Diurnal and seasonal dynamics of soil respiration in desert shrubland of Artemisia Ordosica on Ordos Plateau of Inner Mongolia, China

The diurnal and seasonal dynamics of soil respiration in the A. ordosica shrubland on Ordos Plateau were investigated in the growing season （May-October） of 2006 and their environmental driving factors were also analyzed, Results indicated that diurnal dynamics of soil respiration rate and its temperature dependence showed some discrepancy in two different growth stages （the vegetative growth stage and the reproductive growth stage）. During the vegetative growth stage, the diurnal variation of soil respiration was slight and not correlated with the daily temperature change, but during the reproductive growth stage, the daily respiration variation was relatively large and significantly correlated with the diurnal variation of air and soil temperature. In the growing season, the peak value of soil respiration occurred at July and August because of the better soil water-heat conditions and their optimal deployment in this period. In the shrubland ecosystem, precipitation was the switch of soil respiration pulses and can greatly increase soil respiration rates after soil rewetting. Moreover, the soil respiration rates in the growing season and the air temperature and soil surface water content were closely correlated （p〈0.05） each other. The stepwise regression model indicated that the variation of soil surface moisture accounted for 41.9% of the variation in soil respiration （p〈0.05）.

Human induced dryland degradation in Ordos Plateau,China,revealed by multilevel statistical modeling of normalized difference vegetation index and rainfall time-series

Land degradation causes serious environmental problems in many regions of the world, and although it can be effectively assessed and monitored using a time series of rainfall and a normalized difference vegetation index （NDVI） from remotely-sensed imagery, dividing human-induced land degradation from vegetation dynamics due to climate change is not a trivial task. This paper presented a multilevel statistical modeling of the NDVI-rainfall relationship to detect human-induced land degradation at local and landscape scales in the Ordos Plateau of Inner Mongolia, China, and recognized that anthropogenic activities result in either positive （land restoration and re-vegetation） or negative （degradation） trends. Linear regressions were used to assess the accuracy of the multi- level statistical model. The results show that： （1） land restoration was the dominant process in the Ordos Plateau between 1998 and 2012; （2） the effect of the statistical removal of precipitation revealed areas of human-induced land degradation and improvement, the latter reflecting successful restoration projects and changes in land man- agement in many parts of the Ordos; （3） compared to a simple linear regression, multilevel statistical modeling could be used to analyze the relationship between the NDVI and rainfall and improve the accuracy of detecting the effect of human activities. Additional factors should be included when analyzing the NDVI-rainfall relationship and detecting human-induced loss of vegetation cover in drylands to improve the accuracy of the approach and elimi- nate some observed non-significant residual trends.

regionalclimate Characteristics of regional climate change and pattern analysis on Ordos Plateau

The characteristics of precipitation, temperature and their combination determine the special ecological environment pattern of Ordos Plateau. Analyzing its evolutionary trend attributes to understanding the succession process of the ecological environment of Ordos Plateau and has crucial instructional significance on the ecological restoration research being conducted in this region. Four time scales, arranging from ten days, one month, one season(growing season contrasting to non growing season)to one year were adopted to analyze the climate data which included nearly 30 years and were collected by eight weather stations on Ordos Plateau. The results indicated that the mean annual temperature and the mean monthly temperature of February, September and December, had increased significantly during the late 30 years. The annual precipitation did not show significant changes but its distribution pattern had changed obviously. The ratio of precipitation of major growing season (May October)to annual precipitation had increased distinctively,and five counties' precipitation reached statistically significant level. And the ratio of precipitation of latter growing season (September)to one year decreased significantly while the ratio of non growing season(November next April)to one year changed insignificantly. The results showed that maybe the interaction of increased mean temperature and insignificant change of precipitation in non growing season was one of the reasons why the desertification of the region was deteriorating in recent years. Using some factors closely relating to vegetation succession such as mean annual temperature, mean annual precipitation, distributive pattern of precipitation, mean temperature of the coldest month, mean temperature of the warmest month, precipitation of the warmest month, mean temperature of growing season, precipitation of growing season, potential evapotranspiration(PET)and radiative dryness index(RDI), to synthetically analyze the climate characteristics of Ordos Plateau. The regionalized Ordos Plateau to three synthetical climate types were recognized as follows: Type I, semi humid and low evaporation(including Jungar Banner, Dongsheng City and Ejin Horo Banner), Type II, semi arid, semi humid and moderate evaporation(including Uxin Banner and Dalad Banner), Type III,arid and high evaporation(including Hanggin Banner, Otog Banner and Otog Qian Banner).

