全文获取类型
收费全文 | 195篇 |
免费 | 14篇 |
国内免费 | 17篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 8篇 |
地球物理 | 100篇 |
地质学 | 40篇 |
海洋学 | 45篇 |
综合类 | 3篇 |
自然地理 | 27篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 4篇 |
2020年 | 3篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 5篇 |
2014年 | 7篇 |
2013年 | 12篇 |
2012年 | 8篇 |
2011年 | 14篇 |
2010年 | 14篇 |
2009年 | 19篇 |
2008年 | 29篇 |
2007年 | 11篇 |
2006年 | 13篇 |
2005年 | 8篇 |
2004年 | 4篇 |
2003年 | 10篇 |
2002年 | 8篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
排序方式: 共有226条查询结果,搜索用时 31 毫秒
51.
Grass nitrogen (N) and phosphorus (P) concentrations are direct indicators of rangeland quality and provide imperative information for sound management of wildlife and livestock. It is challenging to estimate grass N and P concentrations using remote sensing in the savanna ecosystems. These areas are diverse and heterogeneous in soil and plant moisture, soil nutrients, grazing pressures, and human activities. The objective of the study is to test the performance of non-linear partial least squares regression (PLSR) for predicting grass N and P concentrations through integrating in situ hyperspectral remote sensing and environmental variables (climatic, edaphic and topographic). Data were collected along a land use gradient in the greater Kruger National Park region. The data consisted of: (i) in situ-measured hyperspectral spectra, (ii) environmental variables and measured grass N and P concentrations. The hyperspectral variables included published starch, N and protein spectral absorption features, red edge position, narrow-band indices such as simple ratio (SR) and normalized difference vegetation index (NDVI). The results of the non-linear PLSR were compared to those of conventional linear PLSR. Using non-linear PLSR, integrating in situ hyperspectral and environmental variables yielded the highest grass N and P estimation accuracy (R2 = 0.81, root mean square error (RMSE) = 0.08, and R2 = 0.80, RMSE = 0.03, respectively) as compared to using remote sensing variables only, and conventional PLSR. The study demonstrates the importance of an integrated modeling approach for estimating grass quality which is a crucial effort towards effective management and planning of protected and communal savanna ecosystems. 相似文献
52.
C. P. C. David Y. Y. Sta. Maria F. P. Siringan J. M. Reotita P. B. Zamora C. L. Villanoy E. Z. Sombrito R. V. Azanza 《Environmental Geology》2009,58(2):447-454
The supply of nitrogen and phosphorus in coastal zones through time is reflected in the nutrients’ concentration in the sediment
record. Five aquaculture sites in the Philippines were investigated in an effort to establish how long-term changes in land
and coastal water use could have led to biogeochemical modifications affecting the coastal ecosystem. Samples from study sites
show a narrow concentration range for nitrogen and did not reveal any significant trend through time. In contrast, phosphorus
concentrations in most sites start at less than 20 ppm in sediments 30 years and older. The phosphorus value continuously
increase in younger sediments, with each site having a different magnitude change as well as timing of when the major increase
happened. The uppermost 10 cm, representing the last 15 years in sites with age control, typically show a 2- to 3-fold increase
in P load values. Historical increase in nutrient load also coincides with harmful algal bloom events in each area; when effective
P input exceeded 130 kg/km2 per year. Lastly, the observed increase may be attributed to several factors including physical attributes of the area, urbanization
of coastal zones, but most importantly in the proliferation of aquaculture activities. 相似文献
53.
