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   175篇
  免费   24篇
  国内免费   31篇
测绘学   4篇
大气科学   8篇
地球物理   86篇
地质学   32篇
海洋学   82篇
综合类   8篇
自然地理   10篇
  2023年   3篇
  2022年   3篇
  2021年   3篇
  2020年   7篇
  2019年   1篇
  2018年   2篇
  2017年   8篇
  2016年   5篇
  2015年   8篇
  2014年   11篇
  2013年   8篇
  2012年   4篇
  2011年   20篇
  2010年   13篇
  2009年   7篇
  2008年   24篇
  2007年   10篇
  2006年   17篇
  2005年   6篇
  2004年   6篇
  2003年   8篇
  2002年   12篇
  2001年   8篇
  2000年   2篇
  1999年   7篇
  1998年   3篇
  1997年   3篇
  1996年   3篇
  1995年   1篇
  1994年   6篇
  1993年   4篇
  1992年   1篇
  1991年   3篇
  1990年   1篇
  1989年   1篇
  1982年   1篇
排序方式: 共有230条查询结果,搜索用时 31 毫秒
111.
112.
113.
A large pool of nitrogen in the sediment pore fluid of a eutrophic lake in Iowa, USA, was mapped in this study. Previously, the lake had supported fishing and boating, but today it no longer supports its designated uses as a recreational water body. In the top 5 cm of the lake bottom, the pore water nitrogen ranges between 3.1 and 1,250 μg/cm3 of sediments, with an average of 160.3 μg/cm3. Vertically, nitrate concentrations were measured as 153 μg/cm3 at 0–10 cm, 162 μg/cm3 at 10–20 cm, and 32 μg/cm3 at 20–30 cm. Nitrate mass distribution was quantified as 3.67 × 103 kg (65%) in the bottom sediments, 172 kg (3%) in suspended particulates, and 1.83 × 103 kg (32%) in the dissolved phase. Soil runoff nutrients arrive at the lake from the heavily fertilized lands in the watershed. Upon sedimentation, a large mass of nitrogen desorbs from mineral particles to the relatively immobile pore fluid. Under favorable conditions, this nitrogen diffuses back into the water column, thereby dramatically limiting the lake’s capability to process incoming nutrients from farmlands. Consequently, a condition of oxygen deficiency disrupts the post-season biological activities in the lake.  相似文献   
114.
In the taiga gypsum karst ecosystems, gypsum soils formed on the hard gypsum substrates predominate in the soil cover. In these soils, the mineral horizons consist of 95–99% gypsum (CaSO4·2H2O) and the litter is the main horizon for nutrient accumulation. For this reason, the ecosystems are vulnerable to fire and erosion by walkers, from which they only recover slowly. Gypsum mining for industrial uses is also leading to the destruction of this unique ecosystem.  相似文献   
115.
太湖地区农田土壤养分动态及其启示   总被引:24,自引:2,他引:24  
高超  吴蔚东  张桃林 《地理科学》2001,21(5):428-432
太湖地区人地关系紧张,环境问题尤其是地表水体富营养化严重,农田生态系统的养分收支和土壤养分状况不仅关系到农业的高产稳产,对区内的水环境质量也有着重要的影响。典型区的实地调查和采样分析结果表明,自20世纪80年代初以来太湖地区农田生态系统中的氮、磷一直处于盈余状态,且盈余量不断增加,土壤钾素则长期亏缺,但近年来有所缓和。区内土壤有机质、全氮、全磷和有效磷含量近20年来都有较大幅度的提高,速效钾含量在20世纪90年代中期以前呈下降趋势,近年来有所回升。主要类型土壤的pH也呈降低的趋势。  相似文献   
116.
