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Litter decomposition plays an important role in the nutrient cycle of terrestrial ecosystems. The alpine wetland has a high litter accumulation rate and a slow degradation rate, which is extremely sensitive to changeable freeze–thaw patterns against the background of global climate change. Freeze-thaw process is a common natural phenomenon in middle-high latitudes and high altitudes. Hydrothermal changes caused by freezing and thawing process affect the survival and physiological characteristics of microorganisms, and then affect the decomposition process of litters. The alpine wetland could be an ideal example for wetland in high altitude to study the mechanism of microbial community structure and function during litter composition during freeze–thaw process. Moreover, seldom has research in the whole process in the fields. The bacterial and fungal communities were analyzed in three different Carex litter decomposition stages—fresh, four-, and six-month—during the freeze–thaw period using 16/18S rDNA pyrosequencing. Phyllosphere microflora had identified rapid changes in the structure and function. Bacteria were mainly influenced by environmental factors (air temperature, soil moisture), and were significantly related to the degradation of litter lignin and the change of C/N ratio. Fungi were affected by both litter quality and environmental factors, and were significantly related to the degradation of cellulose and the change of C/N ratio. Moreover, phyllosphere organisms, including bacteria (Cryobacterium and Paracoccus members) and fungi (Mrakia, Mrakiella, and Naganishia), were replaced by communities with specific metabolic capabilities to adapt to each particular decomposition stage. Ilumatobacter and the fungal genera, Mycosphaerella and Athelia were characteristic of four-month-old litter samples, whereas Brevundimonas, Paracoccus and Nigrospora were characteristic of six-month-old litter samples. Our results suggest that the bacterial community structures of Carex and forest litter during initial decomposition stages may be similar but that their fungal community structures may differ substantially.

Riparian plant diversity is sensitive to changes in groundwater in arid regions. However, little is known about how plant diversity responds to changes in environment along riverside-desert gradients in riparian ecosystem. Our objectives were to (i) identify riparian plant diversity along riverside-desert gradients in Tarim desert riparian forests, (ii) analyze the impact of environment variables on plant diversity, (iii) determine the optimum groundwater depth for different plant life-forms. Six transects 90 quadrats (with each size 100 m × 100 m) distributed vertically to river bed along riverside-desert gradients ~30 km in length were surveyed. At each quadrat, the morphological features of riparian plant communities were measured, and the groundwater depth, soil water, soil salinity, soil nutrient were also monitored at same sites. Three distinct vegetation communities were identified based on cover and richness in the tree, shrub and herb layers: the riparian zone, the transitional zone and the desert margin zone. Twelve species were indicators of the three vegetation communities. Riparian plant diversity was influenced by groundwater depth, distance from river, soil moisture content, soil salinity and soil nutrient by redundancy analysis. In response to groundwater depth, the optimal groundwater depths for species diversity, evenness and shrub cover were 2.8, 2.7 and 3.7 m, respectively. Therefore, maintaining high plant diversity requires managers to ensure stable groundwater depth for different plant life-forms rather than for some of them.

Inland river terminal lakes help maintain the regional ecological environment. However, terminal lakes in arid regions have shrunk dramatically over the past 50 years. We extracted the water areas of Taitema Lake in the arid region of Central Asia and calculated the fractional vegetation coverage (FVC) of the surrounding areas based on Landsat images from 1989 to 2019. We analysed the evolution and driving factors of the FVC and lake area before and after implementing an ecological water conveyance (EWC) project in 2000. The contributions of the driving factors were derived using regression analysis, and the effect coefficients were calculated with a path analysis. Results showed that before EWC (1989–1999), Taitema Lake was dry, the proportions of medium and high FVC areas were small, and FVC varied moderately. The contributions of land use and inflow water from Qarqan River to the regional FVC were 57.4% and 45.3%, respectively, and the effect coefficients of temperature and precipitation on FVC were 0.09 and 0.02, respectively. Human-dominated land-use change was the main factor affecting FVC change. After EWC (2000–2019), the water in Taitema Lake was restored, and the average annual proportions of medium and high FVC were 9.5% and 2.7% higher than the pre-EWC period, respectively. The contributions of land use and river inflow to the water area were 39.1% and 60.9%, and those to FVC were 40.7% and 59.3%, respectively. Precipitation had a slightly stronger effect on the water area and FVC than the temperature in the study area, whereas the influence of river inflow on the water area and FVC was less evident than that of the EWC. EWC can replenish lake water quickly, which is essential for improving the ecological environment of terminal lake areas.

