The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods (14C and OSL (optically stimulated luminescence)) and materials (shells, carbonate, pollen concentrates and bulk organic carbon). In this study, we synthesized 102 14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.

As a main component in water balance, evapotranspiration is of great importance for water saving and irrigation-measure making, especially in arid or semiarid regions. Although studies of evapotranspiration have been conducted for a long time, studies concentrated on oasis-desert transition zone are very limited. On the basis of the meteorological data and other parameters (e.g. leaf area index (LAI)) of an oasis-desert transition zone in the middle stream of Heihe River from 2005 to 2011, this paper calculated both reference (ET0) and actual evapotranspiration (ETc) using FAO56 Penman-Monteith and Penman-Monteith models, respectively. In combination with pan evaporation (Ep) measured by E601 pan evaporator, four aspects were analyzed: (1) ET0 was firstly verified by Ep; (2) Characteristics of ET0 and ETc were compared, while the influencing factors were also analyzed; (3) Since meteorological data are often unavailable for estimating ET0 through FAO56 Penman-Monteith model in this region, pan evaporation coefficient (Kp) is very important when using observed Ep to predict ET0. Under this circumstance, an empirical formula of Kp was put forward for this region; (4) Crop coefficient (Kc), an important index to reflect evapotranspiration, was also analyzed. Results show that mean annual values of ET0 and ETc were 840 and 221 mm, respectively. On the daily bases, ET0 and ETc were 2.3 and 0.6 mm/d, respectively. The annual tendency of ET0 and ETc was very similar, but their amplitude was obviously different. The differences among ET0 and ETc were mainly attributed to the different meteorological variables and leaf area index. The calculated Kc was about 0.25 and showed little variation during the growing season, indicating that available water (e.g. precipitation and irrigation) of about 221 mm/a was required to keep the water balance in this region. The results provide an comprehensive analysis of evapotranspiration for an oasis-desert transition zone in the middle stream of Heihe River, which was seldom reported before.

The well-documented decrease in the discharge of sediment into the Yellow River has attracted con-siderable attention in recent years. The present study analyzed the spatial and temporal variation of sediment yield based on data from 46 hydrological stations in the sediment-rich region of the Yellow River from 1955 to 2010. The results showed that since 1970 sediment yield in the region has clearly decreased at different rates in the 45 sub-areas controlled by hydrological stations. The decrease in sediment yield was closely related to the intensity and extent of soil erosion control measures and rainstorms that occurred in different periods and sub-areas. The average sediment delivery modulus (SDM) in the study area decreased from 7,767.4 t/(km²•a) in 1951–1969 to 980.5 t/(km²•a) in 2000–2010. Our study suggested that 65.5% of the study area with the SDM below 1,000 t/(km²•a) is still necessary to control soil deterioration caused by erosion, and soil erosion control measures should be further strengthened in the areas with the SDM above 1,000 t/(km²•a).

Fractal theory is becoming an increasingly useful tool to describe soil structure dynamics for a better understanding of the performance of soil systems. Changes in land use patterns significantly affect soil physical, chemical and biological properties. However, limited information is available on the fractal characteristics of deep soil layers under different land use patterns. In this study, the fractal dimensions of particle size distribution (PSD) and micro-aggregates in the 0–500 cm soil profile and soil anti-erodibility in the 0–10 cm soil profile for 10 typical land use patterns were investigated in the Zhifanggou Watershed on the Loess Plateau, China. The 10 typical land use patterns were: slope cropland, two terraced croplands, check-dam cropland, woodland, two shrublands, orchard, artificial and natural grasslands. The results showed that the fractal dimensions of PSD and micro-aggregates were all significantly influenced by soil depths, land use patterns and their interaction. The planta-tions of shrubland, woodland and natural grassland increased the amount of larger micro-aggregates, and decreased the fractal dimensions of micro-aggregates in the 0–40 cm soil profile. And they also improved the aggregate state and aggregate degree and decreased dispersion rate in the 0–10 cm soil profile. The results indicated that fractal theory can be used to characterize soil structure under different land use patterns and fractal dimensions of micro-aggregates were more effective in this regard. The natural grassland may be the best choice for improving soil structure in the study area.

