Short-term effects of nitrogen deposition on soil respiration components in two alpine coniferous forests, southeastern Tibetan Plateau

Nitrogen (N) deposition to alpine forest ecosystems is increasing gradually, yet previous studies have seldom reported the effects of N inputs on soil CO2 flux in these ecosystems. Evaluating the effects of soil respiration on N addition is of great significance for understanding soil carbon (C) budgets along N gradients in forest ecosystems. In this study, four levels of N (0, 50, 100, 150 kg N ha^-1 a^-1) were added to soil in a Picea baifouriana and an Abies georgei natural forest on the Tibetan Plateau to investigate the effect of the N inputs on soil respiration. N addition stimulated total soil respiration (Rt) and its components including heterotrophic respiration (Rh) and autotrophic respiration (Ra);however, the promoted effects declined with an increase in N application in two coniferous forests. Soil respiration rate was a little greater in the spruce forest (1.05 μmol CO2 m^-2 s^-1) than that in the fir forest (0.97 μmol CO2 m^-2 s^-1). A repeated measures ANOVA indicated that N fertilization had significant effects on Rt and its components in the spruce forest and Rt in the fir forest, but had no obvious effect on Rh or Ra in the fir forest. Rt and its components had significant exponential relationships with soil temperature in both forests. N addition also increased temperature sensitivity (Q10) of Rt and its components in the two coniferous forests, but the promotion declined as N in put increased. Important, soil moisture had great effects on Rt and its components in the spruce forest (P<0.05), but no obvious impacts were observed in the fir forest (P>0.05). Following N fertilization, Ra was significantly and positively related to fine root biomass, while Rh was related to soil enzymatic activities in both forests. The mechanisms underlying the effect of simulated N deposition on soil respiration and its components in this study may help in forecasting C cycling in alpine forests under future levels of reactive N deposition.

Impacts of climate change on glacial water resources and hydrological cycles in the Yangtze River source region,the Qinghai-Tibetan Plateau,China:A Progress Report

The Yangtze River Source Region has an area of 137,704 km2.Its mean annual runoff of 12.52 billion m3,which was recorded by the Chumda Hydrological Station in 1961–2000,accounts for only 0.13 percent of the Yangtze River's total annual streamflow.The extensive rivers,lakes,wetlands,glaciers,snow fields,and permafrost of the Yangtze River Source Region,as well as the region's vast alpine grasslands,play a critical role in storing and regulating the flow of water not only in the upper Yangtze River watershed of Qinghai,Sichuan,the Tibet Autonomous Region (TAR) (Tibet) and Yunnan,but also throughout the entire lower Yangtze River basin.Climate change has been the dominant factor in recent fluctuation in the volume of the Yangtze River Source Region's glacier resources.The Chumda Hydrological Station on the lower Tongtian River has registered a mean annual glacial meltwater of 1.13 billion m3 for the period 1961–2000,makes up 9 percent of the total annual runoff.Glacial meltwater makes up a significant percentage of streamflow in the Yangtze River Source Region,the major rivers of the upper Yangtze River Source Region:the Togto,Dam Chu,Garchu,and Bi Chu (Bu Chu) rivers all originate at large glaciers along the Tanggula Range.Glaciers in the Yangtze River Source Region are typical continental-type glaciers with most glacial meltwater flow occurring June–August;the close correlation between June–August river flows and temperature illustrates the important role of glacial meltwater in feeding rivers.Glaciers in the source region have undergone a long period of rapid ablation beginning in 1993.Examination of flow and temperature data for the 1961–2000 period shows that the annual melting period for glacial ice,snow,and frozen ground in the Yangtze River Source Region now begins earlier because of increasing spring temperatures,resulting in the reduction of summer flood season peak runoffs;meanwhile,increased rates of glacier ablation have resulted in more uneven annual distribution of runoff in the source region.The annual glacial meltwater runoff in the Yangtze River Source Region is projected to increase by 28.5 percent by 2050 over its 1970 value with the projected temperature increase of 2℃ and a precipitation increase of 29 mm.As a critical source of surface water for agriculture on the eastern Qinghai-Tibet Plateau and beyond,the mass retreat of glaciers in the Yangtze River Source Region will have enormous negative impacts on farming and livestock-raising ac-tivities in upper Yangtze River watershed,as well as on the viability of present ecosystems and even socioeconomic development in the upper Yangtze River Basin.

