Semi-arid Northern China Research Articles

Optimistic growth of marginal region plantations under climate warming: Assessing divergent drought resilience.

Given the context of significant global warming and the intensification of extreme climate events in the last century, large-scale reforestation and afforestation have been recognized as effective strategies to mitigate the climate crisis. Since the 1970s, China has launched several afforestation programs aimed at regional ecological protection, playing an important role in reaching carbon neutrality by 2060. This study provided a detailed analysis of the growth suitability of the main planted conifers (Pinus sylvestris var. mongolica and Pinus tabulaeformis) and broadleaves (Populus spp., Robinia pseudoacacia) in the semi-arid northern China. We compared the radial growth trends of plantations and their responses to extreme droughts from 1980 to 2018. Growth of most plantations has significantly increased over time, but broadleaves showed recent growth reductions in the past decade, which may be related to tree age and reduced soil moisture. Nevertheless, under warmer climate scenarios, the growth of plantations is forecasted to continue increasing. Broadleaves showed a better post-drought recovery, probably linked to their anisohydric behavior, than conifers, which presented a better resistance to drought. Growth of conifers depended more on warmer temperature and better precipitation conditions during the growing season, whereas broadleaves mainly reacted to warm temperature. Additionally, pre-drought growth levels weakened resilience components, while post-drought precipitation compensated for drought-induced growth deficit. Growth and resilience were negatively related to tree age, while higher stand density reduced growth. This assessment and projections of growth and drought resilience indicate the sustainability of most plantations in semi-arid regions, but future warmer and drier conditions may lead to an uncertain future regarding forest health and reduce their carbon sink potential.

Synergistic Effects of Soil-Based Irrigation and Manure Substitution for Partial Chemical Fertilizer on Potato Productivity and Profitability in Semiarid Northern China.

Soil-based irrigation and the partial substitution of chemical fertilizers with manure are promising practices to improve water and nitrogen (N) use efficiency. We hypothesize that their combination would simultaneously benefit potato production, tuber quality and profitability. A two-year experiment was conducted in semiarid northern China to investigate the combined effects of three water treatments [rainfed (W0), soil-based irrigation (W1), conventional irrigation (W2)] and three N treatments [no N (N0), chemical N (N1), 25% manure substitution (N2)] on these indicators, and to perform a comprehensive evaluation and correlation analysis. The results showed that water and N treatments separately affected all indicators except vitamin C content. Compared to W2, W1 significantly increased water productivity by 12% and irrigation water use efficiency (IWUE) by 30% due to 10% lower evapotranspiration and 21% lower water use. However, W1 and W2 negatively affected crude protein content. Conversely, this was compensated by the combination with N1 and N2. There were slight differences between N1 and N2 for all indicators on average across water treatments, while under W1, N2 significantly increased leaf area index (LAI) and N recovery efficiency (REN) by 18% and 29.4%, respectively, over N1. Also, comprehensive evaluations showed that W1N2 performed best, with the highest tuber yield, profit and acceptable quality. This can be explained by the increase in LAI, IWUE and REN due to the positive correlations with tuber yield and net return. Consequently, soil-based irrigation combined with 25% manure substitution had complementary effects on tuber quality and synergistic effects on potato productivity and profitability.

Changes in the lake-grassland ecosystem revealed by multiple proxies in a sediment core from Ganggeng Nur Lake, northern China

