Analysis of impact factors on scrubland soil respiration in the southern Gurbantunggut Desert,central Asia

Monitoring soil CO₂ respiration with chamber measurements and identifying controlling factors such as the diversity of vegetation species, moisture and temperature can help guide desert scrubland management. Soil CO₂ respiration and potential controlling factors at four sites in desert scrubland were examined along the Sangong River Basin (SRB) in northwestern China in 2004. Soil CO₂ respiration descended along the SRB as did the diversity of vegetation species, air temperature and air humidity. The two sites of the field station (FS) and the north desert (ND) and the low reaches of the SRB among these locations were monitored to analyze the effects of pH value, soil organic carbon (SOC), total nitrogen (TN) and calcium carbonate (CaCO₃) on soil CO₂ respiration during the growing season in 2005. The ND site was located at the southern edge of the Gurbantunggut Desert; the FS site was in the border area of the SRB Alluvial Fan. One-way ANOVA was performed. The result showed that air humidity and CaCO₃ content had a strong influence on soil CO₂ respiration; SOC content was a limitation to soil CO₂ respiration in the arid-desert zone. Effective management activities can attenuate soil CO₂ respiration and keep carbon balance trends at a desirabe level in desert scrublands.     

The scale effect on the soil spatial heterogeneity of Haloxylon ammodendron (C. A. Mey.) in a sandy desert

Haloxylon ammodendron Bge (C.A. Mey.) is a dominant shrub species in the Gurbantonggut Desert and plays an important role in preventing wind erosion and combating desertification, typically by developing fertile islands in desert ecosystems; however, such islands often depend on the scales. An experiment was conducted to determine the scale dependence for the soil spatial heterogeneity of H. ammodendron in the Gurbantonggut Desert using the soil pH, electrical conductivity (EC), soil organic carbon (SOC), and total nitrogen (TN). The results showed that the soil EC, SOC and TN were significantly higher at the individual scale than the population scale. Moreover, the coefficients of variation (CV %) of the soil parameters at the individual scale were greater than they were at the population scale, with all except for pH (CV = 4.35 % for individual scale and CV = 2.87 % for population scale) presenting a moderate degree of variability (10 % < CV < 100 %). A geostatistical analysis revealed a strong spatial dependence [C ₀ /(C ₀ + C) < 25 %] within the distance of ranges for the tested parameters at both scales. The kriging interpolation results presented significant accumulation of soil SOC and TN around the shrub center and formed a significant “fertile island” at the individual scale, whereas the soil EC was much lower at the shrub center. At the population scale, patch fragments of the soil chemical properties were observed; however, not all individuals presented significant fertile islands or salt islands, and the soil EC presented a similar distribution as SOC and TN. These differences suggested that different mechanisms controlled the spatial distribution of soil minerals at the two scales and that the spatial heterogeneities are scale-dependent in a desert ecosystem.     

Annual variation of temperature sensitivity of soil organic carbon decomposition in North peatlands: implications for thermal responses of carbon cycling to global warming

Temperature sensitivities of microbial respiration and dissolved organic carbon (DOC) production were investigated by using a novel method, thermal gradient (2–20°C) temperature bar, in two typical peatlands (bog and fen) in North Wales, UK over 12 months. The study indicated that temperature sensitivity of soil organic carbon decomposition in North peatlands was regulated not only by temperature but soil water content, dry–rewet event and phenologies. Potential decreases of Q₁₀ (CO₂) with increasing soil temperature were confirmed in both peatlands, but Q₁₀ (DOC) increase with increasing soil temperature in both bog and fen sites. These results imply, if other factors such as the so-called CO₂ fertilization effect are simultaneously taken into account, that the feedback of global warming induced CO₂ release from peatlands to climate change may be overestimated in current biogeochemical models. However, global warming might have been nonlinearly accelerating DOC thermal production, and therefore it helps explaining the causes of remarkable increase of DOC in surface water in the Northern Hemisphere during last several decades.     

