Deserts In Northwestern China Research Articles

The role of biochar particle size and application rate in promoting the hydraulic and physical properties of sandy desert soil

Enhancing the resistance of sandy desert soil to drought and wind erosion is key to controlling aeolian desertification. Using biochar as an amendment can improve soil quality. This study aimed to determine the impacts of biochar particle size and application rate on the hydraulic and physical properties of sandy desert soil. Biochars were produced by pyrolyzing corn straw at 500 °C with limited oxygen. Samples of sandy desert soil were collected from the Tengger Desert in Northwestern China. Tests were designed with three biochar particle sizes (0–0.25, 0.25–1, and 1–2 mm) and five biochar application rates (wt%: 0, 0.5%, 1%, 2%, and 4%). Soil hydraulic and physical properties were characterized by the soil–water characteristic curve, soil porosity, soil specific surface area (SSA), and soil structural index (SI). The results showed that biochar significantly increased the soil porosity and the number of extreme micropores, micropores, and small pores. As a consequence, the saturated soil water content (θs), permanent wilting point (PWP), field capacity (FC), and available water content (AWC) were significantly enhanced. The significant improvement in SI demonstrated that biochar enhanced the soil aggregate stability, therefore, the resistance of sandy desert soil to wind erosion is enhanced. Biochar dosage was a more important factor that influenced the FC, AWC, PWP, SI, and SSA of the soil more than biochar particle size. The properties of θs, PWP, FC, AWC, and SI were all closely related to SSA, indicating that SSA played a key role in the mechanism affecting the response of the soil to biochar application. The significant increase in water retention and aggregate stability of sandy desert soil with added biochar suggests a novel strategy to control aeolian desertification by amending the soil.

Groundwater Depths Affect Phosphorus and Potassium Resorption but Not Their Utilization in a Desert Phreatophyte in Its Hyper-Arid Environment.

Nutrients are vital for plant subsistence and growth in nutrient-poor and arid ecosystems. The deep roots of phreatophytic plants are necessary to access groundwater, which is the major source of nutrients for phreatophytes in an arid desert ecosystem. However, the mechanisms through which changes in groundwater depth affect nutrient cycles of phreatophytic plants are still poorly understood. This study was performed to reveal the adaptive strategies involving the nutrient use efficiency (NUE) and nutrient resorption efficiency (NRE) of desert phreatophytes as affected by different groundwater depths. This work investigated the nitrogen (N), phosphorus (P), and potassium (K) concentrations in leaf, stem, and assimilating branch, as well as the NUE and NRE of the phreatophytic Alhagi sparsifolia. The plant was grown at groundwater depths of 2.5, 4.5, and 11.0 m during 2015 and 2016 in a desert-oasis transition ecotone at the southern rim of the Taklimakan Desert in northwestern China. Results show that the leaf, stem, and assimilating branch P concentrations of A. sparsifolia at 4.5 m groundwater depth were significantly lower than those at 2.5 and 11.0 m groundwater depths. The K concentrations in different tissues of A. sparsifolia at 4.5 m groundwater depth were significantly higher than those at 2.5 and 11.0 m groundwater depths. Conversely, the NRE of P in A. sparsifolia was the highest among the three groundwater depths, while that of K in A. sparsifolia was the lowest among the three groundwater depths in 2015 and 2016. The N concentration and NUE of N, P, and K in A. sparsifolia, however, were not influenced by groundwater depth. Further analyses using structural equation models showed that groundwater depth had significant effects on the P and K resorption of A. sparsifolia by changing soil P and senescent leaf K concentrations. Overall, our results suggest groundwater depths affect P and K concentrations and resorption but not their utilization in a desert phreatophyte in its hyper-arid environment. This study provides a new insight into the phreatophytic plant nutrient cycle strategy under a changing external environment in a hyper-arid ecosystem.