AbstractAs the main global terrestrial ecosystem component, grasslands are extremely sensitive to global climate change. With increasing human activities over the last century, grassland ecosystems have been degraded to different degrees. However, the evolution of lake-grassland ecosystems in recent centuries remains unclear due to the dearth of high-resolution records. Here, we present high-resolution lacustrine sediment grain size, pollen (Artemisia, Myriophyllum), Pediastrum, and n-alkane records from Ganggeng Nur Lake to investigate vegetation, lake evolution, and human effects in semiarid northern China. Four stages were identified from the last ca. 150 years: (1) the natural evolution stage (AD 1870–1945), in which there was a wet climate around Ganggeng Nur and the lake level rose from increased runoff; (2) the human disturbance stage (AD 1945–1967), in which the regional climate got drier and human activities began having a detectable effect on the grassland ecosystem; (3) the human transformation stage (AD 1967–2005), in which a completely arid climate coupled with the implementation of a series of land reclamation policies resulted in a large reduction in grassland areas, extensive soil erosion, exacerbated climate change, and shrinking lake areas; and (4) the posttreatment stage (AD 2005–2018), in which soil erosion was alleviated by policy implementation and a favorable humid climate.

Late Quaternary evolution of Daihai Lake in northern China and implications to the variation of the East Asian summer monsoon

Global warming over the past decades has resulted in desiccation of lakes in arid and semi-arid northern China. However, the mechanisms responsible for the sharp fluctuations in water levels for these lakes in this region are still unclear. The water level fluctuations in geological time can provide an analogue to better understand current lake levels’ response to global warming. In this study, a combination of quartz and feldspar luminescence dating techniques was applied to establish the chronologies of two sedimentary sequences related to lake level in the catchment of Daihai Lake, located on the fringe of the East Asian summer monsoon (EASM) domain. Our objective is to reconstruct the late Quaternary history of lake level fluctuations and provide the interpretations of their causes. The results indicate that two lake-level highstands existed during 130–120 ka and 8-4 ka, at least ∼38 m and 40 m above the current lake level, respectively. We interpret that the lake level variations on multiple timescales in the monsoonal marginal area have primarily been regulated by the EASM intensity and corresponding monsoonal precipitation, while temperature changes may have played a minor role. Moreover, a compilation of existing lake level data reveals that EASM intensity has a complex response to changes in Northern Hemisphere summer insolation on millennial-orbital timescales. Combining precipitation records in central and southern China, the character of rainfall across East Asia during MIS 5 exhibits a dominant tripolar pattern.

Agricultural development has not necessarily caused forest cover decline in semi-arid northern China over the past 12,000 years

Forest cover significantly affects the global carbon cycle, biodiversity, and human welfare, but is seriously threatened by human activities. Here we found that anthropogenic forces did not necessarily lead to forest cover decline in the marginal agricultural region of northern China based on a quantitative reconstruction of 12,000-years forest cover changes using modern analog technique and random forest. The forest cover was strongly affected by human activities in the core agricultural region, as indicated by the high independent effects of archeological sites (38.3%) and burning area (42.3%). In contrast, climate impacted the forest cover in the marginal agricultural region, where the annual precipitation and summer mean temperature contributed 52.4% and 27.4%, respectively. Agricultural development in the marginal agricultural region likely occurred in the river valley or sunny slopes, not overlapping with zonal forests growing on shady slopes. This study implies much less afforestation potential than previously expected in the marginal agricultural region.

Effects of Returning Different Organic Materials in Combination with Inorganic Fertilizers on the Diversity of Eukaryotic Microorganisms in Semi-Arid Northern China

Soil eukaryotic microorganisms are important to biodiversity, and returning different kinds of organic materials to the field could improve the biodiversity of soil eukaryotic microorganisms. However, no detailed research has been conducted in the northern China semi-arid area in returning the different organic materials to the field and determining the status of eukaryotic microorganisms. Therefore, we explored the effects of various organic materials returning to the soil in combination with inorganic fertilizer on the diversity and community structure of eukaryotic microorganisms in Shanxi province, China. Soil samples were collected from five different fertilization treatments: chemical fertilizer (F), chemical fertilizer + cattle manure (FM), chemical fertilizer + straw (FS), chemical fertilizer + pig manure (FC) and control without fertilizers (CK). High-throughput sequencing was applied to analyze the eukaryotic diversity and community structure. Results showed that the dominant eukaryotic microorganisms among soil samples were Fungi, Viridiplantae, Metazoa and Protist. Although α-diversity was not significantly different among the five treatments, principal coordinate analysis and permutational multivariate analysis of variance illustrated significant differences (p < 0.001) in β-diversity of eukaryotic microorganism under treatments with different organic materials. Redundancy analysis showed that the soil properties, including total potassium, available nitrogen, available potassium and organic matter were the main factors attributed to eukaryotic microorganisms’ community structure in this region.