Effects of nitrogen fertilization on soil respiration in temperate grassland in Inner Mongolia,China

Nitrogen addition to soil can play a vital role in influencing the losses of soil carbon by respiration in N-deficient terrestrial ecosystems. The aim of this study was to clarify the effects of different levels of nitrogen fertilization (HN, 200 kg N ha⁻¹ year⁻¹; MN, 100 kg N ha⁻¹ year⁻¹; LN, 50 kg N ha⁻¹ year⁻¹) on soil respiration compared with non-fertilization (CK, 0 kg N ha⁻¹ year⁻¹), from July 2007 to September 2008, in temperate grassland in Inner Mongolia, China. Results showed that N fertilization did not change the seasonal patterns of soil respiration, which were mainly controlled by soil heat-water conditions. However, N fertilization could change the relationships between soil respiration and soil temperature, and water regimes. Soil respiration dependence on soil moisture was increased by N fertilization, and the soil temperature sensitivity was similar in the treatments of HN, LN, and CK treatments (Q ₁₀ varied within 1.70–1.74) but was slightly reduced in MN treatment (Q ₁₀ = 1.63). N fertilization increased soil CO₂ emission in the order MN > HN > LN compared with the CK treatment. The positive effects reached a significant level for HN and MN (P < 0.05) and reached a marginally significant level for LN (P = 0.059 < 0.1) based on the cumulative soil respiration during the 2007 growing season after fertilization (July–September 2007). Furthermore, the differences between the three fertilization treatments and CK reached the very significant level of 0.01 on the basis of the data during the first entire year after fertilization (July 2007–June 2008). The annual total soil respiration was 53, 57, and 24% higher than in the CK plots (465 g m⁻² year⁻¹). However, the positive effects did not reach the significant level for any treatment in the 2008 growing season after the second year fertilization (July–September 2008, P > 0.05). The pairwise differences between the three N-level treatments were not significant in either year (P > 0.05).     

The variation of soil temperature and water content of seasonal frozen soil with different vegetation coverage in the headwater region of the Yellow River, China

The variation and distribution of temperature and water moisture in the seasonal frozen soil is an important factor in the study of both the soil water cycle and heat balance within the source region of the Yellow River, especially under the different conditions of vegetation coverage. In this study, the impact of various degrees of vegetation coverage on soil water content and temperature was assessed. Soil moisture (θ _v) and soil temperature (T _s) were monitored on a daily basis. Measurements were made under different vegetation coverage (95, 70–80, 40–50 and 10%) and on both thawed and frozen soils. Contour charts of T _s and θ _v as well as a θ _v–T _s coupling model were developed in order to account for the influence of vegetation cover and the interaction between T _s and θ _v. It was observed that soil water content affected both the overall range and trend in the soil temperature. The regression analysis of θ _v versus T _s plots indicated that the soil freezing and thawing processes were significantly affected by vegetation cover changes. Vegetation coverage changes also caused variations in the θ _v–T _s interaction. The effect of soil water content on soil temperature during the freezing period was larger than during the thawing period. Moreover, the soil with higher vegetation coverage retained more water than that with lower coverage. In the process of freezing, the higher vegetation coverage reduced the rate of the reduction in the soil temperature because the thermal capacity of water is higher than that of soil. Areas with higher vegetation coverage also functioned better for the purpose of heat-insulating. This phenomenon may thus play an important role in the environmental protection and effective uses of frozen soil.     

Assessment of soil moisture influence on CO2 flux: a laboratory experiment

Accurate measurements to assess the influence of soil moisture on CO₂ flux requires the absolute estimates of soil CO₂ flux. Thus, it was constructed a calibration system where CO₂ with fixed concentration flowed through the different porous material. Previous to measurement, in order to verify the performance and reliability of a closed dynamic chamber, different discontinuous air-mixing rates and times were tested. The CO₂ flux was estimated through sequential lectures and the best fit for flux measurements was obtained taking short readings every 3 min, during a total time of 12 min (R ² = 0.99). The best mixing rate was attained for 250 mL min⁻¹, allowing 25 s of mixing previous to CO₂ extraction for an infrared gas analyzer. The deviation of the measured values for dry sand from the reference CO₂ flux (0.097 and 0.071 g m⁻² min⁻¹) was 5 and 7%. On dry sandy loam soil (SLS) the deviation was 2%. The measured fluxes decreased 73 and 22% with content moisture of 20 and 10% (sand), and 78% with content moisture of 31% (SLS). This work allowed to estimate how much the measured emission rates deviate from the true ones for the specified chamber and sampling conditions.     

