全文获取类型
收费全文 | 4210篇 |
免费 | 234篇 |
国内免费 | 231篇 |
出版年
2023年 | 23篇 |
2022年 | 52篇 |
2021年 | 55篇 |
2020年 | 41篇 |
2019年 | 52篇 |
2018年 | 68篇 |
2017年 | 95篇 |
2016年 | 108篇 |
2015年 | 94篇 |
2014年 | 184篇 |
2013年 | 226篇 |
2012年 | 207篇 |
2011年 | 257篇 |
2010年 | 203篇 |
2009年 | 240篇 |
2008年 | 264篇 |
2007年 | 277篇 |
2006年 | 249篇 |
2005年 | 179篇 |
2004年 | 176篇 |
2003年 | 138篇 |
2002年 | 88篇 |
2001年 | 93篇 |
2000年 | 89篇 |
1999年 | 98篇 |
1998年 | 85篇 |
1997年 | 83篇 |
1996年 | 75篇 |
1995年 | 72篇 |
1994年 | 68篇 |
1993年 | 70篇 |
1992年 | 59篇 |
1991年 | 61篇 |
1990年 | 66篇 |
1989年 | 47篇 |
1988年 | 41篇 |
1987年 | 43篇 |
1986年 | 36篇 |
1985年 | 38篇 |
1984年 | 41篇 |
1983年 | 25篇 |
1982年 | 46篇 |
1981年 | 43篇 |
1980年 | 42篇 |
1979年 | 33篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1974年 | 3篇 |
1973年 | 5篇 |
排序方式: 共有4675条查询结果,搜索用时 15 毫秒
1.
Many modern cities have strongly invested in the sustainability of their urban water management system. Nordic cities like Stockholm or Copenhagen are amongst pioneers in investments towards integrated urban water management. However, cities can never be fully self-sufficient due to their dependency on external (water) resources. In this paper, we quantify this water dependency with respect to food consumption in nine cities located in the five Nordic countries (Sweden, Denmark, Finland, Norway and Iceland), by means of the water footprint concept. Detailed urban water footprint assessments are scarce in the literature. By analysing national nutrition surveys, we find that urban food intake behaviour differs from national food intake behaviour. In large Nordic cities people eat generally less potatoes, milk products (without cheese), meat and animal fats and they drink less coffee than outside city borders. On the other hand, they generally eat more vegetables and vegetable oils and they drink more tea and alcoholic beverages. This leads consistently – for the six large Nordic cities Stockholm, Malmö, Copenhagen, Helsinki, Oslo and Reykjavik – to slightly smaller food related urban water footprints (−2 to −6%) than national average values. We also analyse the water footprint for different diets based upon Nordic Nutrition Recommendations (NNR) for these cities. We assessed three healthy diet scenarios: 1) including meat (HEALTHY-MEAT), 2) pesco-vegetarian (HEALTHY-PESCO-VEG) and 3) vegetarian (HEALTHY-VEG). This shows that Nordic urban dwellers 1) eat too many animal products (red meat, milk and milk products) and sugar and drink too much alcohol and 2) they eat not enough vegetables, fruit and products from the group pulses, nuts and oilcrops. Their overall energy and protein intake is too high. A shift to a healthy diet with recommended energy and protein intake reduces the urban WF related to food consumption substantially. A shift to HEALTHY-MEAT results in a reduction of −9 to −24%, for HEALTHY-PESCO-VEG the reduction is −29 to −37%, for HEALTHY-VEG the reduction is −36 to −44%. In other words, Nordic urban dwellers can save a lot of water by shifting to a healthy diet. 相似文献
2.
Yue Qiu Zuo-Yang Zhang Wei-Dong Du Lei Ye Song Xu Xian-Bo Zuo Fu-Sheng Zhou Gang Chen Xue-Ling Ma Marion E. Schneider Hong-Zhen Xia Yuan Zhou Ji-Feng Wu Xu Yuan-Hong Xue-Jun Zhang 《Gene》2014
The purpose of this study was to investigate whether risk of gastric cancer (GC) was associated with single nucleotide polymorphisms (SNPs) in a gene cluster on the chromosome 17q12-q21 (ERBB2 amplicon) in the Chinese Han population. We detected twenty-six SNPs in this gene cluster containing steroidogenic acute regulatory-related lipid transfer domain containing 3 (STARD3), protein phosphatase 1 regulatory subunit 1B (PPP1R1B/DARPP32), titin-cap (TCAP), per1-like domain containing 1(PERLD1/CAB2), human epidermal growth factor receptor-2 (ERBB2/HER2), zinc-finger protein subfamily 1A 3 (ZNFN1A3/IKZF3) and DNA topoisomerase 2-alpha (TOP2A) genes in 311 patients with GC and in 425 controls by Sequenom. We found no associations between genetic variations and GC risk. However, haplotype analysis implied that the haplotype CCCT of STARD3 (rs9972882, rs881844, rs11869286 and rs1877031) conferred a protective effect on the susceptibility to GC (P = 0.043, odds ratio [OR] = 0.805, 95% confidence intervals [95% CI] = 0.643–0.992). The STARD3 rs1877031 TC genotype endued histogenesis of gastric mucinous adenocarcinoma and signet-ring cell carcinoma (P = 0.021, OR = 2.882, 95% CI = 1.173–7.084). We examined the expression of STARD3 in 243 tumor tissues out of the 311 GC patients and 20 adjacent normal gastric tissues using immumohistochemical (IHC) analysis and tissue microarrays (TMA). The expression of STARD3 was observed in the gastric parietal cells and in gastric tumor tissues and significantly correlated with gender (P = 0.004), alcohol drinking (P < 0.001), tumor location (P = 0.007), histological type (P = 0.005) and differentiation (P = 0.023) in GC. We concluded that the combined effect of haplotype CCCT of STARD3 might affect GC susceptibility. STARD3 expression might be related to the tumorigenesis of GC in the Chinese population. 相似文献
3.
