全文获取类型
收费全文 | 929篇 |
免费 | 33篇 |
国内免费 | 38篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 9篇 |
2021年 | 15篇 |
2020年 | 26篇 |
2019年 | 28篇 |
2018年 | 27篇 |
2017年 | 28篇 |
2016年 | 24篇 |
2015年 | 22篇 |
2014年 | 26篇 |
2013年 | 93篇 |
2012年 | 16篇 |
2011年 | 13篇 |
2010年 | 16篇 |
2009年 | 18篇 |
2008年 | 24篇 |
2007年 | 21篇 |
2006年 | 39篇 |
2005年 | 36篇 |
2004年 | 48篇 |
2003年 | 39篇 |
2002年 | 33篇 |
2001年 | 40篇 |
2000年 | 29篇 |
1999年 | 37篇 |
1998年 | 22篇 |
1997年 | 26篇 |
1996年 | 25篇 |
1995年 | 12篇 |
1994年 | 20篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 8篇 |
1990年 | 20篇 |
1989年 | 7篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有1000条查询结果,搜索用时 437 毫秒
141.
中药郁金中姜黄素类成分及微量元素分析 总被引:8,自引:0,他引:8
本文报道了用双波长薄层扫描法对5种郁金中姜黄素类化合物的含量测定结果,同时采用原子吸收光谱法、等离子发射光谱法测定和比较了郁金生药中20种微量元素的含量。 相似文献
142.
Philip W. Boyd 《Journal of phycology》2002,38(5):844-861
The factors controlling the distribution of phytoplankton stocks, species composition, and their physiological status in the Southern Ocean are reviewed. In the last decade, the key data sources have been observational and experimental. Together, they provide a framework to understand the complex temporal and spatial patterns of environmental control within the distinct basins and ecological provinces. High resolution remotely sensed observational data have overcome the issue of geographical remoteness. Furthermore, by exploiting seasonal and spatial differences in algal distributions, observational data have enabled the cross‐correlation of such trends with patterns in other environmental properties. Perturbation experiments have offered a mechanistic understanding to help interpret observational data by altering environmental properties under carefully controlled conditions. A consistent set of trends, on the modes of environmental control of phytoplankton processes, is now emerging across the different basins and provinces. The key determinants are light, iron, and silicic acid supply (top‐down control was not considered). However, their interplay in time and space (i.e. simultaneous limitation of phytoplankton processes) is less clear, requires further study, and is discussed. Future challenges include the need to understand better the mode(s) of environmental control on key algal functional groups via more taxon‐ and species‐specific studies. The initiation of more time‐series moorings with “smart” bio‐optical and sampling sensors are needed to define the seasonal distributions of algal taxa. Moreover, new perturbation experiments are required to investigate the influence on phytoplankton processes of projected climate‐mediated alteration of mixed layer depth and nutrient supply as widely predicted by modelers. 相似文献
143.
The immediate effects of tillage on protected soil C and N pools and on trace gas emissions from soils at precultivation levels of native C remain largely unknown. We measured the response to cultivation of CO2 and N2O emissions and associated environmental factors in a previously uncultivated U.S. Midwest Alfisol with C concentrations that were indistinguishable from those in adjacent late successional forests on the same soil type (3.2%). Within 2 days of initial cultivation in 2002, tillage significantly (P=0.001, n=4) increased CO2 fluxes from 91 to 196 mg CO2‐C m?2 h?1 and within the first 30 days higher fluxes because of cultivation were responsible for losses of 85 g CO2‐C m?2. Additional daily C losses were sustained during a second and third year of cultivation of the same plots at rates of 1.9 and 1.0 g C m?2 day?1, respectively. Associated with the CO2 responses were increased soil temperature, substantially reduced soil aggregate size (mean weight diameter decreased 35% within 60 days), and a reduction in the proportion of intraaggregate, physically protected light fraction organic matter. Nitrous oxide fluxes in cultivated plots increased 7.7‐fold in 2002, 3.1‐fold in 2003, and 6.7‐fold in 2004 and were associated with increased soil NO3? concentrations, which approached 15 μg N g?1. Decreased plant N uptake immediately after tillage, plus increased mineralization rates and fivefold greater nitrifier enzyme activity, likely contributed to increased NO3? concentrations. Our results demonstrate that initial cultivation of a soil at precultivation levels of native soil C immediately destabilizes physical and microbial processes related to C and N retention in soils and accelerates trace gas fluxes. Policies designed to promote long‐term C sequestration may thus need to protect soils from even occasional cultivation in order to preserve sequestered C. 相似文献
144.
