchards on the Loess Plateau. The deforestation time ranged from 3 to 15 years. In each of the four selected orchards, there was a standing orchard that was planted at the same time as the deforested one, and therefore the standing orchard was used to benchmark the initial Cl− and soil water profiles of the deforested orchard. In the deforested orchards, piston flow was detected using the migration of the Cl− front, and preferential flow was measured via soil water increase below the Cl− front. Results showed that in the desiccated zone, Cl− migrated to deeper soil after deforestation, indicating that the desiccated soil layer formed by the water absorption of deep-rooted apple trees did not completely inhibit the movement of water. Moreover, there was an evident increase in soil water below the downward Cl− front, directly demonstrating the existence of preferential flow in deep soil under field conditions. Although pore water velocity was small in the deep loess, preferential water flow still accounted for 34–65% of total infiltrated water. This study presented the mechanisms that regulate movement of soil water following deforestation through field observations and advanced our understanding of the soil hydrologic process in deep soil. [ABSTRACT FROM AUTHOR]

Many modeling efforts have been made for shallow soil, but little has been done in deep-rooted ecosystems, especially on the long-term impact of deep-rooted vegetation to understand the impact of vegetation type on hydrological processes. In this study, we used the Community Land Model (CLM) version 4.0 to simulate the soil water dynamics and groundwater recharge with shallow-rooted and deep-rooted vegetation cover in the critical soil zone of 100 m thickness. We selected winter wheat and summer maize to represent shallow-rooted vegetation and apple trees as deep-rooted vegetation growing in the semi-humid Loess Plateau of China over the period of 1901–2015. Our results show that the rooting depth and precipitation dictate the occurrence of disconnected recharge. This occurred in soil depths that were below 75 m in wet years with annual precipitation of over 650, 730, and 1000 mm for the winter wheat field, summer maize field, and apple orchard, respectively. Connected recharge was the major component of groundwater recharge for all three crop fields. The transit time of precipitation ranged from several to hundreds of years via the disconnected recharge that is caused by extreme precipitation and the connected recharge that is generated by other precipitation. Therefore, both rooting depth and growth period of vegetation affect the groundwater recharge and transit time, as well as precipitation. [ABSTRACT FROM AUTHOR]

• Forest age is key to carbon stock assessment but is challenging to assess at large scales. • This is the first study to apply LandTrendr to identify planted forest age. • LandTrendr is efficient and reliable for planted forest age mapping. • The forest on the Loess Plateau is primarily forest over 30 years old. Forest age is a key parameter for understanding the water and carbon cycles and carbon sequestration potential of planted forest ecosystems. However, estimating forest age on a large scale is difficult. This study aims to determine whether the distribution of forest age can be mapped over a large area using the LandTrendr (LT) algorithm. With the LT algorithm, the initial year of forestland (breakpoints) can be determined rather accurately through the yearly trajectory of the normalized burn ratio (NBR) index in the revegetated forest. The results show that the LT algorithm is a convenient, efficient, and reliable method for identifying forests age. Moreover, a comparison with ground truth data on the Chinese Loess Plateau (LP) revealed that the overall accuracy of the error confusion matrix was 89 %, with a root-mean-square error (RMSE) of 2.14 years. Using the LT algorithm, we revealed that the forestland on the Chinese LP in 2020 was dominated by planted forests over 30 years old, accounting for approximately 78.27 %. The forestland area increased by approximately 20,386 km2 from 1990 to 2020, and the increase occurs primarily around the original forestland in the eastern and southern parts of the Chinese LP. This study provides an important parameter for assessing and quantifying biomass and carbon sequestration through afforestation on the Chinese LP. [ABSTRACT FROM AUTHOR]

Highlights • Six >15-m tritium profiles present a unique record of bomb tritium storage. • Thick tritium profiles were used to evaluate tritium-based recharge estimation. • Recharge rates related to reconstructed precipitation tritium were overestimated. • Recharge mechanism between model assumption and local conditions should match. Abstract Understanding the time that water takes to traverse the vadose zone is important to quantify groundwater recharge rates and to assess the vulnerability of aquifers to contamination. Tritium mass balance models provide measurement-driven estimates of soil water velocities, yet uncertainties embedded in these groundwater recharge estimates are rarely investigated. Here we analyze six >15-m tritium profiles collected from thick unsaturated zones located on China's Loess Plateau and compare recharge rates obtained from various calculation approaches. Models used to estimate recharge based on unsaturated zone data include the tritium peak method and the tritium storage method; models used to estimate recharge using saturated zone data include the 'modern groundwater' method and a method that assumes groundwater is well-mixed in the aquifer. The tritium peak method presents reliable recharge; however, the recharge from the tritium storage method is two times of that from the tritium peak method. The well-mixed reservoir method overestimates groundwater renewal rates because the implicit assumption that groundwater is well-mixed in aquifers is not realistic. For potential recharge, considerable uncertainties arise from precipitation tritium reconstruction; while for actual recharge estimation, uncertainties come from the mismatched recharge mechanism between the model assumption and local characteristics. Selection of appropriate tritium mass balance models is of utmost importance for recharge estimation. [ABSTRACT FROM AUTHOR]