Pollen–vegetation relationship based on lake surface sediments of arid and semi-arid northern China; considerations and limitations

In this paper, we investigated pollen assemblages of 92 lake center surface sediments of arid and semi-arid northern China to statistically disclose the pollen–vegetation relationships at the regional scale and explore considerations and limitations for lacustrine pollen-based reconstructions. As a result, the Principal component analysis (PCA), the Indicator species analysis, the Discriminate analysis (DA), the Random forest (RF), and the Supportive vector machine (SVM) generally reveal positive correlations between pollen assemblages and vegetation types or ecotones and suggests capabilities of pollen to represent regional vegetation and climate. However, the DA, RF, and SVM reveal that the samples of desert steppe, typical steppe, and meadow steppe cannot be differentiated by pollen assemblages, suggesting weaknesses to represent regional vegetation and climate. Mental test analyses of pollen assemblages and azonal vegetation of the dune desert and the Gobi desert yield better correlations than that of regional vegetation, suggesting that the pollen of azonal vegetation distort the regional pollen–vegetation relationships and thus weaken the capabilities to represent regional vegetation and climate. Our results present considerations and limitations for pollen-based past vegetation and climate reconstructions and provide references for interpretations of lacustrine fossil pollen records in arid and semi-arid northern China.

An improved method of using two-dimensional model to evaluate the carrying capacity of regional water resource in Inner Mongolia of China

The evaluation of regional water resource carrying capacity has been repeatedly conducted to provide a scientific basis for the local water resource management and the sustainable development, in particular in the water-limited regions. However, the definition of regional water resource carrying capacity and its evaluation method are still arguable. Through a case study of Inner Mongolia, located in the arid and semi-arid northern China, this paper developed an improved method to calculate regional water resource carrying capacity by the combination of the water supply-demand analysis and the S-shaped curve threshold analysis. The spatial and temporal patterns of the regional water resource carrying capacity in Inner Mongolia during 2000–2019 was evaluated at three scales, namely the province scale, the basin scale and the city scale. The results showed that the average regional water resource carrying capacity of the whole province was 0.25 (the full mark is 1.00); at the basin scale, the Yellow River Basin had the lowest regional water resource carrying capacity (0.17) among all the basins, showing that the utilization of the water resources was unreasonable; at the city scale, the average regional water resource carrying capacities in Hulunbuir and Xilingol were both over 0.25, while those in Alxa, BayanNur and Wuhai were below 0.1; Hulunbuir had 25.48 billion m3 water surplus, while BayanNur suffered from an average water deficit of 4.51 billion m3 from 2000 to 2019. This paper has provided a reasonable way to measure the regional water resource carrying capacity using an improved method by incorporating S-shaped curve threshold analysis, which may have a wider application for the clustering and optimization of regional water management. In addition, the spatial and temporal patterns of regional water carrying capacity are beneficial for policymakers in the implementation of the effective water usage.

A comparison of the aerodynamic characteristics of four kinds of land surface in wind erosion areas of northern China

