首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   47篇
  免费   5篇
  国内免费   4篇
大气科学   3篇
地球物理   9篇
地质学   12篇
综合类   12篇
自然地理   20篇
  2022年   2篇
  2021年   1篇
  2019年   1篇
  2016年   2篇
  2015年   2篇
  2014年   7篇
  2013年   1篇
  2012年   3篇
  2011年   5篇
  2010年   5篇
  2009年   2篇
  2008年   1篇
  2007年   2篇
  2006年   5篇
  2005年   4篇
  2004年   3篇
  2003年   2篇
  2002年   1篇
  2001年   2篇
  1999年   2篇
  1997年   1篇
  1995年   1篇
  1994年   1篇
排序方式: 共有56条查询结果,搜索用时 15 毫秒
1.
Land use changes are known to alter soil organic carbon (SOC) and microbial properties, however, information about how conversion of natural forest to agricultural land use as well as plantations affects SOC and microbial properties in the Changbai Moun- tains of Northeast China is meager. Soil carbon content, microbial biomass carbon (MBC), basal respiration and soil carbon mineraliza- tion were studied in five selected types of land use: natural old-growth broad-leaved Korean pine mixed forest (NF); spruce plantation (SP) established following clear-cutting of NF; cropland (CL); ginseng farmland (GF) previously under NF; and a five-year Mongolian oak young forest (YF) reforested on an abandoned GF, in the Changbai Mountains of Northeast China in 2011. Results showed that SOC content was significantly lower in SP, CL, GF, and YF than in NF. MBC ranged from 304.4 mg/kg in CL to 1350.3 mg/kg in NF, which was significantly higher in the soil of NF than any soil of the other four land use types. The SOC and MBC contents were higher in SP soil than in CL, GF, and YF soils, yielding a significant difference between SP and CL. The value of basal respiration was also higher in NF than in SP, CL, GF, and YF. Simultaneously, higher values of the metabolic quotient were detected in CL, GF, and YF soils, indicat- ing low substrate utilization of the soil microbial community compared with that in NF and SP soil. The values of cumulative mineral- ized carbon and potentially mineralized carbon (Co) in NF were significantly higher than those in CL and GF, while no significant dif- ference was observed between NF and SP. In addition, YF had higher values of Co and C mineralization rate compared with GF. The results indicate that conversion from NF into agricultural land (CL and GF) uses and plantation may lead to a reduction in soil nutrients (SOC and MBC) and substrate utilization efficiency of the microbial community. By contrast, soils below SP were more conducive to the preservation of soil organic matter, which was reflected in the comparison of microbial indicators among CL, GF, and YF land uses. This study can provide data for evaluating soils nutrients under different land use types, and serve as references for the rational land use of natural forest in the study area.  相似文献   
2.
A growing body of field, theoretical and numerical modelling studies suggests that predicting river response to even major changes in input variables is difficult. Rivers are seen to adjust rapidly and variably through time and space as well as changing independently of major driving variables. Concepts such as Self‐Organized Criticality (SOC) are considered to better reflect the complex interactions and adjustments occurring in systems than traditional approaches of cause and effect. This study tests the hypothesis that riverbank mass failures which occurred both prior to, and during, an extreme flood event in southeast Queensland (SEQ) in 2011 are a manifestation of SOC. Each wet‐flow failure is somewhat analogous to the ‘avalanche’ described in the initial sand‐pile experiments of Bak et al. (Physical Review Letters, 1987, 59(4), 381–384) and, due to the use of multitemporal LiDAR, the time period of instability can be effectively constrained to that surrounding the flood event. The data is examined with respect to the key factors thought to be significant in evaluating the existence of SOC including; non‐linear temporal dynamics in the occurrence of disturbance events within the system; an inverse power‐law relation between the magnitude and frequency of the events; the existence of a critical state to which the system readjusts after a disturbance; the existence of a cascading processes mechanism by which the same process can initiate both low‐magnitude and high‐magnitude events. While there was a significant change in the frequency of mass failures pre‐ and post‐flood, suggesting non‐linear temporal dynamics in the occurrence of disturbance events, the data did not fit an inverse power‐law within acceptable probability and other models were found to fit the data better. Likewise, determining a single ‘critical’ state is problematic when a variety of feedbacks and multiple modes of adjustment are likely to have operated throughout this high magnitude event. Overall, the extent to which the data supports a self‐organized critical state is variable and highly dependent upon inferential arguments. Investigating the existence of SOC, however, provided results and insights that are useful to the management and future prediction of these features. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
3.
