全文获取类型
收费全文 | 8243篇 |
免费 | 356篇 |
国内免费 | 10篇 |
专业分类
林业 | 337篇 |
农学 | 383篇 |
基础科学 | 85篇 |
1082篇 | |
综合类 | 532篇 |
农作物 | 326篇 |
水产渔业 | 283篇 |
畜牧兽医 | 4468篇 |
园艺 | 138篇 |
植物保护 | 975篇 |
出版年
2023年 | 50篇 |
2022年 | 50篇 |
2021年 | 83篇 |
2020年 | 124篇 |
2019年 | 132篇 |
2018年 | 138篇 |
2017年 | 140篇 |
2016年 | 156篇 |
2015年 | 134篇 |
2014年 | 169篇 |
2013年 | 572篇 |
2012年 | 402篇 |
2011年 | 485篇 |
2010年 | 209篇 |
2009年 | 242篇 |
2008年 | 432篇 |
2007年 | 425篇 |
2006年 | 357篇 |
2005年 | 366篇 |
2004年 | 366篇 |
2003年 | 337篇 |
2002年 | 308篇 |
2001年 | 249篇 |
2000年 | 236篇 |
1999年 | 216篇 |
1998年 | 67篇 |
1997年 | 69篇 |
1996年 | 68篇 |
1995年 | 69篇 |
1994年 | 47篇 |
1993年 | 53篇 |
1992年 | 117篇 |
1991年 | 124篇 |
1990年 | 142篇 |
1989年 | 115篇 |
1988年 | 104篇 |
1987年 | 89篇 |
1986年 | 71篇 |
1985年 | 77篇 |
1984年 | 73篇 |
1983年 | 55篇 |
1982年 | 36篇 |
1981年 | 43篇 |
1980年 | 48篇 |
1979年 | 54篇 |
1976年 | 29篇 |
1975年 | 43篇 |
1974年 | 29篇 |
1972年 | 36篇 |
1971年 | 34篇 |
排序方式: 共有8609条查询结果,搜索用时 29 毫秒
131.
Aidos I van der Padt A Boom RM Luten JB 《Journal of agricultural and food chemistry》2001,49(8):3697-3704
Fish oil has been extracted from byproducts of the maatjes (salted) herring production using a pilot plant consisting of a mincer, heat exchanger, and three-phase decanter. The crude herring oil obtained had an initial peroxide value (PV), anisidine value (AV) and free fatty acids (FFA) level of only 3 mequiv of peroxide/kg of lipid, 8.9, and 2.9%, respectively. 5,8,11,14,17-Eicosapentaenoic acid (EPA) and 4,7,10,13,16,19-docosahexaenoic acid (DHA) were present in considerable amounts (99 and 91 g/kg, respectively). During storage of the oil, no photooxidation could be detected. Storage at room temperature led to significant autoxidation over time, apparent from primary and tertiary oxidation products, measured by a decrease of hydroperoxides and an increase of fluorescent compounds (FC). Storage at 50 degrees C resulted in significant increases in secondary (AV) and tertiary oxidation (FC) products. At all storage conditions, the FFA contents remained low (<3%) and the alpha-tocopherol content remained constant. These results open the possibility for fish oil production of good quality using salted herring byproducts. 相似文献
132.
van der Stegen GH Essens PJ van der Lijn J 《Journal of agricultural and food chemistry》2001,49(10):4713-4715
A commercial lot of green coffee, naturally contaminated with ochratoxin A (OTA), was roasted under various conditions, and the effects on its final OTA content were determined. Precautions were taken in sampling the coffee to cope with OTA inhomogeneity. The roasting conditions were kept within the range of commercial practice. Roasting time was varied from 2.5 to 10 min, and the roast color varied from light medium to dark. The differences in OTA reduction between the different levels of roasting times and colors did not reach statistical significance. However, for all roasting conditions, the reduction was highly significant, 69% reduction over the combined results. In total, nine studies by various authors about OTA reduction during coffee roasting are now available. Seven out of these nine reported that the relevant range of OTA reductions was between 69 and 96%. Among these seven,are all four studies that reported using naturally contaminated beans, a sampling procedure adapted to mycotoxin inhomogeneity, and roasting conditions within the range of actual practice. Three different explanations are available for this reduction: physical removal of OTA with chaff, isomerization at the C-3 position into another diastereomer, and thermal degradation with possible involvement of moisture. All three explanations may play a partial role in the OTA reduction during coffee roasting. 相似文献
133.
