The Loess Plateau of China (LPC) is suffering from the impacts of overexploitation of groundwater, leading to a decline in the regional water table. This is compounded by relatively recent ...large‐scale transformation of land use, which may impact groundwater recharge and threaten future water quality from intensive nitrogen fertilizer application. Understanding the regional unsaturated water flow and nitrate travel time in the deep vadose zone is crucial for the sustainable management of the LPC groundwater system. We develop here a regional‐scale model that exploits an extensive database of soil properties derived from recent intensive soil profile sampling over the LPC, together with climate, vadose zone thickness, and land use data. The model solves the Richards equation and a conservative solute transport model using a multiple 1D column approach. A comparison between model simulations and observations at five sites reveals good model performance. Application over the LPC indicates that areas with dense irrigated agricultural land use contribute significantly to groundwater recharge. By using land use cover information from 1975 to 2008, simulations reveal that the transition from agriculture to forest and grassland accounts for a small reduction of the total annual recharge (6.1%). Modeled nitrate travel times through the deep unsaturated zone range between decades and centuries; however, recent loading of anthropogenic nitrate is expected to reach the aquifer in the near future under irrigated areas or where the vadose zone is relatively thin. The model simulations provide valuable information for assessing future vulnerability of groundwater resources over a regional scale.
Key Points
A regional‐scale modeling approach exploiting intensive sampling of hydraulic properties used to define fluxes of water and nitrate
Recharge decreased 6.1% following transition of crop land to forest and grassland
Modeled nitrate travel time in the loess unsaturated zone ranged from decades to centuries
Understanding nitrate migration through the deep vadose zone is essential for aquifer vulnerability assessments. The effect of variability of physical properties of the deep vadose zone on nitrate ...transport has been scarcely explored. Recently, deep nitrate storage profiles were determined in the vadose zone of the Loess Plateau of China. Using these observations along with measured soil properties, this study investigates the effect of loess vertical heterogeneity on water movement and nitrate transport through the deep vadose zone. Models of different complexity were established and calibrated. First, a simple piston flow and nitrate mass balance approach was calibrated to the observed nitrate storage. The results indicate that the total nitrate storage is estimated well, while the estimation of the distribution of nitrate is relatively poor. Subsequently, Richards' equation and the Advection‐Dispersion equation were evaluated. Three different conceptualizations of the numerical models were calibrated against deep vadose zone nitrate and water content observations: (1) one‐layer model assuming homogenous loess vadose zone; (2) a model that considers a hydraulic conductivity (Ks) decay function and (3) a model where the Miller‐Miller scaling factors are prescribed to account for changes of the hydraulic functions with depth. Accounting for the vertical Ks decay in the numerical models improved water flow performances. The study reveals the adequacy of implementing water flow and nitrate transport numerical models together with a simple representation of the vertical loess variability, for simulating nitrate migration in loess deep vadose zone environments.
Plain Language Summary
Enhanced concentration of nitrate in groundwater is a global problem. The source of such nitrate is commonly linked to agricultural practices, in particular the use of fertilizers. The intensive development of China in the last few decades has put at risk many groundwater systems, such as the unconfined aquifer of the Loess Plateau of China (LPC). Understanding the fate and travel times of nitrate through the LPC vadose zone before its arrival at the water table would help sustain groundwater systems and identify areas requiring changes in land use management. The current study examines various modeling approaches for nitrate transport estimations in the LPC vadose zone. Four modeling approaches with different complexities are tested: A simple mass‐balance model and more sophisticated numerical models that include different methods to represent the vertical variability of loess physical properties. The study reveals that water flow and nitrate transport numerical models together with a simple representation of the vertical loess variability is the preferable approach for describing nitrate transport in loess environments.
Key Points
Examination of how complex a model needs to be for the simulation of water and nitrate movement in deep unsaturated loess
The performance of a combined Richards' equation and Advection‐Dispersion equation approach is superior to a simplistic piston flow model
Using a Ks decay function to represent loess vertical variability in numerical models improves unsaturated water flow predictions
Hazardous heavy metal pollution of soils is an increasingly urgent problem all over the world. The zeolite as a natural amendment has been studied extensively for the remediation of hazardous heavy ...metal-polluted soils with recycling. But its theory and application dose are not fully clear. This paper reviews the related aspects of theory and application progress for the remediation of hazardous heavy metal-polluted soils by natural zeolite, with special emphasis on single/co-remediation. Based on the comments on hazardous heavy metal behavior characteristics in leaching and rhizosphere and remediation with zeolite for heavy metal-polluted soils, it indicated that the research of rhizosphere should be strengthened. Theory of remediation with natural zeolite could make breakthroughs due to the investigation on synthetic zeolite. Co-remediation with natural zeolite may be applied and studied with more prospect and sustainable recycling.