Complex behavioural patterns and ethological analysis of the trace fossil Zoophycos: evidence from the Lower Devonian of South China
下载免费PDF全文
![点击此处可从《Lethaia: An International Journal of Palaeontology and Stratigraphy》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Li‐Jun Zhang Zhao Zhao 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2016,49(2):275-284
The trace fossil Zoophycos is abundant in the shallow‐marine deposits (tempestites) of the Lower Devonian (Emsian) Yangmaba Formation in Ganxi of Sichuan, South China. It often occurs as part of complex trace fossils that comprise different integrated elements: scratch traces, simple to complex spreiten structures with marginal tubes (Zoophycos) and vertical tunnels. The complex Zoophycos burrows consist of spreiten with a marginal tube, preserved as convex hyporeliefs on the sole of an erosion surface. The exquisite, complex spreiten are interpreted to have been formed by deposit‐feeding behaviour, where the animal constructed the trace upwards without leaving faeces in the spreiten. The width of the marginal tube in different whorls is almost constant. The scratches are observed on the wall of the marginal tubes. The Zoophycos intergrades with Spongeliomorpha and Chondrites and was later cut by vertical shafts. All these features together indicate that the Zoophycos‐maker might have been a vermiform polychaete instead of a predator such as a decapod crustacean (Spongeliomorpha producer). Based on stratigraphical and ichnological features, the complex trace fossils resulted from the complex activity of different opportunistic organisms (r‐strategist) that quickly occupied and thrived within the quiet, nutrient‐rich environment after storm events. 相似文献
145.
146.
Reis M. A. Freitas M. C. Alves L. C. Marques A. P. Costa C. 《Biological trace element research》1999,(1):273-280
The region of Lisbon and south of Lisbon (Sado estuary) is densely industrialized, and, therefore, air pollution should be
studied in a more detailed scale there. The topography of the Sado estuary region and the predominant wind direction from
the northwest contribute to the influence in this region of the industries located in the north. The region selected includes
a fuel-fired power station. Transplants of the lichenParmelia sulcata Taylor were suspended in nylon bags within a rectangle 15 km wide and 25 km long on a grid 2.5 km × 2.5 km, centered in the power
station. In each of the 47 sites, 2 sets of 4 transplants each were hung. Care was taken (1) in covering the two sets with
a polyethylene roof to prevent leaching of elements in the lichen, (2) in building a hanging system that could rotate according
to the wind direction, and (3) in orienting one set toward the wind and the other set opposite the wind. For a 1-yr period
and every 3 mo, one transplant of each set is collected. In this work, the results of the first campaign (after 3 mo suspension)
obtained by instrumental neutron activation analysis and proton-induced X-ray emission are shown. Some elemental contents
are mapped and discussed. 相似文献
147.
A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2 总被引:7,自引:0,他引:7
George R. Hendrey David S. Ellsworth Keith F. Lewin JohN. Nagy 《Global Change Biology》1999,5(3):293-309
A free-air CO2 enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO2 concentrations ([CO2]a) without enclosures that alter tree microenvironment. We describe a prototype FACE system currently in operation in forest plots in a maturing loblolly pine (Pinus taeda L.) stand in North Carolina, USA. The system uses feedback control technology to control [CO2] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole-stand CO2 regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO2 enrichment was conducted in the forest during all daylight hours for 96.7% of the scheduled running time from 23 May to 14 October with a preset target [CO2] of 550 μmol mol–1, ≈ 200 μmol mol–1 above ambient [CO2]. The system provided spatial and temporal control of [CO2] similar to that reported for open-top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO2] within the upper forest canopy at the centre of the FACE plot was 552 ± 9 μmol mol–1 (mean ± SD). The FACE system maintained 1-minute average [CO2] to within ± 110 μmol mol–1 of the target [CO2] for 92% of the operating time. Deviations of [CO2] outside of this range were short-lived (most lasting < 60 s) and rare, with fewer than 4 excursion events of a minute or longer per day. Acceptable spatial control of [CO2] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO2] over the exposure season. CO2 consumption by the FACE system was much higher than for open-top chambers on an absolute basis, but similar to that of open-top chambers and branch bag chambers on a per unit volume basis. CO2 consumption by the FACE system was strongly related to windspeed, averaging 50 g CO2 m–3 h–1 for the stand for an average windspeed of 1.5 m s–1 during summer. The [CO2] control results show that the free-air approach is a tractable way to study long-term and short-term alterations in trace gases, even within entire tall forest ecosystems. The FACE approach permits the study of a wide range of forest stand and ecosystem processes under manipulated [CO2]a that were previously impossible or intractable to study in true forest ecosystems. 相似文献
148.
Trace metal/metalloid (As, Ag, Cd, Cr, Cu, Hg, Ni, Pb, Se, Zn) bioaccumulation was measured over a period of ten years (1985–1995) in five species of epibenthic macroinvertebrates (EMI) from the San Pedro Shelf, California. Four of the species are numerical dominants in the Southern California Bight (SCB) and two species are commercially harvested. Among three echinoderms, a predatory sea star yielded highest tissue concentrations compared to a sea cucumber and a sea urchin, and concentrations measured in a mantid shrimp exceeded those from a prawn. Comparison of trace metal bioaccumulation from the study area, including the ocean outfall, and elsewhere indicated that bioaccumulation in local EMI was generally low. Measurements from an outfall station and two reference stations were used to evaluate the spatial and temporal relationship between trace metal bioaccumulation and the wastewater discharge. It was concluded that there was no spatial or temporal relationship between EMI trace metal bioaccumulation and the discharge. 相似文献
149.
150.
On the distribution of trace element concentrations in multiple bone elements in 10 Danish medieval and post‐medieval individuals
下载免费PDF全文
![点击此处可从《American journal of physical anthropology》网站下载免费的PDF全文](/ch/ext_images/free.gif)