全文获取类型
收费全文 | 4896篇 |
免费 | 528篇 |
国内免费 | 231篇 |
出版年
2023年 | 100篇 |
2022年 | 81篇 |
2021年 | 134篇 |
2020年 | 196篇 |
2019年 | 215篇 |
2018年 | 166篇 |
2017年 | 219篇 |
2016年 | 192篇 |
2015年 | 216篇 |
2014年 | 231篇 |
2013年 | 346篇 |
2012年 | 191篇 |
2011年 | 222篇 |
2010年 | 194篇 |
2009年 | 234篇 |
2008年 | 250篇 |
2007年 | 272篇 |
2006年 | 242篇 |
2005年 | 211篇 |
2004年 | 176篇 |
2003年 | 155篇 |
2002年 | 183篇 |
2001年 | 151篇 |
2000年 | 137篇 |
1999年 | 101篇 |
1998年 | 84篇 |
1997年 | 73篇 |
1996年 | 62篇 |
1995年 | 67篇 |
1994年 | 85篇 |
1993年 | 46篇 |
1992年 | 65篇 |
1991年 | 52篇 |
1990年 | 40篇 |
1989年 | 33篇 |
1988年 | 19篇 |
1987年 | 29篇 |
1986年 | 20篇 |
1985年 | 33篇 |
1984年 | 23篇 |
1983年 | 13篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 15篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有5655条查询结果,搜索用时 15 毫秒
1.
Nitrogen yield advantage from grass–legume mixtures is robust over a wide range of legume proportions and environmental conditions 下载免费PDF全文
Matthias Suter John Connolly John A. Finn Ralf Loges Laura Kirwan Maria‐Teresa Sebastià Andreas Lüscher 《Global Change Biology》2015,21(6):2424-2438
Current challenges to global food security require sustainable intensification of agriculture through initiatives that include more efficient use of nitrogen (N), increased protein self‐sufficiency through homegrown crops, and reduced N losses to the environment. Such challenges were addressed in a continental‐scale field experiment conducted over 3 years, in which the amount of total nitrogen yield (Ntot) and the gain of N yield in mixtures as compared to grass monocultures (Ngainmix) was quantified from four‐species grass–legume stands with greatly varying legume proportions. Stands consisted of monocultures and mixtures of two N2‐fixing legumes and two nonfixing grasses. The amount of Ntot of mixtures was significantly greater (P ≤ 0.05) than that of grass monocultures at the majority of evaluated sites in all 3 years. Ntot and thus Ngainmix increased with increasing legume proportion up to one‐third of legumes. With higher legume percentages, Ntot and Ngainmix did not continue to increase. Thus, across sites and years, mixtures with one‐third proportion of legumes attained ~95% of the maximum Ntot acquired by any stand and had 57% higher Ntot than grass monocultures. Realized legume proportion in stands and the relative N gain in mixture (Ngainmix/Ntot in mixture) were most severely impaired by minimum site temperature (R = 0.70, P = 0.003 for legume proportion; R = 0.64, P = 0.010 for Ngainmix/Ntot in mixture). Nevertheless, the relative N gain in mixture was not correlated to site productivity (P = 0.500), suggesting that, within climatic restrictions, balanced grass–legume mixtures can benefit from comparable relative gains in N yield across largely differing productivity levels. We conclude that the use of grass–legume mixtures can substantially contribute to resource‐efficient agricultural grassland systems over a wide range of productivity levels, implying important savings in N fertilizers and thus greenhouse gas emissions and a considerable potential for climate change mitigation. 相似文献
2.
近年来,与医学相关的营养与代谢的研究出现不少新的认识与新发现。本文总结了以下5个方面的新概念:供能的营养物质不仅是供能,还需注意他们对健康的影响;以构成蔗糖分子一半的果糖为例,它竟然是促成代谢综合征、心脑血管病等疾病的因素;食物纤维是极其重要不可或缺的营养素;应高度重视维护正常肠道菌群,因为它与人体的健康与多种疾病密切相关;全食物营养不仅能维持生命,还能增强体质、避免和减少疾病;植物营养素具有抗氧化、抗炎症及抗癌等作用,是全食物营养不可或缺的组成部分。人体营养与物质代谢研究的终极目标是:增强体质,预防和减少疾病,健康长寿。希望本文能够带给读者一些启发。 相似文献
3.