Vegetation changes that are driven by soil conservation measures significantly affect subsurface water flow patterns and soil water status. Much research on water consumption and sustainability of ...newly introduced vegetation types at the plot scale has been done in the Loess Plateau of China (LPC), typically using local scale measurements of soil water content (SWC). However, information collected at the plot scale cannot readily be up‐scaled. Geophysical methods such as electromagnetic induction (EMI) offer large spatial coverage and, therefore, could bridge between the scales. A noninvasive, multicoil, frequency domain, EMI instrument was used to measure the apparent soil electrical conductivity (σa) from six effective depths under four typical land‐covers (shrub, pasture, natural fallow, and crop) in the north of the LPC. Concurrently, SWC was monitored to a depth of 4 m using an array of 44 neutron probes distributed along the plots. The measurements of σa for six effective depths and the integrated SWC over these depths, show consistent behavior. High variability of σa under shrub cover, in particular, is consistent with long term variability of SWC, highlighting the potential unsustainability of this land cover. Linear relationships between SWC and σa were established using cumulative sensitivity forward models. The conductivity–SWC model parameters show clear variation with depth despite lack of appreciable textural variation. This is likely related to the combined effect of elevated pore water conductivity as was illustrated by the simulations obtained with water flow and solute transport models. The results of the study highlight the potential for the implementation of the EMI method for investigations of water distribution in the vadose zone of the LPC, and in particular for qualitative mapping of the vulnerability to excessive vegetation demands and hence, unsustainable land cover.
Core Ideas
An increasing trend in apparent electrical conductivity (ECa) corresponds with the increase in soil water content (SWC) under four typical land‐covers.
Linear relationships between SWC and ECa were established using the cumulative sensitivity model.
Elevated pore water conductivity effects the relationship between conductivity and SWC.
Extensive nitrogen loads at the soil surface exceed plant uptake and soil biochemical capacity, and therefore lead to nitrogen accumulation in the deep vadose zone. Studies have shown that stored ...nitrogen in the vadose zone can eventually reach the water table and affect the quality of groundwater resources. Recently, global scale models have been implemented to quantify nitrate storage and nitrate travel time in the vadose zone. These global models are simplistic and relatively easy to implement and therefore facilitate analysis of the considered transport processes at a regional scale with no further requirements. However, the suitability of applying these models at a regional scale has not been tested. Here, we evaluate, for the first time, the performance and utility of global scale models at the regional scale. Applied to the Loess Plateau of China, we compare estimates of groundwater recharge and nitrate storage derived from global scale models with results from a regional scale approach utilizing the Richards and advection‐dispersion equations. The estimated nitrate storage was compared to nitrate observations collected in the deep vadose zone (>50 m) at five sites across the Loess Plateau. Although both models predict similar spatial patterns of nitrate storage, the recharge fluxes were three times smaller and the nitrate storage was two times higher compared with the regional model. The results suggest that global scale models are a potentially useful screening tool, but require refinement for local scale applications.
N‐NO3 storage maps, for the year 2000, of the Loess Plateau of China (LPC) predicted with the (a) global approach, (b) the regional approach with (pixel size 48 km × 48 km), and (c) the yearly rain in the LPC (right axis) and the estimated nitrate accumulation by the regional approach and the global approach for the period 1958–2000.
Main antioxidants in higher plants include glutathione, ascorbate, tocopherol, proline, betaine, and others, which are also information-rich redox buffers and important redox signaling components ...that interact with biomembrane-related compartments. As an evolutionary consequence of aerobic life for higher plants, reactive oxygen species (ROS) are formed by partial reduction of molecular oxygen. The above enzymatic and non-enzymatic antioxidants in higher plants can protect their cells from oxidative damage by scavenging ROS. In addition to crucial roles in defense system and as enzyme cofactors, antioxidants influence higher plant growth and development by modifying processes from mitosis and cell elongation to senescence and death. Most importantly, they provide essential information on cellular redox state, and regulate gene expression associated with biotic and abiotic stress responses to optimize defense and survival. An overview of the literature is presented in terms of main antioxidants and redox signaling in plant cells. Special attention is given to ROS and ROS-antioxidant interaction as a metabolic interface for different types of signals derived from metabolism and from the changing environment, which regulates the appropriate induction of acclimation processes or, execution of cell death programs, which are the two essential directions for higher plants.