In the Gurbantunggut Desert of Northwestern China, we surveyed the population structure and regeneration characteristics of six Haloxylon ammodendron plantations based on chronosequencing. The results showed that the age class of an individual H. ammodendron plant from youngest to oldest showed growth potential under the present environment. The optimal termination stage of H. ammodendron occurred at 20 years of age, which was accompanied by a regeneration phase at 17 years. The H. ammodendron plantations started to regenerate after 7 years. The maximal height and basal stem diameter of a grade III seedling reached 1.2 m and 1.9 cm in a 17-year-old plantation. In a 33-year-old plantation, the transformation rate of the seedling from grade I to grade III increased up to 92.3%. Although the seedling bank was plentiful in the plantations, the replenishment of the canopy was not continuous and timely. Therefore, measures to further the stability and sustainability of H. ammodendron plantation were necessary.

Ecological restoration of degraded riparian Tugai forests is a key driver to combat desertification in arid regions. Previous studies have focused mainly on changes in groundwater as the underlying mechanisms of Tugai forest’s decline. We evaluated species composition and diversity of Tugai forest and their relationship to groundwater, soil salinity, and soil nutrient. Using 73 quadrats (100 m × 100 m) from 13 transects located perpendicularly to river in the upper reaches of the Tarim River. Eighteen plant species belonging to sixteen genera and eight families were recorded, and the dominant species included Populus euphratica, Phragmites communis, and Tamarix ramosissima. Three P. euphratica stand ages were detected: young stand, mature stand, and old stand. There were significant differences in species diversity, groundwater depth, groundwater salinity, distance from the quadrat to the river channel, soil moisture content, pH, electrical conductivity, total salt, Cl−, SO42−, Ca2−, Mg2+, Na+, K+, soil organic carbon, and soil organic matter across the stand ages. Seven species were identified as indicators of the three stand ages. Redundancy analysis indicated that the Tugai forest diversity indices were negatively correlated with groundwater depth, groundwater salinity, and distance from the river, and positively associated with electrical conductivity, total salt, pH, Cl−, SO42−, CO32−, soil organic matter, soil organic carbon, and soil moisture content. Plant diversity was the highest at 3–6 m groundwater depth, followed by 0–3 m and then 6–9 m, with the lowest recorded at > 9 m. The appropriate groundwater depth for herbs was about 1–4 m, whereas the depth for trees and shrubs was about 3–6 m. The groundwater depth < 6 m was deemed suitable for the growth of desert riparian forests. This results provide a scientific reference for the ecological restoration and protection for Tugai forests in arid areas.

Although split fertilizer applications have been suggested as a logical strategy to reduce nitrous oxide (N2O) emissions by decreasing soil nitrate concentrations, their efficacy remains unclear. A laboratory incubation experiment was performed to determine the effect of split fertilization vs. single fertilization on N2O emissions. The split urea application reduced the peak N2O emissions during the incubation period, and the cumulative N2O emissions were significantly reduced by 28% compared with the single fertilization, although these emissions were influenced by the N fertilizer rate and soil moisture. A higher percentage reduction in cumulative N2O emissions under the split fertilization occurred at the low (45%) compared with the high fertilizer rate (15%). At the low fertilizer rate (200 N), the split fertilization resulted in a significantly greater reduction in N2O emissions under the dry soil moisture regime (53%) compared with the wet soil moisture condition (37%). In addition, the split fertilization reduced the cumulative CO2 emissions by 9% compared with the single fertilization. Therefore, our laboratory results suggest that the split fertilization strategy appears to be a useful method to reduce greenhouse gas emissions in an irrigated agriculture ecosystem.

Desert riparian plants experience high variability in water availability due to hydrological fluctuations. How riparian plants can survive with low water availability has been well studied, however, little is known about the effects of high water availability on plant community structuring. We conducted a mesocosm experiment to test whether seedling competition under simulated high groundwater availability can explain the shift of co-dominance of Populus euphratica and Tamarix ramosissima in early communities to P. euphratica dominance in mature ones along the Tarim River in northwestern China. Seedlings of these two plant species were grown in monoculture and mixture pools with high groundwater availability. Results indicated that the above-ground biomass and relative yield of T. ramosissima were higher than those of P. euphratica. The competitive advantages of T. ramosissima included its rapid response in growth to groundwater enrichment and its water spender strategy, as evidenced by the increased leaf biomass proportion and the inert stomatal response to leaf-to-air vapor pressure deficit (VPD). In comparison, P. euphratica showed a conservative strategy in water use, with a sensitive response to leaf-to-air VPD. Result of the short-term competition was inconsistent with the long-term competition in fields, suggesting that competition exclusion is not the mechanism structuring the desert riparian plant communities. Thus, our research highlights the importance of mediation by environmental fluctuations (such as lessening competition induced by disturbance) in structuring plant communities along the Tarim riparian zones.