Analysis of variation in reference evapotranspiration and its driving factors in mainland China from 1960 to 2016

Understanding the variation in reference evapotranspiration (ETo) is vital for hydrological cycles, drought monitoring, and water resource management. With 1507 meteorological stations and 130 radiation-measured stations, the annual and seasonal ETo were calculated at each site from 1960 to 2016 in mainland China. The phenomenon of coefficient ‘a’ being less than 0.25 and coefficient ‘b’ being greater than 0.50 in the Angstrom–Prescott model occurred in almost the whole country, except for a small area of western and northeastern China. Moreover, the Xiao’s method was more applicable to calculate the net longwave radiation (R nl) and then improve the estimation accuracy of ETo. The annual ETo varied from 538.8 to 1559.8 mm and had a high-value center located in the plateau and desert of northwestern China and a low-value center located in Northeast China and near the Sichuan Basin. The spatial distribution of seasonal ETo was roughly similar to that of annual ETo, except for that in winter when ETo was high in the south and low in the north. In mainland China, the annual ETo decreased by 21.2 mm decade−1 because of the declining sunshine duration before 1993 and increased by 21.1 mm decade−1 due to the decreased relative humidity (RH) after 1993. Generally, the abrupt change of ETo mainly occurred in the southern China rather than northern China (except for Qinghai Tibet Plateau). Basically, the dominant driving factors of annual and seasonal ETo were RH and/or T max after the abrupt change in most parts of China.

The ecological roles of seed mucilage on germination of Lepidium perfoliatum, a desert herb with typical myxospermy in Xinjiang

Myxospermy is considered as an adaptation to unideal habitats, which is characterized as a gelatinous layer of mucilage around the seed upon contact with water. Studies on the ecological roles of seed mucilage should provide valuable information on seed germination strategy. In the present study, the germination properties of Lepidium perfoliatum L., an early spring ephemeral plant with typical myxospermy in desert of northwestern China, were investigated with the intact seeds and demucilaged seeds under different conditions. Results revealed that, firstly, the seed mucilage was essential for intact seeds to germinate quickly and efficiently under abundant or excess water condition, while had no effect on relief of seed germination at low water potential or other stressful conditions; secondly, the presence of mucilage layer stimulated intact seed germination under relatively higher temperature, appropriate light, permeable soil matrix, which helps seed germinate efficiently under favorable conditions; thirdly, the existence of mucilage may limit air diffusion into seed and subsequently reduce germination, which might be a strategy to ensure a proportion of seeds staying in soil seed bank to wait for next favourable conditions. Our results suggest that the seed mucilage of L. perfoliatum applies multiple ecological roles in protection of seed germination to avoid unfavourable conditions, which should be an adaptation strategy for population survival and succession in a harsh desert environment.

Impacts of Transboundary Dust Transport on Aerosol Pollution in the Western Yangtze River Delta Region, China: Insights Gained From Ground‐Based Lidar and Satellite Observations

AbstractDust storms pose a serious threat to air quality and public health through large‐scale, long‐distance transport. In early April 2018, two severe dust storm events occurred in the Taklimakan and Gobi Deserts in northwestern China. The advected air‐masses containing dust traveled eastward over long distances reaching the western Yangtze River Delta region (WYRDR) with local PM10 peak concentrations of 190 and 410 μg/m3 on April 6 and April 11, 2018, respectively. Satellite remote sensing and ground‐based lidar observations showed that the first dust event was transported to WYRDR at an altitude of <5 km, while the second transport height was at an altitude of <3 km. The first one was accompanied with the dry‐cold air mass in the northwest moving eastward and southward. By comparison, in the second event, the dust aerosols were mixed with anthropogenic aerosols from southwestern China. Meanwhile, as southerly warm‐humid air flows prevailed at near‐surface layer over the WYRDR in the second event, the associated higher relative humidity potentially induced hygroscopic growth of polluted dust aerosols, resulting in explosive growth of PM10. In addition, the average depolarization ratio of aerosols over WYRDR on April 6 was 0.3, while on April 11 it was 0.2, indicating that the previous pollution event featured more obvious nonspherical characteristics of aerosols. Our findings provide scientific evidence that transboundary dust events and resultant local PM10 pollution are closely related to various transport trajectories of dust aerosols as well as the local vertical wind profiles and surface humidity conditions.