Aerodynamic characteristics is a crucial factor influencing soil wind erosion, which is closely related to the properties of surface roughness elements. In the expansive arid and semi-arid northern China, grassland, farmland, mobile sandy land, and gobi are the main land surface types that suffer wind erosion to varying degrees. To investigate the aerodynamic characteristics of different surfaces, we used three-dimensional ultrasonic anemometers to observe the near-surface wind speed above grassland, farmland, mobile sand, and two types of gobi surfaces in areas of northern China (a total of five observation sites). We compared the aerodynamic characteristics of the five surfaces and their causes (i.e., the surface roughness length, wind speed pulsation, turbulence intensity, and drag coefficient), and analyzed how surface roughness elements weakened the airflow. The roughness length was greatest for grassland, followed by farmland, sand, fine gobi, and black gobi. The pulsation intensity of near-surface wind speed increased linearly with increasing wind speed at all sites. At a given wind speed, increasing roughness length increased the pulsation intensity of wind speed. The probability distributions for the drag coefficient and turbulence intensity of the five surfaces followed a positively skewed distribution. The drag coefficient and turbulence intensity were greatest for grassland, followed by farmland, sand, fine gobi, and black gobi surfaces. The drag force exerted on the roughness elements increased with increasing wind speed, but the ratio of drag force exerted on the roughness elements to the total drag force (τR/τ) for flexible roughness elements (i.e., plants) increased as the wind speed increased; for rigid roughness elements, τR/τ decreased with increasing wind speed. This confirms that the shelter capability of flexible roughness elements against soil wind erosion increases with the increase of wind speed, while that of rigid roughness elements is opposite.

Habitat selection and food choice of White-naped Cranes (Grus vipio) at stopover sites based on satellite tracking and stable isotope analysis

By combining satellite tracking, land-cover extracted from Landsite 8 images, and the traditional stable isotope analysis, we studied the habitat selection and food preference of a vulnerable migratory waterbird, the White-naped Crane (Grus vipio), in one of its key stagging sites, the Shandian River Basin in the semi-arid northern China, to provide knowledge that is critical for its conservation in the Anthropocene. Our results showed that the White-naped Cranes used both uplands and natural wetlands in the stopover site. While the cranes used farmland and natural land cover equally as night-time roosting grounds, they spent most daytime foraging at farmlands. Despite the extensive usage of croplands as their foraging ground, the Bayesian mixing models based on stable isotopic analysis revealed that crop residues after harvesting, such as Maize (Zea mays) and Naked Oat (Avena chinensis), were only a small fraction of the White-naped Cranes’ diet (∼19%), and their diet composited mainly natural plants, such as Allium ledebourianum, Potentilla anserina, and P. tanacetifoli. Moreover, more than 20% of the total wetlands in the region were modelled as home range of the cranes. On contrast, less than 10% of croplands and about 1% of the unused uplands were identified as home range. In addition, the entire core habitats were located in natural wetlands. Our findings demonstrated the importance of natural wetlands for the survival of the threatened crane. However, the satellite-derived land cover data showed that croplands increased rapidly in the last decade in this area, at the expense of natural wetlands. With the sharp decrease of White-naped Crane population in China, the conservation of stopover sites becomes imperative. Based on our analysis, we recommend the following management actions: conserving adequate natural wetland area, regulating anthropogenic pressures such as the use of herbicides, expanding the duration and extent of current conservation regulations, establishing a comprehensive monitoring program, and initiating basin-scale ecological restoration, for effective conservation of this threatened species. These integrated conservation strategies for migratory waterbirds are necessary, considering the rapid land-cover changes and agricultural expansion that have been occurring in East Asian-Australasian Flyway, especially in the semi-arid temperate zone.

The Elements and Color of Lacustrine Record Revealed Lake Environmental Changes in Semiarid Northern China: A Case Study From Chagan Nur Lake of Southern Mongolian Plateau

The lakes at the southern Mongolian Plateau are sensitive to the variation of EASM changes during the Holocene, and hence the lacustrine records of these lakes provide prospects for understanding how lake environmental changes respond to EASM variation at different timescales. The interpretation of proxy indexes of lacustrine sediments is complex due to the various climatic or/and environmental factors influencing the processes during deposition and after deposition. In this study, the elements and colors of an 11.7-m lacustrine record from Chagan Nur at the southern Mongolian Plateau of northern China were analyzed to constrain the lake environmental changes during the Holocene. The results show that stable elements of Al, Si, K, Ti, Fe, and Rb are more related to surface erosion or/and vegetation coverage, and the variation of Sr is related to the lake-level change in Chagan Nur. The element and color of the lacustrine records show that a small and shallow lake environment occurred at Chagan Nur during 11.7–10.5 ka, the lake level had increased during 10.5–7 ka, and then the lake shrank with strength surface erosion during 7–6.2 ka and from 2 ka to the present. The lake level and surface erosion changes of Chagan Nur have shown a direct response to EASM precipitation changes as reconstructed from the pollen record from the Chagan Nur Lake and other records from northern China.