Employing the Unit Soil Carbon Amount (USCA) approach, soil carbon storage was calculated across the Northeast Plain of China based on the Multi-purpose Regional Geochemical Survey conducted in 2004 – 2006 (MRGS). The results indicated that the soil organic carbon (SOC) storage in topsoil (0 – 0.2 m), subsoil (0 – 1 m) and deep soil (0 – 1.8 m) was 768.1 Mt, 2978.4 Mt and 3729.2 Mt with densities of 3327.8 t/km2, 12,904.7 t/km2 and 16,157.5 t/km2, respectively. These values were consistent with national averages, whereas the soil carbon densities showed a clear increasing trend from the southern area of the Northeast Plain (Liaoning), to the middle (Jilin) and the northern Plain (Heilongjiang) — particularly in terms of topsoil carbon density, which increased from 2284.2, to 3436.7 and 3861.5 t/km2, respectively. In comparison to carbon data obtained from the Second National Soil Survey in 1984 – 1986 (SNSS), the topsoil SOC storage values from the MRGS were found to have decreased by 320.59 Mt (29.4%), with an average annual decline of 16.0 Mt (l.73%) over the 20 years. In the southern, middle and northern areas of the plain, soil carbon densities decreased by 1060.6 t/km2, 1646.4 t/km2 and 1300.2 t/km2, respectively, with an average value of 1389.0 t/km2 for the whole plain. These findings indicate that the decrease in soil carbon density varied according to the different ecosystems and land use types. Therefore, ratios of soil carbon density were calculated in order to study the carbon dynamic balance between ecosystems, and to further explore distribution characteristics, as well as the sequestration potential of SOC.  相似文献   
4.
Rapid urbanization results in the conversion of natural soil to urban soil,and consequently,the storage and density of the soil carbon pools change.Taking Chongqing Municipality of China as a study case,this investigation attempts to better understand soil carbon pools in hilly cities.First,the vegetated areas in the study area were derived from QuickBird images.Then,topsoil data from 220 soil samples(0-20 cm) in the vegetated areas were collected and their soil organic carbon(SOC) densities were analyzed.Using the Kriging interpolation method,the spatial pattern of SOC was estimated.The results show that the SOC density exhibited high spatial variability in the urban topsoil of Chongqing.First,the SOC density in topsoil decreased according to slope in the order 2°-6° < 25°-90° < 0°-2° < 6°-15° < 15°-25°.Second,the newly developed areas during 2001-2010 had a lower SOC density than the areas built before 1988.Third,urban parks and gardens had a higher SOC density in topsoil,residential green land followed,and scattered street green land ranked last.For hilly cities,the variability of terrain affects the distribution of SOC.The Kriging results indicate that Kriging method combining slope with SOC density produced a high level of accuracy.The Kriging results show that the SOC density to the north of the Jialing River was higher than the south.The vegetated areas were estimated to amount to 73.5 km2 across the study area with an SOC storage of 0.192 Tg and an average density of 2.61 kg/m2.  相似文献   
5.
"5·12"汶川特大地震引发的次生山地灾害中以山体崩塌数量最多.本文在对国道213线都江堰至映秀段以及水磨支线公路边坡地震崩塌调查研究的基础上,分析总结了岩土体边坡的崩塌成灾模式、崩塌作用机理以及崩塌自组织临界(SOC)动力学特性.调查范围包括Ⅸ~Ⅺ度地震区,工点105个.对崩塌体方量、崩塌深度进行统计分析后发现,在Ⅸ度区崩塌体方量和崩塌深度都符合负幂律分布,呈现出明显的自组织临界动力学特性;而Ⅹ、Ⅺ度地震区不呈现该崩塌特性,其崩塌动力学性质受地震的强扰作用控制.  相似文献   
6.
洞庭湖区土壤实测碳量的分布规律及其对全球变化的响应   总被引:2,自引:1,他引:1  
研究区域土壤碳库的分布特征对于全球变化研究、土壤肥力和生态系统的功能评价具有重要意义。在缺乏长期实测资料情况下,国内外多以模型、第二次土壤普查资料或较稀的实测数据进行研究。文中利用洞庭湖区生态地球化学调查实测数据,研究影响该区土壤碳分布的因素认为区域土壤碳量处于全国的中、高级水平;地质地貌条件是影响土壤无机碳量分布的最重要因素;土壤类型对土壤碳量有影响;本区水土流失强度对表层土壤总有机碳量影响不大;经济林、竹林的土地利用方式及防治土壤潜育化有利于土壤的碳储存;从TOC等沉积的时空演化规律说明,本区环境变化是对全球环境变化的响应,为区域土地利用、促进土壤碳固定及全球环境变化研究提供了依据。  相似文献   
7.