The current study investigated the remediation effect of lead-polluted garden soil by natural zeolite in terms of soil properties, Pb fraction of sequential extraction in soil and distribution of Pb ...in different parts of rape. Natural zeolite was added to artificially polluted garden soil to immobilize and limit the uptake of lead by rape through changing soil physical and chemical properties in the pot experiment under greenhouse conditions. Results indicated that the addition of natural zeolite could increase soil pH, CEC, content of soil organic matter and promote formation of soil aggregate. The application of zeolite decreased the available fraction of Pb in the garden soil by adjusting soil pH rather than CEC, and restrained the Pb uptake by rape. Data obtained suggested that the application of a dose of zeolite was adequate (≥10
g
kg
−1) to reduce soluble lead significantly, even if lead pollution is severe in garden soil (≥1000
mg
kg
−1). An appropriate dose of zeolite (20
g
kg
−1) could reduce the Pb concentration in the edible part (shoots) of rape up to 30% of Pb in the seriously polluted soil (2000
mg
kg
−1).
•Seasonal variation of energy and water exchanges was related to plant phenology.•Transpiration occupied most of the surface ET and soil evaporation was weak.•Groundwater is a critical factor ...controlling energy and water exchanges.•Yearly ET over the tamarisk stand was approximately 500mm.
Determining the maintenance and control mechanisms of the energy and water exchanges over the desert riparian forest zone in the lower Tarim River of China could improve our understanding of land surface processes of the groundwater-dependent ecosystems in the hinterland area of the Eurasian continent under hyper-arid climate. Nearly three years of continuous measurements of meteorological factors, surface water and energy fluxes, soil water content, and groundwater table were carried out over a riparian Tamarix spp. stand in the lower Tarim River basin. Because of scarce precipitation and low air humidity, the diurnal and seasonal variations of most meteorological factors exhibited simple unimodal dynamics. However, the seasonal variation of latent heat exchange (LE) was distinctly related to plant phenology. The sensible heat exchange (H) exhibited reverse course with the LE seasonal course, which suggested seasonal variation of the H was controlled by the LE course. In the daytime, a good linear relationship existed between evapotranspiration (ET) and reference evapotranspiration. The yearly ET over the tamarisk stand was approximately 500mmyear−1, and the mean daily ET is about 3.85mmd−1 in a vibrant growing season. The groundwater table and the soil water content (SWC) near the groundwater table exhibited obvious seasonal and diurnal variations in the growing seasons but SWC in the shallow soil layer did not, suggesting the groundwater was the water source of the tamarisk. Our analysis indicated that under a hyper-arid climate, the energy and water exchanges over riparian tamarisk stands in the lower Tarim River basin showed certain features: (1) plant transpiration accounted for most of the surface ET, with soil evaporation weak and negligible; (2) the seasonal processes of surface energy and water exchanges were highly related with plant phenology; (3) the diurnal processes of ET resulted from the comprehensive effects of all atmospheric factors; and (4) ET depends on groundwater, rather than precipitation.
Soil‐water content (SWC) is a key factor in restoring degraded vegetation in alpine meadow ecosystems, but it has rarely been spatially simulated on a hectometer scale. We simulated SWC for typical ...dry and wet days in an alpine meadow using multivariate linear regression and autoregressive state‐space equations based on SWC and other soil, terrain, and vegetation parameters to evaluate the efficiency of these two methods in dry and wet soil‐moisture conditions. SWC measured on a typical dry day (SWC‐D) and a wet day (SWC‐W) increased and decreased with depth, respectively, and SWC‐D was similar to SWC‐W at a depth of 50 cm. Both SWC‐D and SWC‐W were significantly correlated with soil bulk density (BD), capillary porosity, silt content (Silt), gravel and stone content (GSC), pH, and organic carbon density (OCD), and both SWC‐D and SWC‐W were significantly auto‐correlated and cross‐correlated with BD, Silt, GSC, pH, and OCD at more than one lag distance. Multivariate linear regression using three variables in both dry and wet conditions had the highest accuracy, and the accuracy was generally higher for dry conditions than it was for wet conditions. The bivariate state‐space model was the most accurate for both dry and wet soil conditions, but the expression variables were totally different, with pH and OCD for dry day and BD and Silt for wet day. The conditions of soil moisture should thus be considered when choosing variables with which to simulate SWC, instead of only considering the relationships between SWC and other variables.
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