Phosphorus in suspended matter and sediments of a hypertrophic lake. A case study: Lake Dianchi,China 总被引:1,自引:0,他引:1
From June 2004 to December 2004, Lake Dianchi, which had large scale of cyanobacterial blooms was investigated in order to
study P-fractionation in the suspended matter and the sediment. The investigation improves our understanding of phosphorus
in Lake Dianchi and the relationship between phosphorus and cyanobacterial blooms. It contributes to the available literature
on the behavior of P in hypertrophic lakes. The distribution of P-fractions in Lake Dianchi was not uniform from northwest
to south, but was closely related to the trophic status of the whole lake. The concentrations of total phosphorus, labile
P (NH4Cl-P), Organic P (NaOH-NRP) and loss on ignition in suspended matter were positively correlated with the strength of cyanobacterial
blooms. Total phosphorus in suspended matter was relatively stable for almost half an year and closely related to Chl. a concentration. The main content of organic phosphorus is in the cyanobacterial blooms. The concentrations of phosphorus bound
to metal oxides and carbonates (NaOH-SRP and HCl-P) in sediment were similar to NaOH-SRP and HCl-P in the corresponding suspended
matter. The latter two forms of P in suspended matter were not affected by cyanobacterial blooms, indicating that the inorganic
phosphorus is derived from the sediment after resuspension from the sediment due to wind and wave action. The contribution
of the different P-fractions to TP in sediment and in suspended matter indicates that NH4Cl-P in the suspended matter is an important buffer for maintaining dissolved phosphorus in water. 相似文献
54.
营养盐对中肋骨条藻和新月菱形藻生长及氮磷组成的影响 总被引:24,自引:4,他引:20
:于1995年5月1996年3月通过一次培养实验,研究了培养介质中营养盐含量变化对中肋骨条藻和新月菱形藻生长速率和藻体氮、磷含量的影响。根据Monod方程,磷酸盐影响中肋骨条藻和新月菱形藻生长速率的半饱和常数分别为0.54μmol/L和0.50μmol/L;硝酸盐影响两种藻生长速率的半饱和常数分别为4.0μmol/L和14.6μmol/L,相比之下新月菱形藻更容易受到氮不足的限制。将Monod方程扩展得到营养限制复合模式,限制程度可用限制作用系数定量化表示,代表氮或磷。培养介质中营养盐浓度是限制浮游植物生长的决定因素,其N/P比值则反映NO_3-N和PO_4-P何者起主要限制作用。藻体氮、磷含量随和有明显变化。当<0.80时较低,磷含量因贮存而明显升高;当<0.80时藻体磷含量下降,氮含量略有增加。藻体氮含量或>0.80时基本上为非限制状态。 相似文献
55.
Seasonal changes in water quality were measured in samples taken at various distances from shallow water across mudflat to mangroves during flood period and from mangroves across mudflat to shallow water during ebb period in a subtropical mangrove estuary (Zhangjiang Estuary, Fujian, China). The TN (total dissolved nitrogen), TP (total dissolved phosphorus), COD (chemical oxygen demand), and DOC (dissolved organic carbon) contents during the flood period were significantly higher than those during the ebb period. In contrast, the opposite was true for the POC (particulate organic carbon) content and transparency. The mangroves at Zhangjiang Estuary may trap nutrients at rates of 90.5 g N/m2/yr, 2.2 g TP/m2/yr, and 13.7 g C/m2/yr in the form of DOC, and export POC at a rate of 81.8 g/m2/yr. Our results support the hypothesis that the maintenance of estuarine water quality by mangroves occurs during flood periods. 相似文献
56.