Temperature-Based and Radiance-Based Validation of the Collection 6 MYD11 and MYD21 Land Surface Temperature Products Over Barren Surfaces in Northwestern China

In this study, two collection 6 (C6) Moderate Resolution Imaging Spectroradiometer (MODIS) level-2 land surface temperature (LST) products (MYD11_L2 and MYD21_L2) from the Aqua satellite were evaluated using temperature-based (T-based) and radiance-based (R-based) validation methods over barren surfaces in Northwestern China. The ground measurements collected at four barren surface sites from June 2012 to September 2018 during the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) experiment were used to perform the T-based evaluation. Ten sand dune sites were selected in six large deserts in Northwestern China to carry out an R-based validation from 2012 to 2018. The T-based validation results indicate that the C6 MYD21 LST product has a better accuracy than the C6 MYD11 product during both daytime and nighttime. The LST is underestimated by the C6 MYD11 products at the four T-based sites during the daytime, with a mean bias of -2.82 K and a mean RMSE of 3.82 K, whereas the MYD21 LST product has a mean bias and RMSE of -0.51 and 2.53 K, respectively. The LST is also underestimated at night by the C6 MYD11 products at the four T-based sites, with a mean bias of -1.40 K and a mean RMSE of 1.72 K, whereas the MYD21 LST product has a mean bias and RMSE of 0.23 and 1.01 K, respectively. For the R-based validation, the MYD11 results are associated with large negative biases during both daytime and nighttime at three sand dune sites and biases within 1 K at the other seven sites, whereas the MYD21 results are more consistent at all ten sand dune sites, with a mean bias of 0.45 and 0.70 K for daytime and nighttime, respectively. The emissivities for these two products in MODIS bands 31 and 32 were compared with each other and then compared with the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) emissivity and laboratory emissivity. The results indicate that the emissivities in MODIS bands 31 and 32 of MYD11 at the four T-based and three of the R-based validation sites are overestimated and result in LST underestimation, whereas the emissivities of MYD21 are more consistent with the laboratory emissivity. Besides, an experiment was carried out to demonstrate that the physically retrieved dynamic emissivity of the MYD21 product can be utilized to improve the accuracy of the split-window (SW) algorithm for barren surfaces, making it a valuable data source for retrieving LST from different remote sensing data.

Monitoring and modeling the coupled movement of water, vapor, and energy in arid areas

In arid areas, vapor flow has been recognized to contribute significantly to the mass and energy transfers and to play a critical role in maintaining surface vegetation and ecosystems. To better understand the continuous spatial and temporal variations in liquid water and water vapor contents under different climatic conditions, soil water contents, temperatures, and micrometeorological variables were observed in-situ in the Mu Us Desert of northwestern China. The collected data were then used to calibrate and validate the Hydrus-1D model simulating either the coupled movement of water, vapor, and energy or only isothermal water flow in soils (the latter as a reference). The results of the coupled model were not only in better agreement with observed data, but also advanced our understanding of underlying mechanisms of soil water flow. For the coupled model, the isothermal liquid flux was the most significant component of the total water flux. Three diurnal stages were identified for thermal liquid and vapor fluxes on dry days, while the vapor flux became almost negligible during rainfall events. The results indicate that isothermal liquid, thermal liquid, and thermal vapor fluxes should be considered simultaneously when evaluating soil water flow in arid regions, while the isothermal vapor flux can be neglected. Vapor flow contributed, on average, about 13% of the total water flux in the uppermost soil layer during the analyzed period at the study area, and this ratio often exceeded 20%.

Stomatal Sensitivity to Vapor Pressure Deficit and the Loss of Hydraulic Conductivity Are Coordinated in Populus euphratica, a Desert Phreatophyte Species.

There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (Kl) via precise control over PLC under variable ΔΨ (water potential differential between soil and leaf) conditions to maintain the −m/b constant (−m: the sensitivity of stomatal conductance to VPD; b: reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD, stomatal conductance (gs), gs responses to VPD, mid-day minimum leaf water potential (Ψlmin), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the −m/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ50 (the xylem water potential with 50% loss of hydraulic conductivity) was −1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ50 and wood density/leaf specific hydraulic conductivity (kl) and between specific hydraulic conductance sensitivity to water potential [d(ks)/dln(−Ψ)] and specific hydraulic conductivity (ks). A modified hydraulic model was applied to investigate the relationship between gs and VPD under variable ΔΨ and Kl conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant −m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field.

Water Addition Prolonged the Length of the Growing Season of the Desert Shrub Nitraria tangutorum in a Temperate Desert.