Effects of precipitation pulses on water and carbon dioxide fluxes in two semiarid ecosystems: measurement and modeling

Modeling of soil?Cwater, ?Cheat and ?Ccarbon (C) fluxes provides an important tool for predicting mass and energy transfers based on a hydraulic-, thermal- and C-mass balance approach. Model predictions were evaluated using measured data from two water-limited study sites, one pasture and one supporting an alfalfa crop, to indentify differences between these ecosystems. Soil water content, temperature, and evapotranspiration (ET) data were used to validate soil water dynamics components of a process-based numerical model. Soil surface CO₂ efflux estimates (i.e., fluxes from soil respiration) were also made to estimate soil CO₂ emissions. The results show that the Hydrus-1D numerical model can be parameterized to simulate the soil hydrodynamics and CO₂ fluxes measured at both locations. Rainfall and irrigation events triggering increases in plant root and microbial respiration rates were simulated to recreate observed pulsed CO₂ fluxes. There were distinct differences in ET and soil CO₂ effluxes between the ecosystems and watering events significantly modified the fluxes. Differences in potential evapotranspiration and soil texture could help explain these discrepancies. The results demonstrate that numerical modeling can be a useful tool for estimating soil surface fluxes in calibrated ecosystems when micrometeorological methods may not be suitable.     

荒漠地区土壤初始状况对水平入渗的影响

通过对沙坡头荒漠地区人工植被固沙区生物土壤结皮与固沙区外天然沙丘沙以及天然植被红砂、珍珠分布区砂粘土在不同初始含水率（θ_i）条件下土壤水平入渗过程的对比试验研究,探讨土壤质地以及θ i对水平入渗系数（λ_f）、剖面平均含水率（θ_m）状况的影响。生物土壤结皮对降水入渗量的截持作用大于沙丘沙,因而将显著地改变降水入渗过程中土壤水分的再分配格局,减少降水对深层土壤（沙丘沙）的有效补给。通过对土壤剖面平均含水率θ_m与距离湿润锋前缘0～5 cm、5～10 cm处体积含水率θ_fA、θ_fB比较,生物土壤结皮的θ_m、θ_fA和θ_fB分别约为沙丘沙的1．5倍。     

全球变化条件下的土壤呼吸效应

土壤呼吸是陆地植物固定CO₂尔后又释放CO₂返回大气的主要途径，是与全球变化有关的一个重要过程。综述了全球变化下CO₂浓度上升、全球增温、耕作方式的改变及氮沉降增加的土壤呼吸效应。大气CO₂浓度的上升将增加土壤中CO₂的释放通量，同时将促进土壤的碳吸存；在全球增温的情形下，土壤可能向大气中释放更多的CO₂，传统的土地利用方式可能是引发温室气体CO₂产生的重要原因，所有这些全球变化对土壤呼吸的作用具有不确定性。认为土壤碳库的碳储量增加并不能减缓21世纪大气CO₂浓度的上升。据此讨论了该问题的对策并提出了今后土壤呼吸的一些研究方向。其中强调，尽管森林土壤碳固定能力有限，但植树造林、森林保护是一项缓解大气CO₂上升的可行性对策；基于现有田间尺度CO₂通量测定在不确定性方面的进展，今后应继续朝大尺度田间和模拟程序方面努力；着重回答全球变化条件下的土壤呼吸过程机理；区分土壤呼吸的不同来源以及弄清土壤呼吸黑箱系统中土壤微生物及土壤动物的功能。当然，土壤呼吸的测定方法尚有待改善。     

