Near-surface wind field characteristics of the desert-oasis transition zone in Dunhuang,China

The desert-oasis transition zone(DOTZ)serves as a buffer area between the desert and oasis.Understanding its wind field characteristics is of great significance for the prevention and control of aeolian disasters in the oasis.In this study,we used meteorological data during 2013–2019 from the portable meteorological stations at five sites(site A on the edge of the oasis,sites B,C,and D in the DOTZ,and site O in the desert)in Dunhuang,China to analyze the near-surface wind field characteristics and their causes,as well as to reveal the key role of the DOTZ in oasis protection.The results showed that the mean wind speed,frequency of sand-driving wind,and directional variability of wind decreased from west to east within the DOTZ,and wind speed was significantly affected by air temperature.The terrain influenced the prevailing winds in the region,mainly from northeast and southwest.Only some areas adjacent to the oasis were controlled by southeasterly wind.This indicated that the near-surface wind field characteristics of the DOTZ were caused by the combined effects of local terrain and surface hydrothermal difference.At site D,the annual drift potential(DP)was 24.95 vector units(VU),indicating a low wind energy environment,and the resultant drift direction(RDD)showed obvious seasonal differences.Additionally,the DOTZ played an important buffering role between the desert and oasis.Compared with the desert,the mean wind speed in the oasis decreased by 64.98%,and the prevailing wind direction was more concentrated.The results of this study will be useful in interpreting the aeolian activity of the DOTZ in Dunhuang.

Species diversity and its relation with soil factors under different site conditions in a desert-oasis ecotone

Vegetation and soil surveys were conducted under different site conditions in 2007-2011 to study species diversity using richness, evenness and diversity indices, in the middle portion of the Heihe River Basin. The relationship between species distribution and soil environmental factors was also studied by Canonical Correspondence Analysis （CCA）. Results show that vegetation coverage and species diversity were the highest in the interdune lowland, and the lowest in the mobile dime. Results of the Hill＇s index （di- versity ordering） shows that species diversity is reduced along decreasing soil water content, and the order of species diversity was interdune lowland, flat slope, fixed dune, semifixed dune and mobile dune. The influence degree of soil factors on vegetation dis- tribution was soil water content 〉 pH 〉 total K 〉 organic matter 〉 available N 〉 total N 〉 available K 〉 total P 〉 saline content 〉 available P. Soil water content and pH were important factors significantly affecting spatial distribution difference of vegetation, the environmental explanation was 98%.

Study on Ecological Landscape Pattern Change of Desert Sewage Discharge Area in Desert-Oasis Ecotone on the Southern Edge of Junggar Basin,Xinjiang

In order to understand the change characteristics of ecological landscape pattern of desert sewage discharge area in the desert-oasis ecotone on the southern edge of Junggar Basin,Xinjiang,based on the remote sensing images of 13 scenes GF1 and GF2 from 2013 to 2019,through image preprocessing,classification interpretation,GIS spatial analysis and landscape pattern index,the conclusions were as follows.From 2013 to 2019,the landscape area of sparse shrub forest land,grassland and saline alkali land increased,while the area of sandy bare land and pond surface increased.The mutual transformation among various types of landscapes was more active,among which the mutual transfer between sandy bare land and sparse shrub forest land was the most frequent.Various landscape dynamics changed significantly with time.All kinds of landscapes and the overall landscape showed the trend of fragmentation,discretization and complexity,and the overall landscape showed the trend of balanced distribution.The above results hoped to provide basic data and technical basis for eco-environmental protection,restoration and treatmen on the southern edge of Junggar Basin,and fill the current research gap.