Climate models often predict that more extreme precipitation events will occur in arid and semiarid regions, where plant phenology is particularly sensitive to precipitation changes. To understand how increases in precipitation affect plant phenology, this study conducted a manipulative field experiment in a desert ecosystem of northwest China. In this study, a long-term in situ water addition experiment was conducted in a temperate desert in northwestern China. The following five treatments were used: natural rain plus an additional 0, 25, 50, 75, and 100% of the local mean annual precipitation. A series of phenological events, including leaf unfolding (onset, 30%, 50%, and end of leaf unfolding), cessation of new branch elongation (30, 50, and 90%), and leaf coloration (80% of leaves turned yellow), of the locally dominant shrub Nitraria tangutorum were observed from 2012 to 2018. The results showed that on average, over the seven-year-study and in all treatments water addition treatments advanced the spring phenology (30% of leaf unfolding) by 1.29–3.00 days, but delayed the autumn phenology (80% of leaves turned yellow) by 1.18–11.82 days. Therefore, the length of the growing season was prolonged by 2.11–13.68 days, and autumn phenology contributed more than spring phenology. In addition, water addition treatments delayed the cessation of new branch elongation (90%) by 5.82–12.61 days, and nonlinear relationships were found between the leaves yellowing (80% of leaves) and the amount of watering. Linear relationships were found between the cessation of new branch elongation (90%), the length of the growing season, and amount of water addition. The two response patterns to water increase indicated that predictions of phenological events in the future should not be based on one trend only.

Biomass partitioning and allometric relations of the Reaumuria soongorica shrub in Alxa steppe desert in NW China

Allometry and partitioning of above- and below-ground biomass are needed to evaluate the sustainability of arid ecosystems. Biomass accumulation, allocation patterns, and allometric relationships for above- and below-ground components of the desert shrub Reaumuria soongorica were studied across five age classes (0–10, 11–20, 21–30, 31–40, and > 40 years) in Alxa desert steppe, northwestern (NW) China, where the biomass of each shrub component (i.e., leaves, branches, and coarse and fine roots) was harvested by destructive sampling and the allocation patterns between leaves (ML), roots (MR), branches (MB) were studied by regression analysis of log10-transformed data, and tested for data classified by age class and pooled data. By using basal diameter, age, height, and canopy area, we can predict the allometry properties of the R. soongorica shrub in terms of age class and/or entire age-sequence. The experimental results indicate that biomass accumulation has an obvious gradient across the five age classes, branch, and coarse root biomass were the main biomass pool as mature shrub. R. soongorica shrub partitioned a greater proportion of total biomass to roots with age in order to adapt to the extreme arid conditions. Shrub allometry was affected by variation of biomass partitioning, where the allometric relationships were strongly related to basal shrub diameter for almost all shrub components. Allometric biomass relationships were improved with the introduction of a second variable crown area.

Provenance of eolian sands in the Ulan Buh Desert, northwestern China, revealed by heavy mineral assemblages

Accurate identification of the provenance of eolian sand in deserts is essential to study sedimentation processes in the earth system. Exposed bedrock surrounding the desert has been regarded as the main source of eolian sand in some deserts of the world. However, few studies have focused on whether paleolacustrine deposits and bedrock outcrops in desert hinterlands are the main sources of eolian sand. In this study, we selected the Ulan Buh Desert in northwestern China as the study area, and a total of 99 samples were collected throughout the desert and nearby regions to study the provenance of the current eolian sand based on their heavy mineral assemblages and contents. The results showed similar heavy mineral assemblages among the eolian sand, the bedrock to the northwest of the desert, and the paleolacustrine deposits and exposed bedrock in the desert hinterlands, suggesting multiple provenance for the eolian sand in the Ulan Buh Desert. The sources of the eolian sand in the Ulan Buh Desert include bedrock to the northwest, gobi to the west, and paleolacustrine deposits and exposed bedrock in the desert hinterlands. Helan Mountains bedrock and Yellow River fluvial deposits near the desert contribute little to the eolian sand. Moreover, the paleolacustrine deposits and exposed bedrock in the desert hinterlands should not be neglected as the potential provenance of eolian sand in other studies. This study enhances understanding the provenance of eolian sand in the deserts of northern China.