Rangeland degradation is a serious problem throughout sub-Saharan Africa and its restoration is a challenge for the management of arid and semi-arid areas. In Lake Baringo Basin of Kenya, communities and individual farmers are restoring indigenous vegetation inside enclosures in an effort to combat severe land degradation and address their livelihood problems. This study evaluated the impact of enclosure management on soil properties and microbial biomass, being key indicators of soil ecosystem health. Six reseeded communal enclosures using soil embankments as water-harvesting structures and strictly regulated access were selected, varying in age from 13 to 23 years. In six private enclosures, ranging from 3 to 17 years in age, individual farmers emulated the communal enclosure strategy and restored areas for their exclusive use. Significant decreases in bulk density, and increases in the soil organic carbon, total nitrogen and microbial biomass contents and stocks were found in the enclosures as compared with the degraded open rangeland. In the private enclosures, the impact of rehabilitation on the soil quality was variable, and soil quality was in general lower than that obtained under communal management. The significant increase of absolute stocks of carbon, nitrogen and microbial biomass compared to the degraded open rangeland indicates the potential for the restoration of soil quality through range rehabilitation. Over-sowing with indigenous legume fodder species could improve total nitrogen content in the soil and nutritional value of the pastures as well.

Determining soil N mineralization response to soil temperature and moisture changes is challenging in the field due to complicated effects from other factors. In the laboratory, N mineralization is highly dependent on temperature, moisture and sample size. In this study, a laboratory incubation experiment was carefully designed and conducted under controlled conditions to examine the effects of soil temperature and moisture on soil N mineralization using soil samples obtained from the Stipa krylovii grassland in Inner Mongolia, China. Five temperature (i.e. 9°C, 14°C, 22°C, 30°C and 40°C) and five moisture levels (i.e. 20%, 40%, 60%, 80% and 100% WHC, where WHC is the soil water holding capacity) were included in a full-factorial design. During the 71-day incubation period, microbial biomass carbon (MBC), ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) were measured approximately every 18 days; soil basal respiration for qCO₂ index was measured once every 2 days (once a week near the end of the incubation period). The results showed that the mineral N production and net N mineralization rates were positively correlated with temperature; the strongest correlation was observed for temperatures between 30°C and 40°C. The relationships between moisture levels and both the mineral N production and net N mineralization rates were quadratic. The interaction between soil temperature and moisture was significant on N mineralization, i.e. increasing temperatures (moisture) enhanced the sensitivity of N mineralization to moisture (temperature). Our results also showed a positive correlation between the net nitrification rate and temperature, while the correlation between the NH₄⁺-N content and temperature was insignificant. The net nitrification rate was negatively correlated with high NH₄⁺-N contents at 80%–100% WHC, suggesting an active denitrification in moist conditions. Moreover, qCO₂ index was positively correlated with temperature, especially at 80% WHC. With a low net nitrification rate and high soil basal respiration rate, it was likely that the denitrification concealed the microbial gross min-eralization activity; therefore, active soil N mineralization occurred in 60%–80% WHC conditions.

Determining the mechanisms underlying the spatial distribution of plant species is one of the central themes in biogeography and ecology. However, we are still far from gaining a full understanding of the autecological processes needed to unravel species distribution patterns. In the current study, by comparing seedling recruitment, seedling morphological performance and biomass allocation of two Haloxylon species, we try to identify the causes of the dune/interdune distribution pattern of these two species. Our results show the soil on the dune had less nutrients but was less saline than that of the interdune; with prolonged summer drought, soil water availability was lower on the dune than on the interdune. Both species had higher densities of seedlings at every stage of recruitment in their native habitat than the adjacent habitat. The contrasting different adaptation to nutrients, salinity and soil water conditions in the seedling recruitment stage strongly determined the distribution patterns of the two species on the dune/interdune. Haloxylon persicum on the dunes had lower total dry biomass, shoot and root dry biomass, but allocated a higher percentage of its biomass to roots and possessed a higher specific root length and specific root area by phenotypic traits specialization than that of Haloxylon ammodendron on the interdune. All of these allowed H. persicum to be more adapted to water stress and nutrient shortage. The differences in morphology and allocation facilitated the ability of these two species to persist in their own environments.