耕作措施对陕西耕作土壤碳储量的影响模拟*   总被引:6,自引:0,他引:6  
张凡  李长生  王政 《第四纪研究》2006,26(6):1021-1028
鉴于农业生态系统土壤有机碳(SOC)平衡对中国农业可持续发展的重要性,文章以陕西农业生态系统为对象,整合农业生物地球化学模型(DNDC)与陕西农业地理信息系统数据库,利用陕西地区气象和作物资料,对陕西省2000年作物生长发育和土壤碳循环进行了模型模拟,实例探讨了耕作管理对土壤碳储量的影响,并由此评价生物地球化学模型在气候变化、土壤性质及农业耕作管理措施对土壤碳含量影响方面的预测能力。敏感性分析表明土壤性状,尤其是初始有机碳含量是影响模型区域尺度模拟的最主要敏感因素。区域模拟使用灵敏系数分析法,分别采用敏感因子的最大、最小值驱动模型在每一模拟单元内的运算,以产生一个土壤有机碳变化的范围值,土壤有机碳变化的真实值应以较大机率包含在这一范围内。分析模拟结果可以得出3点结论: 1)2000年陕西耕作土壤总有机碳储量约为103TgC,是一个大气CO2源,向大气释放碳0.5TgC; 2)提高作物秸秆还田率是提高陕西农田碳库储量的有效可行措施,将作物秸秆还田率从当前的15 % 提高到50 % 或90 % 会使陕西农田土壤从大气CO2源转变为汇,每年分别增加土壤有机碳库储量0.7TgC或2.1TgC; 3)施用有机农肥(500kg/hm2)也会增加土壤碳输入,从而提高土壤碳储量,使陕西农田系统转变为较弱的碳(C)源,每年可多固定0.2TgC。  相似文献   
8.
Introduction Digital seismic observation systems originated from 1970′s. It has developed greatly in the past 30 years up to now. Its performances were improved, dynamic range and resolution increased a lot, the power consumption decreased a lot, and so on (YOU et al, 2003a, b). In a word, the dream of broad frequency-band, big dynamic range, digitalization of seismic observation has come true already. But, the previous digital seismic observation systems only support communica-tion based o…  相似文献   
9.
The presence of a low-level, capping inversion layer will affect the height and structure of the planetary boundary layer (PBL). Results from models of varying levels of sophistication, including analytical, turbulent kinetic energy (TKE), second-order closure (SOC), large-eddy simulation (LES) and direct numerical simulation (DNS) models, are used to investigate this influence for the neutral, barotropic PBL. Predicted and observed profiles of stress and geostrophic departure components, and integral measures, such as the parameters of Rossby-number similarity theory, are compared for the KONTUR, Marine Stratocumulus, JASIN, Leipzig, Pre-Wangara and Upavon field experiments.Analytical models of the equilibrium value of inversion height zi, which depend on the surface friction velocity u*, and both the Coriolis parameter f and the free-flow Brunt-Väisälä frequency N, are found to give reasonable estimates of the PBL height. They also indicate that only the KONTUR and Marine Stratocumulus experiments were strongly influenced by N. More quantitative comparisons would require larger, more comprehensive datasets. The effects of the presence of a capping inversion on the profile structure were found to be insignificant for h* = |f|zi/u* > 0.15.The simple analytical model performed quite well over all values of h*; it predicted the profiles of the longitudinal stress component (in the direction of the surface stress) better than the lateral component. The more advanced models performed well for small values of h* (for flow over the sea), but systematically underestimated the cross-isobaric angle for flow over land. These models predicted the profiles of the lateral stress component better than the longitudinal component. The profiles of the analytical model agreed with those of the advanced models when the constant eddy viscosity of the outer layer was increased.Agreement with DNS was achieved by increasing the eddyviscosity of the analytical model by a factor of 5.Zilitinkevich and Esau(2002, Boundary-Layer Meteorology, 104, 371–379)suggest that the neutral, barotropic values of A and B of Rossby-numbersimilarity theory are not universal constants, but depend on the ratio N/|f|. The dependence for A and B is calculated using the analytical model and TKE models. Over the sea (h* 0.1; N/|f| 100, where we have used the Zilitinkevich-Esau relation to convert between h* and N/|f|) there is agreement between the model predictions and observations; however over land where the equilibrium boundary-layer height is greater (h* 0.35; N/|f| 10) the inconsistency between the advanced model predictions (TKE, SOC, LES, and DNS) and observations, as noted previously by Hess and Garratt, still exists. We attribute this disagreement to violations of the strict assumptions of steady, horizontally homogeneous, neutral, barotropic conditions implicit in the observations. At small values of zi and a strongly stable background stratification (h* 0.04; N/|f| 1000) both the TKE and analytical models predict that A and B depend significantly on h*, however observations are unavailable to confirm these predictions. Zilitinkevich and Esau call this case the `long-lived near-neutral PBL', and state that it is found in cold weather at high latitudes.  相似文献   
10.
分形与混沌理论在地震学中的应用与探讨   总被引:1,自引:2,他引:1  
分形与混沌理论的研究与发展,揭示了自然界中一大类无规几何形体物理过程的内在规律性及其动力演化过程的内在随机性。这给我们探索自然界中分形客体的几何形态及其与内部物理本质的关系提供了一条暂新的途径。特别是这一理论给我们探讨地震问题,例如:探讨地震活动时空分布的确定性图像、地震演化过程的动力学行为、临界特征以及未来大地震的发生等问题,带来了某些希望。本文较详细地综述了近年来分形与混沌理论在地震学中的应用研究。全文共分5部分。第1部分是孕震过程的一般概述。第2部分描述单分形与多分形在地震学中的某些应用.文中第3部分介绍了混沌动力学中的某些研究在测震学与模型中的应用。第4部分综述了自组织临界现象和闪变噪声的研究.最后,第5部分是分形理论与混沌动力学在地球科学中的应用前景与展望.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号