Patterns of zooplankton–phytoplankton interactions in subtropical lakes of the Southern Hemisphere may deviate from those established for north-temperate lakes. We tested the responses of phytoplankton growth to different community structures of zooplankton and nutrient enrichment in a subtropical Australian reservoir for the prediction of potential outcomes of lake biomanipulation. Two zooplankton communities were created in lake enclosures over 4 weeks: a rotifer-dominated community developed in the presence of planktivorous fish (Hypseleotris spp.) and a Ceriodaphnia-dominated community developed in the absence of fish. Biomass gradients of both communities were established in 20 L containers and several separate containers received no additions (controls) or were enriched with nitrogen and/or phosphorus. The growth rate of total phytoplankton significantly increased in response to nutrient enrichment, indicating nutrient limitation. Most phytoplankton taxa were not markedly affected by grazing of either zooplankton community. However, both communities had significant stimulatory effects on the growth of inedible chlorophytes. The ability of zooplankton grazing to negatively affect phytoplankton growth during the summer was counteracted regardless of zooplankton community structure, possibly by nutrients regenerated by zooplankton. We hypothesise that in the subtropical system studied, changes in food web nutrient recycling may be more important for the outcome of biomanipulation than grazing impacts.  相似文献   
117.
We investigated C and N cycling in long-term agroecological experiments initiated over 50 years ago at a cool, semi-arid site on the North American Great Plains. We used isotopes at natural abundance to trace C and N exchange between soils and plants in contrasting cropping systems. Both 13C and 14C indicated that the soil organic matter was isotopically distinct from current plant inputs, suggesting that recently added plant C was cycling independently of much of the soil C pool. For tracing recent C flows, bomb-14C was more sensitive than 13C, and increased more in high – than in low – yielding systems. Analysis of 15N in plant tissues, as an index of 15N in actively cycling soil N, suggested that biological and industrial N fixation both tended to decrease plant 15N, whereas livestock manure addition increased 15N abundance. Collectively, the data suggest that soil organic matter is kinetically heterogeneous, so that a majority of soil C and N inputs and outputs exchange with only the small pool of soil organic matter that is actively cycling. Consequently, recently photosynthesized C and deposited N may not readily enter the old, stable fractions of soil organic matter. Practices to retain CO2 from the atmosphere and prevent leakage of reactive N to non-agricultural systems should therefore focus on management of this active pool.  相似文献   
118.
119.
为探讨中高山区土壤及成土母质理化特征及其控制因素,本文以四川省西昌市为例,系统性总结了该地区的地质建造背景,并在基于地质建造背景划分的成土母质类型基础上,讨论了成土母质、土壤的物理化学性质、土壤含水率、pH值、高程等之间的关系,通过对不同成土母质发育的同种土壤的数据分析及营养元素含量评价,我们认为地质建造背景直接制约了成土母质及土壤的物理化学性质,营养元素在成土母质—土壤之间的迁移存在继承性。本文研究成果为中高山区土壤类型和成土母质之间的关系以及成土母质单元划分方案提供理论依据。  相似文献   
120.
在安龙县耕地质量地球化学调查评价的基础上,通过对不同成土母岩、不同土壤类型和不同耕地地类表层土壤中有机质、N、P、K、Mn、B、Cu、Mo、Zn等营养元素进行分析研究。研究结果表明:(1)营养元素平均含量由高到低依次为:有机质>K>N>Mn>P>Zn>B>Cu>Mo,其中P、Mn、Mo、Cu、Zn平均含量高于贵州和全国背景值;有机质、N、B平均含量高于全国背景值、低于贵州背景值;K平均含量低于贵州和全国背景值。有机质、N、P、K、B 、Cu变异系数小于060,含量变化幅度相对较小; Mn、Mo、Zn变异系数在091110间,含量变化幅度相对较大。(2)不同成土母岩土壤表层营养元素中,有机质与N、B,N与B,Mn与Zn在 001 水平上显著正相关。P与K在 001 水平上显著负相关;Cu与有机质、N、B在 005 水平上显著负相关。(3)不同土壤类型营养元素主要呈微-弱富集或微-弱贫化,仅石灰土中Mn为中等富集、红壤中Cu为中等贫化。耕地表层土壤营养元素富集与贫化受成土母岩、土壤类型和耕地地类影响,成土母岩为主要影响因素。  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