The hydrogen isotopic composition of plant leaf wax (δD_wax) is used as an important tool for paleohydrologic reconstruction. However, the understanding of the relative importance of environmental and biological factors in determining δDwax values still remains incomplete. To identify the effects of soil moisture and plant physiology on δD_wax values in an arid ecosystem, and to explore the implication of these values for paleoclimatic reconstruction, we measured δD values of soil water (δD_water) and δDwax values in surface soils along two distance transects extending from the lakeshore to wetland to dryland around Lake Qinghai and Lake Gahai on the north¬east Qinghai-Tibetan Plateau. The results showed that the δD_water values were negatively correlated with soil water content (SWC) (R²=0.9166), and ranged from –67‰ to –46‰ with changes in SWC from 6.2% to 42.1% in the arid areas of the Gangcha (GCh) and Gahai (GH) transects. This indicated that evaporative D-enrichment in soil water was sensitive to soil moisture in an arid ecosystem. Although the shift from grasses to shrubs with increasing aridity occurred in the arid area of the GH transect, the δD_wax values in surface soils from the arid areas of the two transects still showed a negative correlation with SWC (R²=0.6835), which may be due to the controls of primary evaporative D-enrichment in the soil water and additional transpirational D-enrichment in the leaf water on the δDwax values. Our preliminary research suggested that δDwax values can potentially be applied as a paleo-humidity indicator on the northeast Qinghai-Tibetan Plateau.

Black locust (Robinia pseudoacacia L.) and Chinese pine (Pinus tabulaeformis Carr.) are two woody plants that are widely planted on the Loess Plateau for controlling soil erosion and land desertification. In this study, we conducted an excavation experiment in 2008 to investigate the overall vertical root distribution characteristics of black locust and Chinese pine. We also performed triaxial compression tests to evaluate the root cohesion (additional soil cohesion increased by roots) of black locust. Two types of root distribution, namely, vertical root (VR) and horizontal root (HR), were used as samples and tested under four soil water content (SWC) conditions (12.7%, 15.0%, 18.0% and 20.0%, respectively). Results showed that the root lengths of the two species were mainly concentrated in the root diameter of 5–20 mm. A comparison of root distribution between the two species indicated that the root length of black locust was significantly greater than that of Chinese pine in nearly all root diameters, although the black locust used in the comparison was 10 years younger than the Chinese pine. Root biomass was also significantly greater in black locust than in Chinese pine, particularly in the root diameters of 3–5 and 5–10 mm. These two species were both found to be deep-rooted. The triaxial compression tests showed that root cohesion was greater in the VR samples than in the HR samples. SWC was negatively related to both soil shear strength and root cohesion. These results could provide useful information on the architectural characteristics of woody root system and expand the knowledge on shallow slope stabilization and soil erosion control by plant roots on the Loess Plateau.