Influence of short-term experimental warming on heat-waterprocesses of the active layer in a swamp meadow ecosystemof the Qinghai-Tibet Plateau

Climate change is now evident in the Qinghai-Tibet Plateau(QTP), with impacts on the alpine ecosystem, particularly on water and heat balance between the active layer and the atmosphere. Thus, we document the basic characteristics of changes in the water and heat dynamics in response to experimental warming in a typical alpine swamp meadow ecosystem. Data sets under open top chambers(OTC) and the control manipulations were collected over a complete year. The results show that annual(2008) air temperatures of OTC-1 and OTC-2 were 6.7 °C and 3.5 °C warmer than the control. Rising temperature promotes plant growth and development. The freeze-thaw and isothermal days of OTCs appeared more frequently than the control, owing to comparably higher water and better vegetation conditions. OTCs soil moisture decreased with the decrease of soil depth; however, there was an obviously middle dry aquifer of the control, which is familiar in QTP. Moreover, experimental warming led to an increase in topsoil water content due to poorly drained swamp meadow ecosystem with higher organic matter content and thicker root horizons. The results of this study will have some contributions to alpine cold ecosystem water-heat process and water cycle under climate change.

Two Ultraviolet Radiation Datasets that Cover China

Ultraviolet（UV） radiation has significant effects on ecosystems, environments, and human health, as well as atmospheric processes and climate change. Two ultraviolet radiation datasets are described in this paper. One contains hourly observations of UV radiation measured at 40 Chinese Ecosystem Research Network stations from 2005 to 2015. CUV3 broadband radiometers were used to observe the UV radiation, with an accuracy of 5%, which meets the World Meteorology Organization＇s measurement standards. The extremum method was used to control the quality of the measured datasets. The other dataset contains daily cumulative UV radiation estimates that were calculated using an all-sky estimation model combined with a hybrid model. The reconstructed daily UV radiation data span from 1961 to 2014. The mean absolute bias error and root-mean-square error are smaller than 30% at most stations, and most of the mean bias error values are negative, which indicates underestimation of the UV radiation intensity. These datasets can improve our basic knowledge of the spatial and temporal variations in UV radiation. Additionally, these datasets can be used in studies of potential ozone formation and atmospheric oxidation, as well as simulations of ecological processes.

Desertification evolution prediction for lake area of Minqin oasis based on integration of GIS and cellular automata

The Minqin oasis is surrounded on three sides by the Tengger Desert and the Badanjilin Desert, and it prevents these two deserts from converging. However, in recent years it has become the worst ecological environment in the Lake area due to deficient water resources, continual declines in the groundwater level and quality （increasing mineralization and salination）, which are causing in- creasing desertification. In this study, Landsat Thematic Mapper （TM） remote images from 1992, 1998, 2002, and 2006 of the Lake area of the Minqin oasis are interpreted to analyze the desertification evolution. A combination of an ArcObjects module and a cellular automata model is used to build a model simulating the desertification dynamics; the forecasting accuracy of this model is shown to reach up to 90%. The desertification situation in 2012 is forecasted by this model, and the results showed that, from 2006 to 2012, the green land area will be reduced by 999.92 hm2 （l.59 percent of the total oasis area）, the desertification land area will be reduced by 3,000.68 hrn2 （4.78 percent of the total oasis area）, and sand land area will increase by 4,000.6 hm2 （6.37 per- cent of the total oasis area）. The sand land is predicted to become more widespread, and more than 18% sand land will be distrib- uted in the center of green land in the Lake area. In other words, more and more abandoned green land （mined farm land） will be transformed into sand land, and this will intensify the desertification.

Study on the impact of vegetation degeneration to hydrology characteristic of the Alpine soil

Alpine soil infiltration process is an important part of the hydrological characteristics of alpine soil in permafrost. This research is carried out in the source region of the Yellow River where the permafrost is severely degraded, using various methods for choosing typical sample areas, and to experiment, study and simulate the soil water curve, soil saturated hydraulic conductivity, soil infiltration and soil moisture under different characteristics of degraded vegetation. The results indicate that the empirical equation θ=AS-B, proposed by Gradner and Visser, is very reliable in simulating the soil moisture curve; soil saturated hydraulic conductivity and soil infiltration are significantly different under different vegetation coverage： in the soil surface within 0-10 cm, the saturated hydraulic conductivity and infiltration intensity of Black Beach are the strongest; respectively, in soil layers below 30 cm, vegetation has almost no impacts on the saturated hydraulic conductivity, infiltration intensity and soil moisture content. Significant reduction of soil moisture occurs in soil surfaces with degraded vegetation. The more serious the degradation, the more water loss, and it can be up to 38.6% in the worst situation. Soil moisture of developed vegetation root systems in depths within 10-20 cm has the greatest impact on the soil environment, and the loss of moisture induces difficulty in the restoration of degraded meadows. Through a comparative study, the Kostiakov infiltration equationf（t） = at-b is more applicable for studies on the process of soil moisture infiltration of the alpine meadow in the source region of the Yellow River.