Effects of climatic factors and soil salinity on the distribution of vegetation types containing <Emphasis Type="Italic">Anabasis aphylla</Emphasis> in Iran: a multivariate factor analysis

In order to identify factors affecting the distribution of Anabasis aphylla in rangelands of Iran, 132 climatic parameters which contributed to the species distribution (based on the ecological conditions of the study area) were selected. Factor analysis was then applied to determine the most important factors affecting species distribution. In the next stage, the spatial distribution map of An. aphylla throughout Iran was prepared. According to the results of factor analysis, ten factors with Eigenvalues greater than one explained 92.96% of the total variance. These factors were temperature below zero, winter rainfall, summer rainfall, wind, sunlight, warm season dust, rainfall in fall, thunderstorms, relative humidity in September, and cloudy days in winter and explained 34.34, 9.71, 9.69, 8.85, 5.99, 5.35, 4.97, 4.78, 4.73, and 4.51% of the total variance, respectively. Moreover, six vegetation types containing An. aphylla were identified throughout the country. These types were distributed in the central areas of Iran and also in patches within the southeastern parts of the country. The results showed that in addition to the nine above climatic factors, soil variables particularly salinity influence establishing the vegetation types in their distribution areas and the vegetation types are located in low to relatively high salinity, and soils with these salinity levels (low to relatively high salinity) are suitable for these types of plants.     

Soil quality changes in land degradation as indicated by soil chemical, biochemical and microbiological properties in a karst area of southwest Guizhou, China

Not only the nutritional status and biological activity but also the soil ecological functioning or soil health has been impacted profoundly by land degradation in the karst area of southwest China where the karst ecosystems are generally considered as extremely vulnerable to land degradation under intensified land-use changes. The objectives of this study are to elucidate the changes in overall soil quality by a holistic approach of soil nutritional, biological activity, and soil health indicators in the karst area as impacted by intense cultivation and vegetation degradation. Topsoil samples were collected on selected eco-tesserae in a sequence of land degradation in a karst area of southwest Guizhou in 2004. The soil nutrient pools of organic carbon (C_org), extractable extracellular carbon (C_ext), total soil nitrogen (N_t), alkali-hydrolyzable nitrogen (N_ah), total phosphorus (P_t), available phosphorus (P_a) were analyzed by wet soil chemistry. The soil biological properties were studied by means of measurements of microbial biomass carbon (both by fumigation–extraction, FE-C_mic, and by calculation from substrate-incubation respiration, SIR-C_mic) of respiration [respiration without addition of substrates, basal respiration (BR), and potential respiration (PR) with substrate-incubation] and of soil enzyme activities (invertase, urease, and alkaline phosphatase). Soil health status was assessed by simple indices of C_mic/C_org and BR/C_mic in conjunction with bacterial community structures determined by polymerase chain reaction and denaturing gradient gel electrophoresis. While the nutritional pool parameters, such as C_org and C_ext, described basically the changes in soil life-supporting capacity with cultivation interference and vegetation declined, those parameters of biological activity such as FE-C_mic, SIR, and SIR-C_mic as well as bacterial community structures measured by molecular method evidenced well the changes in soil functioning for ecosystem health with the land degradation.     

Bayesian hierarchical models for soil CO<Subscript>2</Subscript> flux and leak detection at geologic sequestration sites

Proper characterizations of background soil CO₂ respiration rates are critical for interpreting CO₂ leakage monitoring results at geologic sequestration sites. In this paper, a method is developed for determining temperature-dependent critical values of soil CO₂ flux for preliminary leak detection inference. The method is illustrated using surface CO₂ flux measurements obtained from the AmeriFlux network fit with alternative models for the soil CO₂ flux versus soil temperature relationship. The models are fit first to determine pooled parameter estimates across the sites, then using a Bayesian hierarchical method to obtain both global and site-specific parameter estimates. Model comparisons are made using the deviance information criterion (DIC), which considers both goodness of fit and model complexity. The hierarchical models consistently outperform the corresponding pooled models, demonstrating the need for site-specific data and estimates when determining relationships for background soil respiration. A hierarchical model that relates the square root of the CO₂ flux to a quadratic function of soil temperature is found to provide the best fit for the AmeriFlux sites among the models tested. This model also yields effective prediction intervals, consistent with the upper envelope of the flux data across the modeled sites and temperature ranges. Calculation of upper prediction intervals using the proposed method can provide a basis for setting critical values in CO₂ leak detection monitoring at sequestration sites.     