Transcriptomic and metabolomic analyses of Lycium ruthenicum and Lycium barbarum fruits during ripening

Red wolfberry (or goji berry, Lycium barbarum; LB) is an important agricultural product with a high content of pharmacologically important secondary metabolites such as phenylpropanoids. A close relative, black wolfberry (L. ruthenicum; LR), endemic to the salinized deserts of northwestern China, is used only locally. The two fruits exhibit many morphological and phytochemical differences, but genetic mechanisms underlying them remain poorly explored. In order to identify the genes of interest for further studies, we studied transcriptomic (Illumina HiSeq) and metabolomic (LC-MS) profiles of the two fruits during five developmental stages (young to ripe). As expected, we identified much higher numbers of significantly differentially regulated genes (DEGs) than metabolites. The highest numbers were identified in pairwise comparisons including the first stage for both species, but total numbers were consistently somewhat lower for the LR. The number of differentially regulated metabolites in pairwise comparisons of developmental stages varied from 66 (stages 3 vs 4) to 133 (stages 2 vs 5) in both species. We identified a number of genes (e.g. AAT1, metE, pip) and metabolites (e.g. rutin, raffinose, galactinol, trehalose, citrulline and DL-arginine) that may be of interest to future functional studies of stress adaptation in plants. As LB is also highly suitable for combating soil desertification and alleviating soil salinity/alkalinity/pollution, its potential for human use may be much wider than its current, highly localized, relevance.

Response of ecosystem functioning to environmental variations in an artificial sand-binding vegetation desert in northwestern China.

The establishment of artificial sand-binding vegetation is one of the main means for restoring damaged ecosystems that are impacted by global change. This study was conducted to evaluate the influence of environmental factors on ecosystem function (net ecosystem exchange (NEE), evapotranspiration (ET), and water use efficiency (WUE)) in an artificial sand-binding vegetation desert (with dominant shrubs, such as Artemisia ordosica and Caragana korshinskii, and herbaceous plants) in northwestern China. NEE, ET, and meteorological data were observed with the eddy covariance (EC) technique. The random forest (RF) method was used to identify major environmental factors that affected NEE, ET, and WUE. Our results showed that the mean annual NEE, ET, and WUE values were - 112.4g C m-2, 232.1mm, and 0.49g C kg-1 H2O, respectively, during the 2015 to 2018 growing seasons. At the weekly scale, the most important drivers of NEE were the normalized difference vegetation index (NDVI) and soil water content (SWC). Rainfall, SWC, and NDVI were important drivers of ET. WUE was mainly controlled by rainfall and SWC. Linear regression showed that NEE had significant negative relationships with the NDVI and SWC. ET had positive relationships with rainfall, SWC, and the NDVI. WUE had significant negative relationships with SWC and rainfall. These findings indicate that drought inhibited ET more than carbon absorption, thus promoting the WUE of the ecosystem to some extent. The close relation of the ecosystem function to SWC implies that this ecosystem may be critically regulated by future climate change (specifically, changes in rainfall patterns).

A dryland re‐vegetation in northern China: Success or failure? Quick transitions or long lags?

AbstractIntensified anthropogenic activities have drastically altered many ecosystems, motivating the use of restoration to regain key ecosystem functions and services and to stem biodiversity losses. Restoration is particularly difficult when human activities have pushed an area into a new state with self‐reinforcing feedbacks. This study investigated a long‐term restoration project in a dryland ecosystem of the Tengger Desert in northwestern China, initiated in 1956. We analyzed shrub and grass cover for 49 yr after the installation of restoration infrastructure that altered external conditions (i.e., using packed straw to reduce wind erosion) and system state (by planting shrubs). After 37–40 yr, the re‐vegetation project was successful in restoring the system to a state similar to native vegetation, with high grass cover (30–50%), low shrub cover (8–10%), and a thick biological soil crust (biocrust). However, the shift to high grass cover did not begin until year 37, before which shrub cover was high (15–20%) and grasses were subdominant (usually <10%). The shift from shrub to grass dominance was abrupt, registering significant nonlinear changes over time and relative to a key driver of vegetation dynamics, estimated biocrust thickness. Biocrust thickness increased gradually over time, which reduced rainfall infiltration into deep soil and thus increased soil moisture available for the shallow‐rooted grasses. The shift from the bare soil state to the steppe state exhibited a long time lag, suggesting that it can take decades to determine whether dryland restoration efforts succeed. The results indicate that persistence might be critical to forcing desired state transitions and that dryland restoration can proceed as a series of time lags, punctuated by abrupt changes in ecosystem state.