青藏高原冻土区高寒草甸能量分配及蒸散发影响因子

高寒草甸的能量分配和蒸散发对青藏高原多年冻土区水循环至关重要。然而,能量分配、蒸散发及其驱动因素的季节变化(冻融循环)仍需要明确。因此,本研究在位于青藏高原风火山流域的高寒草甸进行了为期4年的能量通量(包括潜热和感热)观测,并估算了大气边界参数(包括表面导度,解耦系数和Priestley-Taylor系数)。研究结果表明,研究区日均潜热(27.45±23.89 W/m^(2))和显热(32.51±16.72W/m^(2))分别占可利用能量的31.71%和50.14%。在降雨期,更多可利用能量被分配到潜热;而在冻结期,67.54±28.44%的可利用能量分配给显热。显热在降雨期间是潜热的一半,而由于冻结期较低的土壤水分含量及植被盖度,显热在冻结期间是潜热的7倍。研究区年均蒸散发为347.34±8.39 mm/year,接近年均降水量。较低的日均解耦系数(0.45±0.23)和Priestley-Taylor系数(0.60±0.29)表明高寒草甸的蒸散发受水分供应限制。然而,在降雨期由于降水充足,蒸散发受到可利用能量的限制。在过渡期,蒸散发和降水之间存在较大差异,表明在该季节上游冰川和雪的融水通过侧向流动补给到土壤中。本研究的结果表明,在未来模拟多年冻土区水和能量通量时应考虑大气边界参数的季节变化。

陆地植被生态水文过程前沿进展:从植物叶片到流域

陆地植被生态水文过程是生态学、水文学和全球变化研究关注的前沿领域,更是生态水文学的关键理论基础之一.近年来围绕植被生态与水相互作用的研究范畴涵盖了从植物细胞到大陆尺度的几乎所有空间尺度,在不同尺度上分别在生态学和水文学各自视角取得了较大进展.但从生态水文学交叉学科角度,迫切需要整合生态学与水文学多尺度相关研究进展,系统性地归纳和总结跨尺度理论和方法进展,梳理理论前沿热点问题.为此,本文从近年来关于陆地植被生态水文过程与模拟研究进展中,系统提炼和总结了以植物水分利用与调控机制、碳氮水耦合循环过程与模拟、水循环关键过程的生态作用、植被生态作用下的径流形成与变化、植被生态水循环变化对大气降水过程的反馈影响等为主要研究热点的理论前沿方向,系统总结了这五方面在不同尺度及跨尺度方面所取得的主要进展,特别关注了基于过程机理的定量模型方面的发展状态.基于这些前沿热点面临的挑战性难题,提出了未来需要重点关注的核心问题,包括探索植物水分利用与调控的多尺度关联机制、发展基于过程机制的碳氮水耦合循环精准模拟方法、径流形成与动态变化的生态因素甄别与定量刻画等.对深入理解生态水文学国际前沿发展趋势,引导我国生态水文学学科的发展方向等有参考意义.

Linkage between permafrost distribution and river runoff changes across the Arctic and the Tibetan Plateau

River runoff in the Arctic and the Tibetan Plateau(TP) change significantly in recent decades. However, the mechanisms of the physical processes of permafrost river runoff change remain uncertain across large scale. This study investigated the mainstreams and tributaries of main Arctic and TP rivers dominated by permafrost and assessed the linkage between hydrological regime change and permafrost. The results show that the effects of permafrost on river runoff are highly dependent on the permafrost coverage of a watershed. For the past decades, the majority of the Arctic and TP basins showed increased discharge, while all of the studied basins showed increased baseflow, with faster increasing speed than total discharge.Both total discharge and baseflow annual change rate(ΔQ and ΔBF) increased with permafrost coverage, indicating the increments of streamflow are enhanced with high permafrost coverage. Meanwhile, the annual change of precipitation showed weak connection with total discharge and baseflow change. The high permafrost coverage basins showed high annual maximum/minimum discharge ratio(Qmax/Qmin), while the Qmax/Qminchanged slightly in low permafrost cover basins. Our results highlight the importance of permafrost coverage on streamflow regime change for permafrost basins across the northern hemisphere. Due to these linkage between permafrost extent and runoff regime change and the increasing changes of permafrost, more attention should be paid to the change of hydrological processes in permafrost-underlain basins.