Using food network unfolding to evaluate food–web complexity in terms of biodiversity: theory and applications 下载免费PDF全文
Yoshikazu Kato Michio Kondoh Naoto F. Ishikawa Hiroyuki Togashi Yukihiro Kohmatsu Mayumi Yoshimura Chikage Yoshimizu Takashi F. Haraguchi Yutaka Osada Nobuhito Ohte Naoko Tokuchi Noboru Okuda Takeshi Miki Ichiro Tayasu 《Ecology letters》2018,21(7):1065-1074
Food–web complexity often hinders disentangling functionally relevant aspects of food–web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food–web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (DH), vertical diversity (DV) and range diversity (DR), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H′), where H′ = DH + DV ? DR. Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within‐site changes in food–web structures that could not be captured with H′ alone. 相似文献
4.
5.
P. Bourke D. Ziuzina L. Han P.J. Cullen B.F. Gilmore 《Journal of applied microbiology》2017,123(2):308-324
There is a diverse range of microbiological challenges facing the food, healthcare and clinical sectors. The increasing and pervasive resistance to broad‐spectrum antibiotics and health‐related concerns with many biocidal agents drives research for novel and complementary antimicrobial approaches. Biofilms display increased mechanical and antimicrobial stability and are the subject of extensive research. Cold plasmas (CP) have rapidly evolved as a technology for microbial decontamination, wound healing and cancer treatment, owing to the chemical and bio‐active radicals generated known collectively as reactive oxygen and nitrogen species. This review outlines the basics of CP technology and discusses the interactions with a range of microbiological targets. Advances in mechanistic insights are presented and applications to food and clinical issues are discussed. The possibility of tailoring CP to control specific microbiological challenges is apparent. This review focuses on microbiological issues in relation to food‐ and healthcare‐associated human infections, the role of CP in their elimination and the current status of plasma mechanisms of action. 相似文献
6.
T. Santos J. A. Díaz J. Pérez-Tris R. Carbonell & J. L. Tellería 《Animal Conservation》2008,11(1):46-56
Organisms often face a higher risk of local extinction in fragmented than in continuous habitat. However, whether populations are affected by reduced size and connectivity of the habitat or by changes in habitat quality in fragmented landscapes remains poorly investigated. We studied the regional distribution and microhabitat selection of the lacertid lizard Psammodromus algirus in a fragmented landscape where the existence of deciduous and evergreen woodlands brought about variation in habitat quality. Lizards never occupied any fragment smaller than 0.5 ha. However, above that limit fragment size no longer predicted lizard occurrence, which was explained by woodland type instead, with lizards being more frequently found in deciduous than in evergreen woodlands. Lizards selected microhabitats that had structural features favouring thermoregulation, foraging and predator avoidance, and we identified better conditions for thermoregulation and food acquisition in deciduous than in evergreen woodlands. Our results support the idea that variation in habitat quality can sometimes override the effect of habitat fragmentation on animal populations. We consider the implications of our study for the conservation of Mediterranean lizards, discussing our results in a broader context framed by previous studies conducted in nearby areas. 相似文献
7.
Assigning functional information to hypothetical proteins in virus genomes is crucial for gaining insight into their proteomes.
Human adenoviruses are medium sized viruses that cause a range of diseases. Their genomes possess proteins with
uncharacterized function known as hypothetical proteins. Using a wide range of protein function prediction servers, functional
information was obtained about these hypothetical proteins. A comparison of functional information obtained from these servers
revealed that some of them produced functional information, while others provided little functional information about these
human adenovirus hypothetical proteins. The PFP, ESG, PSIPRED, 3d2GO, and ProtFun servers produced the most functional
information regarding these hypothetical proteins. 相似文献
8.