Inverse Estimation of Spatiotemporal Flux Boundary Conditions in Unsaturated Water Flow Modeling

AbstractAccurate estimation of boundary conditions is essential to the success of numerical schemes for modeling soil water flow processes. The flux‐type boundary condition, which is integration of many boundary fluxes and subsurface conditions, is traditionally estimated from hydrologic and meteorological information. However, such information could be rapidly variable in space and time and difficult to measure and quantify accurately. This study inversely estimates upper flux boundary conditions by calibrating interior observations of soil moisture. Unknown flux boundaries with both spatiotemporal and temporal only variabilities are studied. In the case of spatiotemporal variability, we formulate the inverse model by extending a Tikhonov regularization framework proposed in our previous study, which calibrated a macroscopic root water uptake model in one‐dimensional soil water flow. The developed method is efficient because it takes advantage of the strategy in the previous study that the inversion is transformed to a variational problem, and also because it incorporates locally one‐dimensional operator splitting technique to overcome the computation complexity when dealing with multidimensional flow processes. When flux boundaries vary with time only (special case), we have found that the inversion degenerates into a well‐posed problem and formulated the inversion explicitly without use of regularization. Numerical tests are conducted through both one‐dimensional (well‐posed) and two‐dimensional (ill‐posed) heterogeneous soil profiles to illustrate the effectiveness of the proposed inverse formulation. Particularly, the applicability of the inverse formulation in the special case is further validated in field experiments using in situ measurements in arid and semiarid northern China.

Nitrogen addition elevated autumn phosphorus retranslocation of living needles but not resorption in a nutrient-poor Pinus sylvestris var. Mongolica plantation

Continuously increasing atmospheric nitrogen (N) deposition has aggravated phosphorus (P) limitation in many forests worldwide, but how trees change nutrient use efficiency to adapt to the aggravated P limitation induced by N deposition, and what is the role of understory vegetation in mediating the responses are largely unknown. Here, the primary nutrient use strategies, i.e., nutrient resorption during leaf senescence and seasonal nutrient retranslocation in living tree leaves, were investigated in response to 5 years of N addition (none vs. 10 g N m−2 year−1) and understory removal (understory vegetation was intact vs. removed) in a nutrient-poor Mongolian pine (Pinus sylvestris var. mongolica) plantation in semiarid northern China. Soil available P concentration was significantly decreased due to N addition, but such decrease was counteracted by the understory removal. Neither P resorption efficiency nor P proficiency was significantly affected by these treatments, with an average P and N resorption efficiency of 60% and 57%, respectively. N addition decreased P concentration by 17% but did not affect total P content in a needle of all aged classes, regardless of the understory vegetation manipulation. Furthermore, N addition significantly elevated the autumn P retranslocation of all aged living needles only when understory vegetation was intact, with an average of 55% of autumn P retranslocation, compared to an average of 15% in the other treatments. The autumn P retranslocation of Mongolian pine was positively correlated with soil available P concentration. Understory removal alone marginally decreased leaf P concentration only in the current-year needles, and significantly attenuated the spring and autumn P retranslocation. Our results highlight the importance of seasonal P retranslocation in elevating tree P use efficiency under the condition of enhanced P deficiency, and role of understory vegetation in mediating seasonal P dynamics and their responses to increasing N enrichment.