The ecosystem along the lower reach of Tarim River has been undergoing remarkable changes due to the ecological water conveyance activity since 2000. To improve the scientific basis of riparian restoration, the clearing understanding of the dynamic of eco-hydrological system and the feedback between hydrological and ecological components are needed. A fully distributed eco-hydrological model combines a hydrology component based on the modified MODFLOW, with a vegetation dynamic component described by Verhulst logistic equation. The validation of the eco-hydrological model using groundwater well observation, river stage observation and remote sensing data demonstrated that the good capability of the model in simulating the dynamic of the eco-hydrological system in lower reach of Tarim River. The interaction between the hydrological component and ecological component has been revealed through water balance analysis and scenario studies. Vegetation has a significant impact on groundwater evapotranspiration, groundwater storage change and groundwater depth, while groundwater depth is a critical feedback variable that determines the vegetation dynamics, which is shown by observation and simulation result. The eco-hydrological model represented in this study could help to groundwater dependent ecosystem restoration and protection, particularly for riparian ecosystem.

Questions: What are the effects of a shrub (Haloxylon ammodendron) on spatial patterns of soil moisture in different seasons? How does productivity of understorey annuals respond to these effects? Are such effects always positive for annuals under shrubs? Location: South Gurbantunggut Desert, northwest China. Methods: Using geostatistics, we explored seasonal patterns of topsoil moisture in a 12 × 9-m plot over the growing season. To determine spatial patterns of understorey annuals in response to H. ammodendron presence, biomass of annuals was recorded in four 0.2 × 5.0-m transects from the centre of a shrub to the space between shrubs (interspace). We also investigated vertical distribution of root biomass for annuals and soil moisture dynamics across soil profiles in shrub-canopied areas and interspaces. Results: Topsoil moisture changed from autocorrelation in the wet spring to random structure in the dry season, while soil moisture below 20 cm was higher in shrub-canopied areas. Across all microhabitats, soil moisture in upper soil layers was higher than in deeper soil layers during the spring wet season, but lower during summer drought. Topsoil was close to air-dry during the dry season and developed a ‘dry sand layer’ that reduced evaporative loss of soil water from deeper layers recharged by snowmelt in spring. Aboveground biomass of understorey annuals was lowest adjacent to shrub stems and peaked at the shrub margin, forming a ‘ring’ of high herbaceous productivity surrounding individual shrubs. To acclimate to drier conditions, annuals in interspaces invested more root biomass in deeper soil with a root/shoot ratio (R/S) twice that in canopied areas. Conclusions: Positive and negative effects of shrubs on understorey plants in arid ecosystems are commonly related to nature of the environmental stress and tested species. Our results suggest there is also microhabitat-dependence in the Gurbantunggut Desert. Soil water under H. ammodendron is seasonally enriched in topsoil and deeper layers. Understorey annuals respond to the effect of shrubs on soil water availability with lower R/S and less root biomass in deeper soil layers and develop a ‘ring’ of high productivity at the shrub patch margin where positive and negative effects of shrubs are balanced.

Water and energy balance models were developed at the local and basin scale by aggregating a HYDRUS Model across scales in a topographic framework. A spatially distributed approach was followed to aggregate the HYDRUS Model to basin scale. The effects of spatial variability on the estimation of evapotranspiration at basin scale are emphatically discussed. In the present work the main emphasis was focused on the estimation of the spatial variability of both evapotranspiration and the calculation of statistical parameters of soil water storage from local scale to basin scale. Our results illustrate that the changes of the local and area-averaged potential evapotranspiration (ETp), actual evapotranspiration (ET) show a strong seasonal trend where the mean and standard deviation (STD) approximately fit the two-harmonics of Fourier series. Moreover, ETp and ET are random variables which follow the normal distribution with mean and deviation changing with season. A sensitivity analysis of the results shows that to a reasonable degree, the incorporated model can be used to determine the relationship between the actual area-averaged evapotranspiration and the main controlling factors for both bare soil and vegetated surfaces.

Years of intensive irrigated farming in the Oasis area of Xinjiang in north-western China has caused secondary salinization of vast areas of land since the mid 1980s. Soil salinity/sodicity surveys in the area, conducted after the problem surfaced, showed relatively high levels of the sodium adsorption ratio (SAR) in the upper soil layers. In one study, 92% of cored soil samples of the affected fields showed SAR values above 5, while 42% had values above 15. One ameliorative approach, bio-drainage using different desert vegetations, was used to control the high water-tables and reverse the observed salinity/sodicity trends in the border area of the Manas Alluvial Fan. The ground-water table dropped to below 2.28 m in the summer of the fifth year, which was sufficient to prevent secondary salinization. A related plot study showed that Lycium barbarum and Puccinellia Chinamponsis consumed large amounts of soil water while leaving most of the salts behind. The salt contents of leaves and young branches were different for different vegetation. Soil profile salt distributions for the deep-rooted and shallow-rooted systems were different in two of the three years after planting. Commercial forestry of L. barbarum and other timber products under current conditions, and without irrigation, appears a viable agricultural alternative that could bring higher returns than for wheat or cotton. Results of this study indicate that both conventional and biological drainage are attractive options for controlling the local hydrology and limiting or diminishing salinity–sodicity trends.