Application of allochthonous organic carbon and phosphorus forms in the interpretation of past environmental conditions 总被引:2,自引:0,他引:2
Changwei Lü Jiang He Huimin Sun Hongxi Xue Ying Liang Saruli Bai Ying Sun Lili Shen Qingyun Fan 《Environmental Geology》2008,55(6):1279-1289
Organic matter in sediments, for instance, carbon, nitrogen and phosphorus, can be used to reconstruct the paleoecological
and pollution history of lakes and their catchment basins. In this paper, the contents of allochthonous organic carbon (allochthonous
OC) and autochthonous organic carbon (autochthonous OC) in sediment cores taken from Wuliangsuhai Lake and Daihai Lake in
northern China are quantified by using a binary model, and phosphorus forms in the sediment cores from the two lakes are extracted
by sequential extraction techniques. The results indicate that the palaeoenvironment and paleoclimate of Daihai Lake and
its catchment basin in the recent 250 years can be well reconstructed based on the content of allochthonous OC. The climate
was relatively humid and warm in the period of 1865–2005, while relatively dry and cold in the period of 1765–1865. The sedimentary
information of allochthonous OC in the 22–42-cm portion of the sediment cores in Daihai Lake corresponds to the final cold
fluctuation of the Little Ice Age that occurred since the Middle Holocene. The difference of phosphorus forms in the sediment
cores between the two lakes indicates that phosphorus input to the lakes and the correlation between phosphorus forms and
distribution and the changes of environment are influenced by the eutrophication mechanisms and environmental conditions of
the two lakes. 相似文献
57.
58.
A high sensitivity, low volume HPLC method to determine soluble reactive phosphate in freshwater and saltwater 总被引:1,自引:0,他引:1
A reverse phase High Performance Liquid Chromatography (HPLC) method was developed to detect soluble reactive phosphate (SRP) in marine and freshwater samples. This technique is scalable over a range of sample volumes from 10 μl to 10 ml, requires minimal sample preparation, and responds linearly from 1 nM to 100 μM (R2>0.964). Standard Murphy–Riley molybdenum blue reagents were used to react with phosphate in samples for analysis. Solid phase extraction tubes were used to clean the mixed complexing reagent prior to combining the reagent with samples. The method was applied to freshwater samples collected in July 2001 along an Andean mountain stream located in the Eastern Cordillera (2400 m) of Peru and marine samples collected in January 2002 within the low marsh of the estuarine Nueces Delta System, Texas Coastal Bend. For method comparison, field samples were analyzed using a standard manual spectrophotometric method. Field sample phosphate concentrations produced from each method were nearly identical. This method offers the advantages of low sample volume (1 ml or less) with high sensitivity, precision, and potential automation. 相似文献
59.
太湖现代沉积物中磷的沉积通量及空间差异性 总被引:5,自引:0,他引:5
在水文状况、水质状况、生态类型均显著不同的三个太湖典型湖区, 利用^210Pb定年测定了近百年来沉积物及磷的沉积通量.结果发现, 三个湖区磷的沉积通量及其变化规律相差很大.发生蓝藻水华暴发的梅梁湾近百年来沉积物和磷的沉积通量都比较小, 只是在近十几年来才有明显增加的趋势; 太湖上游的夹浦沉积物沉积速率近百年来持续增加, 只有表层有所下降, 但表层12cm沉积物中磷的沉积通量却显著下降; 处于太湖下游草型化的胥口湾湖区, 在20世纪60年代以前沉积物沉积通量呈增加趋势, 之后呈下降趋势, 磷的沉积通量则呈振荡下降趋势.研究表明, 太湖沉积物中磷的累积通量具有明显的空间异质性. 相似文献
60.
Viant MR Pincetich CA Walton JH Tjeerdema RS Hinton DE 《Marine environmental research》2002,54(3-5):553-557
In vivo nuclear magnetic resonance spectroscopy (NMR) is a powerful technique for characterizing the sublethal actions of physical and chemical stressors in live, intact organisms. In particular, 31P NMR is ideal for observing perturbations to cellular energetics since critical metabolite concentrations, including phosphagens, ATP and inorganic phosphate (Pi), can be measured non-invasively and in real time. This technique's versatility is demonstrated not only in the diversity of organisms that can be studied, but also in its broad-ranging applicability to environmental research. Illustrative studies include the actions of copper in adult red abalone (Haliotis rufescens) and changes in energetically important metabolites in developing medaka embryos (Oryzias latipes). Advantages and disadvantages of in vivo NMR will be discussed. 相似文献