A fully integrated NPS PI model in R: An option for coupling nonpoint source models and GIS

The coupling of a geographic information system (GIS) and nonpoint source (NPS) models has significantly promoted NPS modeling and output visualization. This study reported an approach using R to fully integrate the GIS and a NPS Phosphorus indicator model, as well as to build an interactive web interface for the model". A case study in a semiarid northern China subwatershed showed that the proposed method was feasible, flexible and effective. Our experiences demonstrated that developing a fully-coupled GIS-NPS model system in R could simplify NPS modeling across computation platforms, promote modeling efficiency, implement dynamic simulations, enhance model inputs/outputs display and provide readily interactivity. We envision that the experiences could provide a promising option for NPS modeling.

Changes of Sabina vulgaris growth and of soil moisture in natural stands and plantations in semi-arid northern China

To better understand the adaptability of Sabina vulgaris in the southeastern margin of China’s Mu Us Sandy Land to climate change, we observed plant growth and soil moisture to a depth of 100 cm in natural stands and plantations for 2 years. The soil water content gradually decreased from northeast to southwest in the study area. Topography obviously affected soil moisture, with the soil water content on windward slopes and on leeward slopes significantly higher (P < 0.05) than that at dune tops. The density of Sabina vulgaris had no significant effect on soil water content in young plantations. Cumulative plant growth was negatively correlated with soil water storage to a depth of 100 cm. With increasing shrub growth, soil water content decreased. Our results confirmed that the growth of S. vulgaris depended on the soil water content and differed between young (artificial) and old (natural) stands, with the older plants being more vulnerable to low soil water content. In addition, S. vulgaris plantations appear to be well adapted to local climate conditions, so cultivating plantations of this species may improve the vegetation cover in the Mu Us Sandy Land, thereby functioning as a windbreak and promoting sand fixation. If the government intends to continue its large-scale revegetation projects, more attention should be paid to S. vulgaris.

Impact of anthropogenic activities on the enrichment of fluoride and salinity in groundwater in the Yuncheng Basin constrained by Cl/Br ratio, δ18O, δ2H, δ13C and δ7Li isotopes

Substantial concerns have been raised about the degradation of groundwater quality and their harmful effects due to salinization processes and endemic fluorosis issues in China. Yuncheng Basin is an economically important area situated in semi-arid northern China, which has suffered from severe groundwater contamination and intensive anthropogenic activities (agricultural, industrial and domestic). Twenty water quality parameters were surveyed at 85 groundwater and salt lake water sites during August 2013–September 2015. The combining using of Cl/Br mass ratio, δ2H, δ18O, δ13C, and δ7Li isotopes was employed to investigate the potential impact of anthropogenic activities which are supposed to contribute to groundwater salinization and fluoride enrichment in the Yuncheng Basin to a large extent. Results show that shallow groundwater are more vulnerable to contamination from anthropogenic processes of irrigation leaching, wastewater drainage, fertilizer utilization and Salt Lake water intrusion. Vertical irrigation recharge has been an important source of salt and fluoride for the shallow groundwater in the runoff area between Kaolao lowland and the central Basin, revealed by the high salinity (>4000 mg/L), high Cl/Br ratios (>2000), moderate high δ7Li (19.7–26.1‰), moderate δ13C (−8.1~−10.3‰) and constant δ18O and δ2H values. The contribution of over-utilization of fertilizers on high fluoride shallow saline groundwater is found in the crop land in the west part of the Salt Lake with a moderate high nitrate concentration (>10 mg/L), moderate Cl/Br ratios (332~1000) and low δ7Li value (13.8‰). Shallow groundwater in the northern part of the Salt lake polluted by wastewater, evidenced by boron concentration (>2000 μg/L), Na/K (molar ratio > 0.6), δ13C (−11~−11.1‰), and δ7Li (20.4~22.5‰), show highest fluoride concentrations. Due to over-pumping of groundwater, Salt Lake water intrusion mainly occurs in the northern shore of the lake and results in the enrichment of fluoride and salinity in some shallow groundwater with low Cl/Br ratios (around 300), high δ7Li value (c.a. 27‰), and moderate high δ18O (−8.2~−8.7‰) and δ2H (−62.2~−66.1‰) values. The influence of anthropogenic activities on deep groundwater is reflected in two possible scenarios. Increased extraction of groundwater leads to downward leakage of shallow high fluoride saline groundwater, increasing the pollution risk of deep groundwater within the depression cone in the west of Yuncheng City. The rapidly-developed drilling activities of geothermal water (δ7Li, 11~14.3‰; δ13C, −7.9~−8.4‰; [B], 709~3523 μg/L; [F], 2.1~8.2 mg/L) bring about the contamination of fresh groundwater by the underground leaking or discharge of geothermal water. Finally, a conceptual model has been developed to demonstrate the influences of anthropogenic activities on groundwater contamination along a northwest to southeast cross-section.