辽西半干旱地区深松中耕对作物产量的影响及其作用机理研究 总被引:15,自引:2,他引:13
对辽宁西部半干旱地区深松中耕对作物产量影响及作用机理的研究表明,深松打破了犁底层,减小了土壤容重及穿透阻力,增加了表层及亚表层土壤的孔隙度.深松土壤水分人渗可达80cm,大豆根系深度增加3.6~4.0cm,大豆产量和水分利用效率分别增加27.7~34.5%和16.1%,在干旱年份玉米增产7.9%. 相似文献
4.
Jaime Cavelier 《Trees - Structure and Function》1990,4(3):155-163
Summary Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m-2 day-1 during cloudy days and between 532 and 944 g m-2 day-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage. 相似文献
5.
Hamed A. El-Serehy Hala S. Abdallah Fahad A. Al-Misned Saleh A. Al-Farraj Khaled A. Al-Rasheid 《Saudi Journal of Biological Sciences》2018,25(7):1247-1256
Lake Timsah is considered as the biggest water body at Ismailia City with a surface area of 14?km2. It is a saline shallow water basin lies approximately mid-way between the south city of Suez and the north city of Port Said at 30o35′46.55“N and 32o19′30.54″E. Because it receives water with high and low salinities, salinity stratification is producing in the Lake Timsah, with values of 14–40‰ for the surface water and over 40‰ for the bottom water. The temperature of the lake water decreased to below 19 °C in the winter and rose to above 29?°C in the summer; the concentration of dissolved oxygen ranged between 6.5 and 12.2?l?1 and the pH fluctuated between 7.9 in its lower value and 8.2 in its higher value. Water transparency was very low as indicated by Secchi disc readings recorded during this study and varied between 0.3 and 2.7?m. The main chemical nutrient (phosphorus) reached its highest levels of 96?µg?l?1 in winter and their lowest values of 24?µg?l?1 during summer. This nutrient concentration is high especially by comparing with those of unpolluted marine waters, but is typical of the more eutrophic coastal waters worldwide. The composition and abundance of phytoplankton with dominancy of diatoms and increased population density (20,986 cell l?1) reflect the eutrophic condition of the lake. The intensive growth of phytoplankton was enriched by high concentration of chlorophyll a with annual values ranged between 6.5 and 56?µg?l?1. The objective of the present work was quantitative assessment of the quality of the water of the Lake Timsah using different approaches. During the present study, three different approaches were applied for the quantitative assessment of Lake Timsah water quality: the trophic state index (TST); trophic level index (TLI) and water quality index (WQI). Application of the trophic state and trophic level indices (TSI & TLI) revealed that the Lake Timsah has trophic indices of 60 and 5.2 for TSI and TLI, respectively. Both indices reflected the eutrophic condition of the lake waters and confirmed that the eutrophication is a major threat in the Lake Timsah. On the other hand, the WQI calculated for the Lake Timsah during the present study with an average of 49 demonstrated that the water of the Lake Timsah is bad and unsuitable for main and/or several uses. Moreover, WQI allows accounting for several water resource uses and can serve a more robust than TSI and/or TLI and can be used effectively as a comprehensive tool for water quality quantification. In conclusion, the three subjective indices used for the assessment process for the lake water are more suitable and effective for needs of the sustainable water resources protection and management of the Lake Timsah. 相似文献
6.
7.