• Agroforestry stand age affects soil water patterns significantly. • Over 20 years, stand age reduces soil water movement. • New and existing soil water interplay adds complexity in agroforestry. Understanding the relationship between agroforest age and soil water dynamics is crucial for effective land and water resources management. However, the complexities of these dynamics, such as soil water recharge and depletion, hamper in-depth understanding, particularly in water-scarce regions. In this study, we examined soil water recharge and depletion in relation to the stand age of apple trees, a widely planted and representative deep-rooted agroforest, over four years in a semi-arid region on China's Loess Plateau (CLP). We collected soil cores to >20 m depth from four apple orchards (referred to as 'agroforests') with variable stand ages (established in 2008, 2005, 1998, and 1994). For comparison, we selected adjacent cropland as land use prior to agroforestry practices ('control'). We measured soil water content and tritium distributions to model soil water dynamics and estimate water ages across different soil profiles. Our results show that recharge amounts (and depths) in shallow soils were 298.4 mm (7 m), 303.4 mm (6.6 m), 300.6 mm (5.4 m), and 483.1 mm (7.6 m), whereas deep soils had net depletions of 111.1 mm, 391.9 mm, 192.8 mm, and 108.9 mm for AP2008, AP2005, AP1998, and AP1994, respectively. The tritium peak depths, which indicate the 1963 bomb peak depth, significantly differed between agroforested and non-agroforested plots. In particular, agroforestation reduced the seepage velocity of soil water over 20 years. Furthermore, our tritium tracer water age model suggests that the age of transpired deep soil water exceeded 200 years in the oldest orchard. These findings highlight a complex interaction between newly infiltrated water and existing water, possibly due to variations in soil pore size distributions. The results of this study offer valuable insights into the ecohydrological impacts of agroforestation on the CLP and in similar climatic regions. [ABSTRACT FROM AUTHOR]

Clarifying the water-root-carbon nexus in the entire root zone is crucial for unlocking the potential of afforestation in mitigating climate change. But the nexus in deep soil (depth > 1 m) remains poorly understood. Here we report contrasts in deep soil water and root distributions across 72 paired sites of adjacent farmlands, representing typical pre-afforestation conditions, and tree plantations, representing modern afforestation across the Loess Plateau of China. Ranging from 6 to 25 m of depth, these profiles included plantations of 13 tree species ranging from 1 to 25 years of age. The observations revealed sustained water mining in deep soil following afforestation with mean soil water decline of 75.2 ± 9.8 mm yr−1 that were accompanied by root deepening rates of 1.00 ± 0.06 m yr−1 with an associated biomass input of 0.18 ± 0.04 Mg C ha−1 yr−1. A water for carbon trade-off in deep soil become evident, likely involving a single pulse of C gains and water losses as no signs of soil rewetting under tree plantations where observed total soil water exhaustion that accumulated the equivalent of up to 2.8 years of mean annual precipitation inputs. The reported water-root-carbon nexus reveals overlooked hydrological costs and, more importantly, over-optimistic expectations of sustained C sequestration under afforestation that may rather represent a single-pulsed C gain supported by deep soil water exhaustion. • Afforestation on the Loess Plateau led to continuous water loss in deep soil. • Water loss led trees to root deeper, causing over 50% of fine roots growing in deep soil. • Root biomass increased as soil water declined, implying a water for carbon trade-off. • Limited soil water replenishment led to a single pulse of water for carbon trade-off. [ABSTRACT FROM AUTHOR]