Forage nitrogen yield and soil nitrogen in artificial grasslands with varied Medicago seedling proportion

ABSTRACTThis study aimed to understand the effects of Medicago spp proportion on symbiotic and non-symbiotic nitrogen (N) utilization of plants, and subsequent forage production and soil N status in an artificial Leymus chinensis grassland. By a two-year field experiment conducted in semi-arid northern China, it was found that the corresponding biomass proportions of legume in swards were 0, 39, 63, 83 and 98% when legume seedling proportions at sowing were 0, 25, 50, 75 and 100%, respectively. Increased Medicago seedling proportion (from 25 to 100%) decreased legume N2 fixation capacity from 53 to 21%, as a consequence, this reduced total symbiotic N2 fixation and its contribution to forage production with Medicago seedling proportion increasing from 50 to 75% or more. However, as increased Medicago seedling proportion enhanced legume biomass and sward uptake to soil mineral N, higher legume stands still led to the greater biomass and N yield. The cultures with 50% seedling of legume had 4–13% greater soil N concentration than the cultures with 0, 25, 75 and 100% seedling of legume. We concluded that pure Medicago stands led to the greatest forage yield, while medium Medicago stands could lead to the greater symbiotic N fixation and soil N concentration.

Recent greening (1981–2013) in the Mu Us dune field, north‐central China, and its potential causes

AbstractDesertification of arid and semiarid land is a severe environmental problem. Better understanding of the desertification process, in the context of changing climate and growing human activities, is essential for policy‐making on desertification control strategies and land‐use management in these environmentally sensitive areas. In this study, we analyze vegetation change and dune activity over the years 1981–2013 in Mu Us, a semiarid dune field in north‐central China, using remote sensing techniques and geomorphic analysis. During this period, we have observed an increasing trend in vegetation greenness, indicated by the Normalized Difference Vegetation Index, which is consistent with the decreasing trend of the dune mobility index. Changes in dune morphologies in the study area indicate that dunes are being stabilized under vegetation growth, and this morphological change could potentially be used to detect the long‐term trend in desertification. Based on a detailed analysis of both climatic and socioeconomic data, we found that the decadal changes in wind strength have significant impact on the long‐term vegetation rehabilitation in the study area and that the short‐term variations in vegetation growth are associated with interannual fluctuations in precipitation. Policy‐driven, large ecological restoration projects implemented in recent decades can exert a positive impact on vegetation restoration, especially under favourable climatic conditions. This study implies that such climatic conditions may offer a window of opportunity for land regeneration in semiarid northern China.

Soil bacterial community responses to revegetation of moving sand dune in semi-arid grassland.