Impact of water table on hierarchically nested groundwater flow system

Water table configuration gives rise to hierarchically nested groundwater flow systems.However,there remains a lack of comprehensive understanding regarding the controlling factors of water table and its impact on flow systems.Moreover,it remains challenging to identify characteristics of water table space variation through limited groundwater observations at the regional scale.Based on two ideal two-dimensional cross-section analytical models,this study presents a simplified approach to preliminarily assess the nonlinear interactions between water table variation and three driving factors:Topography,geol-ogy and climate.Two criteria,C1 and C2,are utilized to address issues at different scales ranging from basin to local:(i)the influence of various factors on water table configuration;and(ii)the influence of water table on groundwater flow pattern.Then,the Ordos Plateau is taken as an example to explore the role of the water table in nested groundwater systems using the provided approach and criterion.The applica-tion of this approach in the Ordos Plateau demonstrates its appropriateness as a practical method for prelim-inarily determining the characteristics of water table configuration and its impact on flow systems.The study explores the mechanism influencing spatial variation in the water table and improves understanding of the interaction between topography,geology,and climate on groundwater flow patterns.

Fertile island effect in the sedimentary process of Tetraena mongolica Maxim nebkhas in steppe–desert ecotones on the Inner Mongolia Plateau,China

Phytogenic mounds(nebkhas)formed by shrubs are a common phenomenon in arid and semiarid areas.The formation of nebkhas is accompanied by the appearance of the fertile island effect.Quantitative evaluation of the shrub sand compost island effect is a key link in preventing soil erosion and nutrient loss.This study took the typical shrub Tetraena mongolica in desert areas as the research object and quantified the sand-trapping capacity of the shrub.We revealed the influence of sediment texture and volumetric soil water content on fertile islands during the development of T.mongolica nebkhas.The results showed that(1)the single shrub intercepted large amounts of sediments due to the high density of branches of T.mongolica,forming nebkhas that were positively correlated with the shrub size.(2)The overall soil nutrient content below the shrub was greater than the soil nutrient content outside the shrub,forming a typical fertile island effect.The soil organic carbon(SOC),alkaline hydrolytic nitrogen(AHN),available phosphorus(AP),and available K(AK)content all increased gradually with increasing nebkha volume.Compared with the deep soil of nebkhas,the nutrient content of the surface soil was generally higher.(3)There was a positive correlation between the volumetric water content and nutrient content in nebkhas.(4)The semi-ellipsoid shape of T.mongolica enabled it to intercept large amounts of coarse-grained material.Fine sand(100–250μm)was the main particle size in the sediment aggregates inside the nebkhas.The fine sand content generally increased with increasing shrub size.Redundancy analysis(RDA)revealed that the fine sand content of the nebkha sediments had a strong positive correlation with the soil nutrient content.This paper provides an example for evaluating the fertile island effect during the deposition process of nebkhas in the desert transition zone.