Change in Characteristics of Soil Carbon and Nitrogen during the Succession of Nitraria Tangutorum in an Arid Desert Area

The shrub Nitraria tangutorum is distributed widely in arid desert areas, and plays a critical role in the desert–oasis ecosystem. This study quantified varying characteristics of carbon (C) and nitrogen (N) in the soil at four stages—the initial stage (IS), stable stage (SS), degradation stage (DS), and severe degradation stage (SDS)—in a steppe ecosystem in the desert of northwestern China. The results indicated that N. tangutorum experienced both expansion and deterioration as a decline of 50.7% occurred in the available soil water due to agricultural utilization, and the plant community transformed from being shrub-dominated to annual herb-dominated. At soil layer depths between 0–100 cm in the N. tangutorum nebkha dune ecosystem, organic C and total N storage was 1195.84 g/m2 and 115.01 g/m2 during the SDS, respectively, with an increase of 11.13% and 12.59% from the IS. In addition, the storage of C and N in the soil increased during the IS as well as the SS, when most of the C and N were accumulated, and the storage decreased during the DS and SDS, as the N. tangutorum communities declined. At soil layer depths between 0–100 cm in the desert steppe ecosystem, the highest storage levels of C and N were 8465.97 g/m2 and 749.29 g/m2 during the SS, and the lowest were 1076.12 g/m2 and 102.15 g/m2 during the IS, respectively. The changes and accumulation of C and N were greater in the deeper (40–100 cm) layer than in the surface layer of soil (0–40 cm). Lastly, changes in soil organic carbon (SOC) as well as in the total nitrogen (TN) were strongly related to the coverage degree, water content in soil, and the ratio of fine soil particles (silt and clay). To sum up, the intensive development of water resources has vastly reduced the ability of N. tangutorum vegetation to sequester C and N in the desert of Minqin. Efforts to perform ecological restoration and reverse desertification in the Minqin Desert should focus on preventing the unreasonable exploitation of water resources in order to maintain stable N. tangutorum communities.

Genetic diversity patterns in an endangered desert shrub from north-western China: implications for conservation

Tamarix taklamakanensis is an endangered shrub endemic to the Tarim Basin and adjacent Kumtag Desert in north-western China. Here, we used two chloroplast DNA sequences, namely, psbA-trnH and trnS-trnG, to examine the genetic diversity patterns of this species across its entire covered range. A total of nineteen haplotypes were detected. The total gene diversity within the species is high. Genetic variation mainly occurred among populations, SAMOVA groups, and geographic regions. The test for isolation-by-distance showed a significant correlation between genetic and geographical distances, and the genetic landscape shape analysis showed a significant genetic divergence between the Tarim Basin and Kumtag Desert. T. taklamakanensis likely had a potential geographic range during the Last Glacial Maximum period that was much smaller than the present range predicted by ecological niche modelling. The cold and dry climate during the glacial periods of the Quaternary might be a driver of the genetic isolation and divergence detected within T. taklamakanensis, and climatic oscillations might account for the habitat fragmentation of the species. Within the inner of the basin, populations have a higher level of genetic diversity and harbor most of this genetic diversity, thus a nature reserves should be set up in this area for the in situ conservation. In addition, five genetically distinct groups within T. taklamakanensis should be treated as different management units (MUs) when implementing conservation activities.

NO3− circulation and associated driving factors in the unsaturated zone of southwestern Tengger Desert, Northwestern China

In the southwestern Tengger desert in northwestern China, nitrate circulation processes in the unsaturated zone and associated driving factors were studied using water chemistry and stable isotope techniques. At unvegetated sand sites, NO3− content increases from the south to north in the desert. Additionally, a negative correlation (R2 = 0.90, P < 0.005) was evident between the stability of soil NO3− and elevation, suggesting that NO3− storage in soil is significantly influenced by precipitation and temperature. And a significant positive correlation between mNO2−/Cl− in unsaturated zone and elevation (R2 = 0.50, P < 0.0001), indicating both nitrification and denitrification are strongly in the south of desert, however, the nitrification plays a dominant role in the north of desert. Isotopic analyses (δ15N-NO3−, δ18O-NO3−, and δ18O-H2O, δ2H-H2O) of soil pore-water showed that the two dominant NO3− sources were the mixing of atmospheric deposition and soil nitrification, and that the NO3− circulation at the vegetated sand sites was affected by vegetation and animals. Additionally, local environmental conditions lead to complex driving factors, such as the abiotic process of ammonia volatilization of NO3− circulation, which resulted in the 15N enrichment and 18O depletion of residual nitrate in the unsaturated zone of one of the soil profiles. Denitrification was noted in the deep unsaturated zone of two profiles, indicating that the N loss resulted from abiotic versus biotic processes. Analyses of the stable isotope results and the distribution characteristics of NO3− and Cl− suggests leaching of soil NO3− as a result of NO3− in unsaturated zone poses a potential threat to groundwater quality in Dengmaying region. This study suggests that NO3− circulation in the unsaturated zone responds to climate characteristics in the desert ecosystem, and to provide useful information for the protection of groundwater quality in arid desert regions.