Grasslands in semi-arid Northern China are widely desertified, thus inducing the formation of a large area of moving sand lands. Revegetation of the sandy land is commonly adopted to restore degraded grasslands. The structure of the soil microbial community might dramatically change during degradation and recovery because microorganisms are one of the major drivers of ecological process through their interactions with plants and soil. Assuming that soil properties are the key determinants of the structure of soil bacterial community within the same soil type, whether the vegetation type causes the significant difference in the structure of soil bacterial community during revegetation and restoration of the degraded grasslands remains poorly understood. Our study aimed to (1) investigate the response of soil bacterial communities to the changes during vegetation degradation and recovery and (2) evaluate whether the soil bacterial communities under plantations return to their native state. We detected the shifts in diversities and compositions of the soil bacterial communities and the relative abundance of dominant bacterial taxa by using the high-throughput Illumina MiSeq sequencing technique in an area covered by 32-year-old Caragana microphylla, Artemisia halodendron, Hedysarum fruticosum, Pinus sylvestris var. mongolica, Populus simonii, and Salix gordejevii sand-fixing plantations and in the native community (NC) dominated by elm, and moving sandy dune (MS). We found that the obtained operational taxonomic units by 16S rRNA gene sequencing and diversity index in MS were all significantly lower than those in NC, and the number and composition of dominant genera were significantly different between NC and MS. Interestingly, the compositions of bacterial communities and the dominant genera in different sand-fixation plantations (C. microphylla, A. halodendron, H. fruticosum, P. sylvestris var. mongolica, P. simonii, and S. gordejevii) were all similar to those of the native soil of NC, suggesting that the plantation type and soil properties exhibit a minimal effect on the compositions of soil microbial communities within a continuous landscape. These results revealed that the structure of the soil bacterial community of degraded sandy grassland (even degenerated into a mobile sand dunes) in semi-arid region can be reversibly restored by planting indigenous shrub or semi-shrub plantation on human time scales.

Eolian dust forcing of river chemistry on the northeastern Tibetan Plateau since 8 Ma

Eolian dust is one of the most important factors controlling fluvial hydrological evolution in modern arid and semi-arid central Asia. Here, we present the bulk carbonate Ca–Mg–Sr concentrations and Sr isotopic compositions recorded in water soluble salts, carbonate and silicate fractions, as well as the Nd isotopic compositions in the silicate fraction of a Late Miocene (12.2–5.1 Ma) fluvial sequence exhibiting paleosol development, in the Linxia Basin on the northeastern Tibetan Plateau (TP). Bulk carbonate Mg–Sr systematics show a distinct pattern in log–log plots of Mg/Ca versus log Sr/Ca ratios, and clearly higher 87Sr/86Sr ratios since ∼8 Ma. These findings cannot be adequately explained by the mechanism of prior calcite precipitation (PCP) – this latter process results in a positive correlation between the Mg/Ca and Sr/Ca ratios in authigenic calcite, evident in the fixed gradient in their log–log plots and has been isolated as being the major factor controlling carbonate Mg–Sr systematics before 8 Ma. Nor can these findings be explained by other mechanisms related to the catchment's provenance/sedimentation. The dramatic changes in carbonate Sr contents, Sr isotopes, and Sr/Mg ratios since ∼8 Ma may therefore be inferred to have been triggered by significant inputs of eolian dust via the dissolution of dust carbonates and evaporites in the paleowaters where fluvial and paleosol carbonates precipitated. This process of eolian dust input can be reliably illustrated using a binary mixing model corresponding to a series of varying PCP fluxes (identical to processes affecting the area before 8 Ma) combined with a constant eolian influx calculated from the co-variations between Mg/Ca, Sr/Ca and Sr isotopic ratios. Eolian dust also leaves a fingerprint in the carbonate and silicate minerals of bulk sediments, as revealed respectively by their Sr and Nd isotopic compositions. Eolian dust compositions for the ∼8–5 Ma on the northeastern TP can thus be taken to be characterized by a εNd value ≤−10.5 for the silicate fraction, and by a 87Sr/86Sr value ≥0.7115 for the labile faction (carbonates and evaporites). Eolian dust is now recognized to have made a significant impact on the hydrological evolution of arid and semi-arid northern China as far back as ∼8 Ma, suggesting an enhanced regime of dust activity and regional aridification in central Asia at that time.