• Three specific water ages were determined in the unsaturated zone. • Temporal origin of transpiration was partitioned into pre- and post-plantation. • Perennial deep-rooted vegetation consumed decades-old water. • Temporal origin of transpiration was mainly derived from post-planting precipitation. Deep-rooted vegetation transpires a considerable amount of deep soil water with different ages in the unsaturated zone. However, the tradeoffs between new water of transpiration (temporally originating from post-planting precipitation) and old water of transpiration (temporally originating from pre-planting precipitation) across the vegetation lifespan are poorly understood. In this study, we collected soil samples from beyond 28 m soil depth on the Loess Plateau of China to investigate the influence of deep-rooted vegetation on the age of soil water and analyze the proportion of new and old water of transpiration in the unsaturated zone under grassland, 22-year-old apple orchard, and 17-year-old peach orchard. Water isotopes (2H, 18O, and 3H), solutes (chloride, nitrate, sulfate), and soil water content were used to identify the critical water ages in the unsaturated zone (one-year water age, water age corresponding to stand age, and the maximum water age of transpiration), and to determine soil water deficit, soil evaporation loss fraction, and potential groundwater recharge. The results showed that soil water mainly moved as piston flow in these soil profiles, and deep soil water largely came from heavy precipitation. Deep-rooted vegetation restrained new pore water velocity and potential groundwater recharge. New pore water velocity declined from 0.40 m yr−1 to 0.14 m yr−1 and 0.34 m yr−1 for apple and peach, respectively. Deep-rooted vegetation decreased groundwater recharge by 9.46 % for apple and 7.04 % for peach, compared to grassland. Over the vegetation lifespan, annual average transpiration was 500.56 mm yr−1 and 468.89 mm yr−1 with maximum water age of 63 years and 45 years for apple and peach, respectively. The transpiration of deep-rooted vegetation mainly used new water—94.97 % for apple and 97.47 % for peach. The total old water of transpiration was 553 mm for apple and 209 mm for peach. Our results identify the temporal sources of vegetation water use, offering new insights into the transpiration process of deep-rooted vegetation. [ABSTRACT FROM AUTHOR]

• Forestland had significantly lower soil water contents than grassland. • Different vegetation restoration types altered the soil texture. • Afforestation promoted the formation of deep roots. • Forestland did not increase SOC content in the soil profile. • Apple orchard had significantly lower MWD than the others. Vegetation restoration on the Loess Plateau has decreased soil erosion. However, the comprehensive effects of different vegetation restoration types on soil properties are not clear. Evaluating the soil properties of the main economic and ecological tree species on the Loess Plateau can provide crucial guidance for regional ecological environment restoration. Therefore, we compared soil water content (SWC), soil particle composition, root distribution, soil organic carbon (SOC) content, soil bulk density (BD), total porosity (TP), saturated hydraulic conductivity (Ks), soil water retention curves (SWRCs), and soil aggregate stability of peach (Prunus persica), walnut (Juglans regia L.), and apple (Malus pumila) orchards, black locust (Robinia pseudoacacia), and grassland of similar age on the semi-arid Loess Plateau. Forestland, especially black locust, had significantly lower SWC than grassland. The different vegetation restoration types altered the soil texture; grassland had the highest clay content, while black locust had the lowest. Afforestation promoted deep root formation, reaching>10 m, with>50 % of the roots below 2 m. Compared with grassland, forestland did not increase SOC content in the soil profile. Soil BD increased with increasing soil depth from 0 to 80 cm for the five vegetation types but was relatively stable below 80 cm; grassland had the greatest BD, while black locust had the lowest. The walnut orchard had the lowest Ks in the 0–80 cm soil layer. The apple orchard had the highest residual soil water content and field capacity. The apple orchard had significantly lower mean weight diameter (MWD) than the others, indicating low surface soil stability due to tillage management, increasing the risk of soil erosion. Overall, these results suggested that black locust had greater negative effects on soil properties than economic trees. Therefore, black locust establishment or planting density should be reduced in future vegetation restoration processes. [ABSTRACT FROM AUTHOR]

Summary The study of the hydrological processes in the transition zone is important, but more complex compared with the homogenous land use units. A typical farmland–forest–desert transition zone in the Heihe River Basin was selected to study the hydrological processes and eco-hydrological effects among these land use units by monitoring the soil water content (SWC), groundwater level (GWL), and vegetation dynamics. Results showed that the sharp fluctuations of daily SWC and GWL in the farmland and the forest were primarily attributed to the irrigation events (7 and 6 times for the farmland and forest, respectively). The hydrological links among the three land use units were exhibited in three patterns. First, the soil water of the upper soil layer near the interface of two land use units moved from the irrigated land use unit to the non-irrigated one under soil water potential gradients through physical diffusion (the lateral water flow rate was less than 1 cm d −1 ). Second, the water flowed from the irrigated land use unit to the non-irrigated one under GWL gradients through groundwater flow (the lateral groundwater flow rate was less than 10 cm d −1 ). Third, a portion of the soil water in the farmland was utilized by the extended root system of the trees. The water exchange between the farmland and the forest resulting from one irrigation event was 5–30 mm, which caused increased GWLs for 1 week. At the forest–farmland boundary, the impacts of the extended tree roots reduced maize growth and extended 10–15 m into the farmland. By contrast, no obvious impacts were observed at the forest–desert boundary. Irrigating the farmland and the forest separately and reducing the width of the forest by 15–20 m would be more beneficial for irrigation water efficiency. These results would be useful for soil water management in terms of water balance impact on ecological construction and implementation of water-saving agriculture, as well as for optimal design of land-use patterns and efficient protection of oasis ecosystems to preserve limited water resources. [ABSTRACT FROM AUTHOR]