土壤碳蓄积量变化的影响因素研究现状

土壤碳库的动态平衡影响作物产量和土壤肥力的高低，是土壤肥力保持和提高的重要研究内容。简要评述了土壤理化特性、温度和降水变化、大气CO₂浓度上升、人类的农业活动对土壤有机碳蓄积量的影响，介绍了当前对土壤碳蓄积量动态变化的研究进展，认为应加强气候变化和土地利用/土地覆被变化与土壤碳循环研究的结合，提高对陆地生态系统碳循环变化的认识，并需要从生态环境保护的利益和可持续发展的理论出发，进一步加强土地管理方式的改变，促进土壤有机质的积累，提高土壤对碳的固定。     

Analysis of an oasis microclimate in China’s hyperarid zone

The microclimate of a desert oasis in China’s hyperarid zone was monitored, analysed and compared to that of nearby forested lands. Factors associated with differences in photosynthetically active radiation (PAR) between clear, cloudy and dust storm days are discussed. Desert oases were shown to fulfill ecological functions such as altering solar radiation, adjusting near-ground and land surface temperatures, reducing temperature differences, lowering wind velocity, and increasing soil and atmospheric humidity. Total solar radiation within the oasis was roughly half that above the forest canopy. During the growing season, air temperatures in Populus euphratica Olivier and Tamarix ramosissima Ledeb. woodlands were, on average, 1.62 and 0.83°C lower, respectively, than that in surrounding woodlands. The greater the forest cover, the greater was the difference in temperature. Air temperature was higher at the upper storey than that at the lower storey of the community, i.e., air temperature increased with increasing height above the soil surface. During the growing season, relative humidity was higher in woodlands than in surrounding areas: relative humidity in P. euphratica and T. ramosissima woodlands were, on average, 8.5 and 4.2% higher, respectively, than that in the surrounding area. Mean wind velocity in the P. euphratica forest land was 0.33 m/s, 2.31 m/s lower than that in the surrounding area. On dust storm days PAR and total radiation, Q, were significantly lower than that on cloudy or clear days. Their ratio, η _Q  = PAR/Q, was larger and much more variable on dust storm days than that on clear or cloudy days.     

塔里木河流域干旱区棉田土壤呼吸的温度敏感性研究

利用阿克苏绿洲农田试验站连续测定的棉田土壤呼吸值及同步测定的土壤温度与湿度数据, 构建通用的土壤呼吸温度敏感性模型. 结合构建的模型, 对5个常用的土壤呼吸温度模型所模拟的研究区的土壤呼吸变化的温度敏感性进行评析. 结果表明: Arrhenius模型能较好的预测研究区的土壤呼吸温度敏感性的变化; 研究区土壤呼吸温度敏感性随温度的升高呈下降的趋势. 两类试验样地土壤呼吸温度敏感性存在明显的差异性, 这可能与两类试验样地土壤呼吸组分的差异有关, 这是以后需要重点关注的地方.     

Statistical analysis of the temporal stability of soil moisture in three desert regions of northwestern China