Using euhalophytes to understand salt tolerance and to develop saline agriculture: Suaeda salsa as a promising model 总被引:5,自引:0,他引:5
Background As important components in saline agriculture, halophytes can help to provide food for a growing world population. In addition to being potential crops in their own right, halophytes are also potential sources of salt-resistance genes that might help plant breeders and molecular biologists increase the salt tolerance of conventional crop plants. One especially promising halophyte is Suaeda salsa, a euhalophytic herb that occurs both on inland saline soils and in the intertidal zone. The species produces dimorphic seeds: black seeds are sensitive to salinity and remain dormant in light under high salt concentrations, while brown seeds can germinate under high salinity (e.g. 600 mm NaCl) regardless of light. Consequently, the species is useful for studying the mechanisms by which dimorphic seeds are adapted to saline environments. S. salsa has succulent leaves and is highly salt tolerant (e.g. its optimal NaCl concentration for growth is 200 mm). A series of S. salsa genes related to salt tolerance have been cloned and their functions tested: these include SsNHX1, SsHKT1, SsAPX, SsCAT1, SsP5CS and SsBADH. The species is economically important because its fresh branches have high value as a vegetable, and its seed oil is edible and rich in unsaturated fatty acids. Because it can remove salts and heavy metals from saline soils, S. salsa can also be used in the restoration of salinized or contaminated saline land.Scope Because of its economic and ecological value in saline agriculture, S. salsa is one of the most important halophytes in China. In this review, the value of S. salsa as a source of food, medicine and forage is discussed. Its uses in the restoration of salinized or contaminated land and as a source of salt-resistance genes are also considered. 相似文献
9.
A hybrid anaerobic solid-liquid (HASL) bioreactor is an enhanced two-phase anaerobic system, that consists of a solid waste reactor as the acidification reactor and a wastewater reactor, i.e. an upflow anaerobic sludge blanket (UASB) reactor as the methanogenic reactor. Food waste digestion in HASL bioreactors with pre-acidification and HASL operation stages was investigated in two separate runs. After 8 days of pre-acidification in Run A and 4 days in Run B, total volatile fatty acid (TVFA) and chemical oxygen demand (COD) concentrations in the leachates of both acidification reactors were similar. During HASL operation stage, TVFA and COD removal in the methanogenic phase were 77–100% and 75–95%, respectively. Some 99% of the total methane generated was from the methanogenic phase with a content of 68–70% methane. At the end of operation, about 59–60% of the added volatile solids (VS) were removed with a methane yield of 0.25 l g–1 VS. 相似文献
10.
I. Zimmer E. Usleber H. Klaffke R. Weber P. Majerus H. Otteneder M. Gareis R. Dietrich E. Märtlbauer 《Mycotoxin Research》2008,24(1):40-52
In order to calculate the dietary fumonisin intake of the German consumer, a large survey was carried out on a variety of
potentially contaminated products in the period between December 1998 and July 2001. A total of 1960 food samples comprising
all known relevant groups of products were analysed for fumonisins. Furthermore, 272 of these samples were also analysed for
hydrolysed fumonisins (HFB). For routine analysis enzyme immunoassay was used, confirmatory and control analyses were performed
using HPLC-FLD after precolumn derivatisation, or by LC-MS/MS. Daily intake of fumonisins was calculated by combining fumonisin
contamination data obtained in this study with available food consumption data for Germany. In a “mean case” scenario, median
fumonisin levels in foods and mean food intake values were used. To generate a “bad case” scenario, the 90th percentile of fumonisin levels in foods and mean food intake values were combined. The overall daily fumonisin intake by
the German consumer was 1.1 μg in the “mean case” scenario, and 21 μg in the “bad case” scenario. It was concluded that in
general there is no increased risk for the German consumer in aspects of exceeding the recommended tolerable daily intake
of fumonisins (2 μg/kg body weight). However, certain products (and certain brands of products) were repeatedly found to contain
elevated fumonisin levels, which in a “worst case” scenario (“high” food intake of maize-based products) could pose a potential
risk for the consumer, in particular concerning foods for infants and young children. High fumonisin levels were found in
infant foods in 1999, but contamination levels decreased strongly in the following years. HFBs (mostly HFB1) were frequently found in processed cereals such as corn flakes, but in relatively low concentrations. According to our findings,
the new European Union maximum levels for fumonisins are suitable to eliminate peak contamination levels of fumonisins in
foods, but would lead to a regular excess of the TDI for infants and young children if these maximum levels would indeed be
exhausted.
Financial support: This work was financially supported by the German Federal Ministry for Nutrition, Agriculture and Consumer
Protection, research grant 415-6080-1/60 (BMG alt). 相似文献