• Water use strategies of three economic tree species were investigated. • Peach, walnut, and apple trees have different water use strategies. • Walnut trees have greater ecological plasticity by absorbing more deep soil water. • Peach and apple trees have higher water use efficiencies than walnut trees. Economic tree species growing in the semi-arid Loess Plateau of China are important for maintaining sound economic viability and protecting the fragile hydro-ecological environment. However, their water use strategies and ecological adaptation to dryland environments remain poorly understood. In this study, we measured variations in soil water content, root distribution, water isotopic signature for hydrogen (δ2H) and oxygen (δ18O) in xylem and soil water of peach (Prunus persica), walnut (Juglans regia L.), and apple (Malus pumila) trees, as well as carbon isotopic signature (δ13C) in their leaves during two consecutive growing seasons—2018 (normal year) and 2019 (wet year). The results showed all species consumed less deep soil water in 2019 than in 2018 due to the increased precipitation. Peach trees used more shallow soil water compared to walnut and apple trees that used more deep soil water during high water-demanding periods. Walnut trees depended more on deep soil water throughout the growing season, contributing 46.2% in 2018, compared to 26.0% for peach trees and 24.0% for apple trees. Moreover, walnut trees had lower deep soil water and leaf δ13C than peach and apple trees. The findings suggest differences in phenology periods of the three rainfed economic tree species to affect the water source used. The three tree species differed in their water use strategies: walnut trees had ecological plasticity for reducing water stress by absorbing deep soil water, and peach and apple trees had higher water use efficiencies. This study has significance for evaluating water use characteristics and drought stress responses of economic trees in water-limited environments. [ABSTRACT FROM AUTHOR]

Deep soil water (DSW) plays a pivotal role in tree growth, susceptibility to drought-induced mortality, and belowground carbon and nutrient cycling. Assessing DSW depletion is essential for evaluating the resilience and sustainability of planted forests. But, due to the poor accessibility of deep soil layers, little is known about large scale DSW depletion. In this study, we leverage the concept that "plants are reliable indicators of deep soil water" to estimate DSW depletion in planted forests within the arid and semi-arid regions of the Chinese Loess Plateau (CLP). Our approach involves establishing a model that correlates forest age with DSW depletion. We then employ this model to estimate DSW depletion across the region, utilizing readily available data on the distribution of forest age and utilize the boundary models to consider the variability of DSW depletion estimated with forest age. Our results indicate that the model effectively estimates DSW depletion in planted forests, demonstrating a strong fit with an R 2 of 0.71 and a low root mean square error (RMSE) of 332 mm. Notably, a substantial portion of the planted forest areas on the CLP has experienced DSW depletion from 800 mm to 1600 mm, and totaling 2.41 × 10 10  m 3 DSW depletion from 1995 to 2020 based on the general model. However, the available DSW in the existing planted forests on the CLP is estimated at only 1.73 × 10 10  m 3 by 2038. This suggests that there is potential risks and unsustainability for further afforestation efforts and carbon sequestration on the CLP under the current continuous afforestation measures. Our study holds significant implications for sustainable regional ecological management and quantifying water resources for carbon trading through afforestation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this study., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Biochar application has been a promising approach to improve soil quality but their optimal amount in improving physical and hydraulic properties remains contradictory and inconclusive. The objective of this study was to examine and propose an optimal biochar application amount in saline alkali soil considering their impact on soil physical and hydraulic properties. A three-year field experiment was conducted in the saline-alkali soils under plastic film-mulched drip irrigation in Xinjiang, China. The studied physical and hydraulic properties included bulk density, soil porosity, saturated soil water content (θ s ), permanent wilting point (PWP), field capacity (FC), plant available water (PAW), spatial distribution of soil water content, planar soil water storage (PSWS), and soil evaporation. The treatments included biochar application amounts of 0 (CK), 10 (B10), 50 (B50), and 100 t ha -1 (B100) in 2018. Additional two treatments with 25 t ha -1 (B25) and 30 t ha -1 (B30) were added in 2019 and 2020, respectively. A four-parameter Gaussian function was fitted to the single-peak curves of the studied hydraulic properties vs. biochar application amounts to determine the most optimal biochar application amount. The results indicated that: (1) All of the biochar treatments significantly decreased bulk density and increased soil porosity over CK; (2) B10 and B25 treatments significantly increased θ s , FC, PAW, PWP, and PSWS of root zones in the film-mulched zones over CK, but reverse results were observed in the B50 and B100 treatments; (3) Daily and cumulative soil evaporation were increased in no mulch zones of all biochar treatments over CK; (4) A dose of 21.9 t ha -1 was recommended as the most optimal biochar application amount for improving physical and hydraulic properties of saline-alkali soil. This research provided useful information on biochar application amounts for improving physical and hydraulic properties in saline-alkali soil., Competing Interests: Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted., (Copyright © 2021 Elsevier B.V. All rights reserved.)