Seasonal Patterns of Soil Respiration in Three Types of Communities along Grass-Desert Shrub Transition in Inner Mongolia, China

The seasonal dynamics of soil respiration in steppe （S. bungeana）, desert shrub （A. ordosica）, and shrubperennial （A. ordosica ＋C. komarovii） communities were investigated during the growth season （May to October） in 2006; their environmental driving factors were also analyzed. In the three communities, soil respiration showed similar characteristics in their growth seasons, with peak respiration values in July and August owing to suitable temperature and soil moisture conditions during this period. Meanwhile, changes in soil respiration were greatly influenced by temperatures and surface soil moistures. Soil water content at a depth of 0 to 10 cm was identified as the key environmental factor affecting the variation in soil respiration in the steppe. In contrast, in desert shrub and shrub-perennial communities, the dynamics of soil respiration was significantly influenced by air temperature. Similarly, the various responses of soil respiration to environmental factors may be attributed to the different soil textures and distribution patterns of plant roots. In desert ecosystems, precipitation results in soil respiration pulses. Soil carbon dioxide （CO2） effluxes greatly increased after rainfall rewetting in all of the ecosystems under study. However, the precipitation pulse effect differed across the ecosystem. We propose that this may be a result of a reverse effect from the soil texture.

Quantification of groundwater recharge and evapotranspiration along a semi-arid wetland transect using diurnal water table fluctuations

Groundwater is a vital water resource in arid and semi-arid areas.Diurnal groundwater table fluctuations are widely used to quantify rainfall recharge and groundwater evapotranspiration(ET_(g)).To assess groundwater resources for sustainable use,we estimated groundwater recharge and ET_(g) using the diurnal water table fluctuations at three sites along a section with different depths to water table(DWT)within a wetland of the Mukai Lake in the Ordos Plateau,Northwest China.The water table level was monitored at an hourly resolution using a Keller DCX-22 A data logger that measured both the total pressure and barometric pressure,so that the effect of barometric pressure could be removed.At this study site,a rapid water table response to rainfall was observed in two shallow wells(i.e.,Obs1 and Obs2),at which diurnal water table fluctuations were also observed over the study period during rainless days,indicating that the main factors influencing water table variation are rainfall and ET_(g).However,at the deep-water table site(Obs3),the groundwater level only reacted to the heaviest rainfalls and showed no diurnal variations.Groundwater recharge and ET_(g) were quantified for the entire hydrological year(June 2017–June 2018)using the water table fluctuation method and the Loheide method,respectively,with depth-dependent specific yields.The results show that the total annual groundwater recharge was approximately 207 mm,accounting for 52%of rainfall at Obs1,while groundwater recharge was approximately 250 and 21 mm at Obs2 and Obs3,accounting for 63%and 5%of rainfall,respectively.In addition,the rates of groundwater recharge were mainly determined by rainfall intensity and DWT.The daily mean ET_(g) at Obs1 and Obs2 over the study period was 4.3 and 2.5 mm,respectively,and the main determining factors were DWT and net radiation.

新石器时代以来鄂尔多斯高原人类活动遗址研究

Settlements are excellent spatiotemporal indicators for studying historical human activities and environmental change.This paper discusses the spatial and temporal changes of sites on the Ordos Plateau in China since the Neolithic Age,based on an analysis of spatiotemporal distribution characteristics of the human settlements.The frequency of human settlements on the Ordos Plateau presented a phased fluctuation process,and the sizes were mainly small and medium.The spatial distribution of human settlements was fractal,and the D value of the aggregation dimension was generally small,indicating that the spatial distribution of the sites was agglomerated.Affected by the desert,the sites were mainly distributed in the south and east of the Ordos Plateau.The spatiotemporal distribution pattern of human settlements in the Ordos Plateau resulted from the combined action of natural and human factors,especially climate change.Moreover,local livelihood patterns significantly affected the frequency of human settlements.The number of human settlements in the farming period was large,and the distribution was concentrated.In contrast,the number of sites in the nomadic period was small and scattered.The central government’s policies and the conflicts between farming and nomadic groups further affected the spatiotemporal distribution of human settlements.This study may contribute to a better understanding of historical environmental change and human-land relationships in the Ordos Plateau.