The effect of hydrologic process changes on NDVI in the desert-oasis ecotone of the Hexi Corridor

In the arid inland river basins of northwestern China,human activities control almost all the surface hydrologic processes.The potential effects of these altered hydrologic processes are gradually becoming clear,especially since the 2000 implementation of the integrated water resources management projects in the Shiyang River,the Heihe River,the Tarim River,and the Shuler River.While the appearance of these eco-hydrology changes and consequent environmental effects in the oasis has attracted broad attention,related research is still lacking.Eco-hydrological process changes in the desert-oasis ecotone were investigated in the Pingchuan irrigation district in the middle reaches of the Heihe River.The results showed that the annual average amount of surface water irrigation during the past 20 years has decreased by 1.498×107 m3,while the annual average amount of well irrigation has increased by 1.457×107 m3,since 2000,when the State Council of China approved the water diversion scheme for the Heihe River Basin.The groundwater depth before the water diversion scheme generally varied between2.44–3.19 m(average 2.73±0.24 m),while that after the water diversion scheme has varied between 3.08–4.01 m(average3.79±0.62 m).The distribution area of<3 m groundwater depth decreased from 3612 to 394 hm2;while the distribution area of>3 m groundwater depth increased from 853 to 3843 hm2.However,although the hydrologic processes changed dramatically,no significant effects on vegetation productivity in the desert-oasis ecotone were detected during the study period.

Increase in medium-size rainfall events will enhance the C-sequestration capacity of biological soil crusts

Biological soil crusts(BSCs) play important roles in the carbon(C) balance in arid regions. Net C balance of BSCs is strongly dependent on rainfall and consequent activation of microbes in the BSCs. The compensation-rainfall size for BSCs(the minimum rainfall amount for a positive net C balance) is assumed to be different with BSCs of different developmental stages. A field experiment with simulated rainfall amount(SRA) of 0, 1, 5, 10, 20, and 40 mm was conducted to examine the C fluxes and compensation-rainfall size of BSCs in different parts of fixed dunes in the ecotone between the Badain Jaran Desert and the Minqin Oasis. We found algae-lichen crust on the interdunes and crest, algae crust on the leeward side, and lichen-moss crust on the windward. Even a small rainfall(1 mm) can activate both photosynthesis and respiration of all types of BSCs. The gross ecosystem production, ecosystem respiration, and net ecosystem exchange were significantly affected by SRA, hours after the simulated rainfall, position on a dune, and their interactions. The rapid activation of photosynthesis provides a C source and therefore could be responsible for the increase of C efflux after each rewetting. C-uptake and-emission capacity of all the BSCs positively correlated with rainfall size, with the lowest C fluxes on the leeward side. The compensation rainfall for a net C uptake was 3.80, 15.54, 8.62, and 1.88 mm for BSCs on the interdunes, the leeward side, the crest, and the windward side, respectively. The whole dune started to show a net C uptake with an SRA of 5 mm and maximized with an SRA of about 30 mm. The compensation-rainfall size is negatively correlated with chlorophyll content. Our results suggest that BSCs will be favored in terms of C balance, and sand dune stabilization could be sustained with an increasing frequency of 5-10 mm rainfall events in the desert-oasis transitional zone.-

The establishment and development of Haloxylon ammodendron promotes salt accumulation in surface soil of arid sandy land

Haloxylon ammodendron, a representative C_4 succulent xerophyte and salt-secreting plant, is widely used in vegetation reestablishment programs to stabilize shifting sand, and is one of the dominant shrubs in the shelter belt used to control desertification in the desert-oasis ecotone in northwestern China. In this study, we collected soil samples in an age sequence of 0-, 2-, 5-, 13-, 16-, 31-, and 39-year-old H. ammodendron plantations to assess the effects of the shrub on soil fertility and salinity. Results show that SOC and total N concentrations increased significantly with increasing plantation age and increased 5.95-(in the interspaces) to 9.05-fold(under the canopy) and 6.15-to 8.46-fold at the 0-5 cm depth at the 39-year-old plantation compared with non-vegetated sandy land. Simultaneously, H. ammodendron establishment and development resulted in significant salt accumulation in the surface layer. On average, total soil salt content at the 0-5 cm and 5-20 cm depth increased 16.8-fold and 4.4-fold, respectively, compared with non-vegetated sandy land. The increase of total salt derived mostly from the accumulation of SO_4^(2-), Ca^(2+) and Na^+ with H. ammodendron development. The accumulation in salinity was more significant than the increase in fertility, suggesting that improved soil fertility did not limit the impact of salinization. The adverse effect of salt accumulation may result in H. ammodendron plantation degradation and impact community stability in the long run.