 Estimation of the transpiration rate for a tree is generally based on sap flow measurements within the hydro-active stem xylem. In this study, radial variation of sap flow velocity (Js) was investigated at five depths of the xylem (1, 2, 3, 5 and 8 cm under the cambium) in three mature Xinjiang poplar (Populus alba L. var. pyramidalis) trees grown at the Gansu Minqin National Studies Station for Desert Steppe Ecosystem from May to October 2011. Thermal dissipation probes of various lengths manufactured according to the Granier’s design were installed into each tree for simultaneous observation of the radial patterns of J_s through the xylem. The radial patterns were found to fit the four-parameter GaussAmp equation. The peak Js was about 27.02±0.95 kg/(dm²•d) at approximately 3 to 5 cm deep from the cambium of the three trees，and the lowest J_s appeared at 1 cm deep in most of the time. Approximately 50% of the total sap flow in Xinjiang poplar occurred within one-third of the xylem from its outer radius, whereas 90% of the total sap flow occurred within two-fifth of the xylem. In addition, the innermost point of the xylem (at 8-cm depth), which appeared as the penultimate sap flow in most cases during the study period, was hydro-active with J_s,8 of 7.55±3.83 kg/(dm²•d). The radial pattern of Js was found to be steeper in midday than in other time of the day, and steeper diurnal fluctuations were recorded in June, July and August (the mid-growing season). Maximum differences between the lowest Js (J_s,1 or J_s,8 ) and the highest Js (J_s,3 or J_s,5) from May through October were 12.41, 17.35, 16.30, 18.52, 12.60 and 16.04 g/(cm²•h), respectively. The time-dependent changes of Js along the radial profile (except at 1-cm depth) were strongly related to the reference evapotranspiration (ET₀). Due to significant radial variability of J_s, the mean daily sap flow at the whole-tree level could be over-estimated by up to 29.69% when only a single probe at depth of 2 cm was used. However, the accuracy of the estimation of sap flow in Xinjiang poplar could be significantly improved using a correction coefficient of 0.885.

C₃ plant Reaumuria soongorica and C₄ plant Salsola passerina are super xerophytes and coexist in a mixed community in either isolated or associated growth, and interspecific facilitation occurs in associated growth. In the present study, the root traits including root distribution, root length (RL), root surface area (RSA), root weight (RW) and specific root length (SRL) of both species in two growth forms were investigated to clarify their response to facilitation in associated growth. Six isolated plants of each species, as well as six associated plants similar in size and development were selected during the plant growing season, and their roots were excavated at 0–10, 10–20, 20–30, 30–40 and 40–50 cm soil depths at the end of the growing season. All the roots of each plant were separated into the two categories of fine roots (<2 mm diameter) and coarse roots (≥2 mm diameter). Root traits such as RL and RSA in the fine and coarse roots were obtained by the root analyzing system WinRHIZO. Most of the coarse roots in R. soongorica and S. passerina were distributed in the top 10 cm of the soil in both growth forms, whereas the fine roots of the two plant species were found mainly in the 10–20 and 20–30 cm soil depths in isolated growth, respectively. However, the fine roots of both species were mostly overlapped in 10–20 cm soil depth in associated growth. The root/canopy ratios of both species reduced, whereas the ratios of their fine roots to coarse roots in RL increased, and both species had an increased SRL in the fine roots in associated growth. In addition, there was the increase in RL of fine roots and content of root N for S. passerina in associated growth. Taken together, the root growth of S. passerina was facilitated for water and nutrient exploration under the inter-action of the overlapped roots in both species in associated growth, and higher SRL allowed both species to more effectively adapt to the infertile soil in the desert ecosystem.

This study aims to investigate the protective roles of photosynthetic characteristics and antioxidative systems in the desiccation tolerance of Sophora moorcroftiana and Caragana maximovicziana as they adapt to arid environments. A variety of physiological and biochemical parameters in the leaves of two Leguminosae species were monitored for 1, 7, 14, 21, and 28 d of drought stress. Soil water content decreased from 38.58% to 7.33% after exposure to 28 d of water stress. The photosynthetic carbon-assimilation rates of the two Leguminosae plants decreased for non-stomatal limitation with processing water stress. The malondialdehyde content and cell membrane relative conductivity of the two species increased significantly from 1 to 21 d and then decreased. S. moorcroftiana showed higher superoxide dismutase and peroxidase activities than C. maximovicziana during the 28 d treatment period. However, the catalase activities and proline content of C. maximovicziana were higher than those of S. moorcroftiana before the water stress treatment reached 21 d. Nine physiological and biochemical parameters were selected to comprehensively evaluate the two species’ drought-resistance by the membership function values (MFV). The mean MFV indicated that S. moorcroftiana has a relatively stronger drought defense capability than C. maximovicziana. S. moorcroftiana mainly uses carbon-assimilation rate and osmotic adjustment to combat water deficiency.