Soil moisture and its variations are key factors for understanding hydrological processes, which are characterized by a high temporal variability at different scales. The study was conducted at three field stations in the desert regions of northwestern China, where soil moisture measurements with gravimetric method were used to characterize the temporal stability of soil moisture using various statistical parameters and an index of temporal stability (ITS). The soils are a gray–brown desert soil at the Linze station, an aeolian sandy soil at the Fukang station, and a brown desert soil at the Cele station. Soil textures are accordingly sandy loam at Linze and Cele, and loamy sand at Fukang. The dynamic variation in soil moisture depends strongly on the rainfall pattern (amount and frequency) in these desert ecosystems. Soil moisture content is low and significantly different among the three desert ecosystems, with the maximum at the Linze station (6.61 ± 2.08 %), followed by the Cele (4.83 ± 0.81 %) and Fukang (3.46 ± 0.47 %) stations. The temporal pattern exhibits high variability because soil moisture is characterized by low temporal stability and a high coefficient of variation (CV). The standard deviation, CV, and ITS increase significantly with increasing soil moisture. Soil moisture displays a skewed frequency distribution that follows a logarithmic function at lower soil moisture but a log-normal distribution at higher values.     

Soil CO2 concentration in biological soil crusts and its driving factors in a revegetated area of the Tengger Desert,Northern China

Biological soil crusts (BSCs) are an important cover in arid desert landscapes, and have a profound effect on the CO₂ exchange in the desert system. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the soil CO₂ concentration in individual layers of the soil profile. In this study, the spatiotemporal dynamics of CO₂ concentration throughout the soil profile under two typical BSCs (algae crusts and moss crusts) and its driving factors were examined in a revegetated sandy area of the Tengger Desert from Mar 2010 to Oct 2012. Our results showed that the mean values of the vertical soil CO₂ concentrations under algal crusts and moss crusts were 600–1,200 μmol/mol at the 0–40 cm soil profiles and increased linearly with soil depth. Daily CO₂ concentrations showed a single-peak curve and often had a 1–2 h time delay after the maximum soil temperature. During the rainy season, the mean soil CO₂ concentration profile was 1,200–2,000 μmol/mol, which was 2–5 times higher as compared to the dry season (400–800 μmol/mol). Annually, soil moisture content was the key limiting factor of the soil CO₂ concentration, but at the daily time scale, soil temperature was the main limiting factor. Combined with infiltration depth of crusted soils, we predicted that precipitation of 10–15 mm was the most effective driving factor in arid desert regions.     

Experimental modeling of the influence of the rise in average summer temperatures on carbon circulation in tundra ecosystems

A sealed vegetation chamber was designed and constructed for physical simulation of climate conditions in the Subarctic zone during the spring–summer time. The small laboratory tundra-simulating ecosystem (TSE) was created for comparative evaluation of the rates of soil respiration and of the total balance of carbon fluxes in tundra ecosystems. The test experiment was performed to study the TSE response to a temperature rise in air and soil by 2°C in terms of the intensity of the СО₂ flux. It was shown that this increase in temperature would cause a pronounced shift in the balance of СО₂ production and utilization in the ecosystem from near-zero values to a stable generation of 24 μmol/h of CO₂ per 1 kg of dry biomass.     

CODA Q estimates for Kumaun Himalaya

CodaQ (Q _c) estimates for the Kumaun Himalaya region have been obtained in high frequency range. Local earthquakes, recorded by a digital seismic network in the region, which fall in the epicentral distances range of 10 to 80 km and with a local magnitude range of 1.4 to 2.8, have been used. The coda waves of 30 sec window length, filtered at seven frequency bands centered at 1.5, 3, 6, 9, 12, 18 and 24Hz, have been analysed using the single backscattering model. The values ofQ _c estimates vary from 65 to 283 at 1.5 Hz to 2119 to 3279 at 24.0 Hz which showed thatQ _c is frequency dependent and its value increases as frequency increases. A frequency-dependentQ _c relationship,Q _c = (92 ± 4.73)f^(1.07±.023), is obtained for the region representing the average attenuation characteristics of seismic waves for Kumaun Himalaya region.     

Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ¹³C and δ¹⁸O values of carbonates from the Atacama are the highest of any desert in the world. δ¹³C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO₂ production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ¹³C value of soil carbonate in the Atacama, whereas the fraction C₃ to C₄ biomass at individual sites has a subordinate influence. The high average δ¹³C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO₂.δ¹⁸O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ¹³C values. δ¹⁸O values for soil carbonate predicted from local temperature and δ¹⁸O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ¹⁸O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.