为研究施用生石灰对池塘浮游细菌群落结构和多样性的影响,采用基于16S rRNA的高通量测序技术比较分析了施用生石灰前后精养池塘浮游细菌群落结构和多样性差异。研究结果显示,施用生石灰进行处理1d后,池塘优势浮游细菌在门和属水平上均与施用前相同,但相对丰度产生变化。在门水平上,蓝细菌门(Cyanobacteria)的相对丰度由53.80%显著降低至47.57%,拟杆菌门(Bacteroidetes)的相对丰度由7.00%显著降低至5.24%,变形菌门(Proteobacteria)的相对丰度由19.72%显著降低至17.60%,而放线菌门(Actinobacteria)的相对丰度由6.76%显著上升至13.47%,浮霉菌门(Planctomycetes)的相对丰度由8.24%显著上升至11.10%。另外,在属水平上,分枝杆菌属(Mycobacterium)的相对丰度由0.73%显著降低至0.49%,浮丝藻属(Planktothrix)的相对丰度由0.041%显著降低至0.0074%。施用生石灰后池塘浮游细菌群落的物种丰富度指数(ACE和Chao 1)和Shannon多样性指数均显著提高,且Simpon指数显著降低(P < 0.05)。研究结果可为施用生石灰管理池塘水质和进行疾病预防提供理论解释,并可为更加科学合理地利用生石灰管理池塘提供科学指导。 相似文献
8.
W. Ellis Penning Marit Mjelde Bernard Dudley Seppo Hellsten Jenica Hanganu Agnieszka Kolada Marcel van den Berg Sandra Poikane Geoff Phillips Nigel Willby Frauke Ecke 《Aquatic Ecology》2008,42(2):237-251
Aquatic macrophytes are one of the biological quality elements in the Water Framework Directive (WFD) for which status assessments
must be defined. We tested two methods to classify macrophyte species and their response to eutrophication pressure: one based
on percentiles of occurrence along a phosphorous gradient and another based on trophic ranking of species using Canonical
Correspondence Analyses in the ranking procedure. The methods were tested at Europe-wide, regional and national scale as well
as by alkalinity category, using 1,147 lakes from 12 European states. The grouping of species as sensitive, tolerant or indifferent
to eutrophication was evaluated for some taxa, such as the sensitive Chara spp. and the large isoetids, by analysing the (non-linear) response curve along a phosphorous gradient. These thresholds
revealed in these response curves can be used to set boundaries among different ecological status classes. In total 48 taxa
out of 114 taxa were classified identically regardless of dataset or classification method. These taxa can be considered the
most consistent and reliable indicators of sensitivity or tolerance to eutrophication at European scale. Although the general
response of well known indicator species seems to hold, there are many species that were evaluated differently according to
the database selection and classification methods. This hampers a Europe-wide comparison of classified species lists as used
for the status assessment within the WFD implementation process. 相似文献
9.
Benthic insect communities (Ephemeroptera, Plecoptera, Coleoptera, Trichoptera) were studied together with water temperature
and environmental parameters in streams between June 2000 and June 2001. The sampling area consisted of 20 sites in small
and medium-sized streams located in the lower mountainous area of Central Europe. Temperature was recorded nearly continuously
and several physicochemical and environmental variables were assessed. Macroinvertebrates were sampled both in spring and
summer. Data-sets of species abundance and occurrence were analysed using multivariate techniques and were correlated to the
thermal and environmental conditions of the streams. The temperature preferences of the species were compared to published
data-sets on their autecological characteristics. Up to 29% of the variability in the Ephemeroptera, Plecoptera, Trichoptera
and Coleoptera community was explained by summer temperature variation in the data-sets for both small and medium-sized streams.
A smaller, but significant part of the variability in species distribution was explained by conductivity, substratum type,
and the percent coverage of local riparian forest. Compared to small streams, temperature was less important for the macroinvertebrate
composition in medium-sized streams. This result is likely due to the more tolerant, eurythermic species composition in larger
streams. A total of 33 Ephemeroptera, Plecoptera, Coleoptera and Trichoptera taxa were positively correlated and 28 taxa were
negatively correlated to summer temperature patterns. The temperature preferences of taxa considered in this study were related
to species traits, such as egg dormancies and life cycle plasticity. 相似文献
10.
In order to test the response of phytoplankton to anthropogenic pressure, data of chlorophyll a concentration, phytoplankton abundance, and composition are analyzed in relation to anthropogenic pressure gradient and environmental variables such as temperature, salinity and nutrients. Investigated sites encompassed wide tropic range according to a preliminary determination of anthropogenic pressure, quantified through the LUSI index. Statistical analyses indicated nitrates and silicates as proxies of freshwater influence, and phytoplankton single metrics such as concentrations of chlorophyll a and abundances as indicators of anthropogenic pressure. Boundary values for different water quality classes for coastal waters under indirect freshwater influence (Type II) are obtained according to gradient between concentration of chlorophyll a and pressure index (LUSI), which empirically fit to exponential equation. The response of phytoplankton diversity was not linear, as the highest diversity was observed in the area with intermediate disturbance level. CCA analysis identified Skeletonema marinoii, Scrippsiella trochoidea, Guinardia flaccida, Leptocylindrus spp., Prorocentrum spp., Proboscia alata, Eutreptiella spp., and Pseudonitzschia spp. as local eutrophication indicators, whose abundances increased with nutrients loads. 相似文献