Effects of Updated RegCM4 Land Use Data on Near-Surface Temperature Simulation in China

Biogeophysical effects of land use and land cover (LULC) changes play a significant role in modulating climate on various spatial scales. In this study, a set of recent LULC products with a spatial resolution of 500 m was developed in China for update in RegCM4 (regional climate model version 4). Two sets of comparative numerical experiments were conducted to study the effects of LULC changes on near-surface temperature simulation. The results show that after LULC changes, areas of crops and mixed woodlands as well as urban areas increase over entire China, accompanied with greatly expanded mixed farming and forests/field mosaics in southern China, and reduced areas of 1) irrigated crops and short grasses in northern China and the Tibetan Plateau, and 2) semi-desert and desert in northwestern China. Improvements in the LULC data clearly result in more accurate simulations of the near-surface temperature. Specifically, increasing latent heat and longwave albedo due to enhanced LULC in certain areas lead to reduction in land surface temperature (LST), while changes in shortwave albedo and sensible heat also exert a great influence on the LST. Overall, these parameter adjustments reduce the biases in near-surface temperature simulation.

Evapotranspiration of xerophytic shrub Salsola passerina and Reaumuria soongorica in an arid desert ecosystem of NW China

AbstractUnderstanding the actual evapotranspiration (ET) variation of the sparsely distributed xerophytic shrubs is crucial to accurately upscale community ET to ecosystem scale. Here we quantified the actual ET of two dominant xerophytic shrubs of the Tengger Desert in northwestern China, i.e. Salsola passerina and Reaumuria soongorica, by using four large weighing lysimeters. The results showed that with the increase in precipitation from 140 to 171 mm in the year 2015/2016, the daily mean evaporation (E) of the bare area, and ET of the single shrub communities of S. passerina, R. soongorica, and the associated shrub community (S. passerina + R. soongorica) increased 50, 60, 44, and 47%, respectively; correspondingly, the total E and ET increased 49, 61, 44, and 47%, respectively. The variation of soil moisture within 0–40 cm depth plays a vital role in regulating the E and ET. The new shoot length, as one of important parameters of the xerophytic shrub, was significantly exponentially related to the cumulative ET. From the long- and short-term perspective, event-based precipitation and wind speed are the dominant driving factors behind changes in E and ET, respectively. Relative humidity is the main influencing factor for E and ET after a large rainfall event within 8 days.

Phenological responses to nitrogen and water addition are linked to plant growth patterns in a desert herbaceous community.

Increases in nitrogen (N) deposition and variation in precipitation have been occurring in temperate deserts; however, little information is available regarding plant phenological responses to environmental cues and their relationships with plant growth pattern in desert ecosystems. In this study, plant phenology and growth of six annuals in response to N and water addition were monitored throughout two consecutive growing seasons in 2011 and 2012 in a temperate desert in northwestern China. The effects of N and water addition on reproductive phenology differed among plant species. N and water addition consistently advanced the flowering onset time and fruiting time of four spring ephemerals; however, their effects on two spring‐summer annuals were inconsistent, with advances being noted in one species and delays in another. N and water addition alone increased plant height, relative growth rate, leaf number, flower number, and individual biomass, while their combinative effects on plant growth and reproductive phenology were dependent on species. Multiple regression analysis showed that flowering onset time was negatively correlated with relative growth rate of two species, and negatively correlated with maximum plant height of the other four species. Our study demonstrates that phenological responses to increasing precipitation and N deposition varied in annuals with different life histories, whereby the effects of climate change on plant growth rate were related to reproductive phenology. Desert annuals that were able to accelerate growth rate under increasing soil resource availability tended to advance their flowering onset time to escape drought later in the growing season. This study promotes our understanding of the responses of temperate desert annuals to increasing precipitation and N deposition in this desert.