Nineteen cultivars, with large differences in heading date, were evaluated for their response to septoria tritici blotch in
two experimental setups in Njoro, Kenya. Due to the more or less constant temperatures during the growing season and the overhead
irrigation applied the epidemic conditions were similar over the whole observation period for the early and late cultivars.
In experiment 1 the cultivars were assessed for disease severity at the same moment irrespective of the developmental stage,
while in experiment 2 the cultivars were assessed at the same developmental stage. Measured at the same time, the disease
severity was highest in the early maturing cultivars and lowest in the late maturing cultivars (r = –0.78). When assessed
at the same development stage the disease build up was independent of heading date (r = –0.10) but strongly dependent on resistance
level. There were no indications that early heading cultivars were more susceptible than late heading cultivars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
134.
Angela Y.Y. Kong Steven J. Fonte Chris van Kessel Johan Six 《Soil & Tillage Research》2009,104(2):256-262
Few studies address nutrient cycling during the transition period (e.g., 1–4 years following conversion) from standard to some form of conservation tillage. This study compares the influence of minimum versus standard tillage on changes in soil nitrogen (N) stabilization, nitrous oxide (N2O) emissions, short-term N cycling, and crop N use efficiency 1 year after tillage conversion in conventional (i.e., synthetic fertilizer-N only), low-input (i.e., alternating annual synthetic fertilizer- and cover crop-N), and organic (i.e., manure- and cover crop-N) irrigated, maize–tomato systems in California. To understand the mechanisms governing N cycling in these systems, we traced 15N-labeled fertilizer/cover crop into the maize grain, whole soil, and three soil fractions: macroaggregates (>250 μm), microaggregates (53–250 μm) and silt-and-clay (<53 μm). We found a cropping system effect on soil Nnew (i.e., N derived from 15N-fertilizer or -15N-cover crop), with 173 kg Nnew ha−1 in the conventional system compared to 71.6 and 69.2 kg Nnew ha−1 in the low-input and organic systems, respectively. In the conventional system, more Nnew was found in the microaggregate and silt-and-clay fractions, whereas, the Nnew of the organic and low-input systems resided mainly in the macroaggregates. Even though no effect of tillage was found on soil aggregation, the minimum tillage systems showed greater soil fraction-Nnew than the standard tillage systems, suggesting greater potential for N stabilization under minimum tillage. Grain-Nnew was also higher in the minimum versus standard tillage systems. Nevertheless, minimum tillage led to the greatest N2O emissions (39.5 g N2O–N ha−1 day−1) from the conventional cropping system, where N turnover was already the fastest among the cropping systems. In contrast, minimum tillage combined with the low-input system (which received the least N ha−1) produced intermediate N2O emissions, soil N stabilization, and crop N use efficiency. Although total soil N did not change after 1 year of conversion from standard to minimum tillage, our use of stable isotopes permitted the early detection of interactive effects between tillage regimes and cropping systems that determine the trade-offs among N stabilization, N2O emissions, and N availability. 相似文献
135.
Nicola C. Okes Philip A.R. Hockey Lorien Pichegru Carl D. van der Lingen Robert J.M. Crawford David Grmillet 《Biological conservation》2009,142(10):2361-2368
In the southern Benguela upwelling ecosystem off the west coast of South Africa, seabird populations are decreasing dramatically because of reduced availability of pelagic fish. We tested the hypothesis that the west coast fishing industry is competing for the remaining stocks of anchovy and sardine with the largest colony of Vulnerable Cape Gannets (Morus capensis) along the Atlantic coast. Using GPS-tracking of the birds, echo-sounding of pelagic fish, and vessel log books, we located overlap areas between bird foraging ranges, pelagic fish distribution, and fishing activities. We then compared fish catches by gannets and vessels within their joint foraging zones. In October 2007, purse-seine fishing grounds and gannet foraging areas overlapped by only 13%. However, for a 1-month period, the amount of fish removed from this area by purse-seine boats amounted to 41% of the food requirements of the 72,000 gannets breeding on Malgas Island (25% of the world population). The fishery’s catch in this area is significant in terms of its potential impact on gannets, but comprises only 3.6% of total fishery catch. Based on this finding, the rapidly decreasing size of the gannet colony and the stated objectives of South Africa’s Marine Living Resources Act of 1998, the case for considering and experimenting with at-sea ‘no-take’ areas for the purse-seine fishery is strong. Efforts to establish whether ‘no-take’ fishing zones increase food availability for top predators is an important next step in conservation of the southern Benguela Ecosystem. 相似文献
136.
Linking habitat suitability and seed dispersal models in order to analyse the effectiveness of hydrological fen restoration strategies 总被引:1,自引:0,他引:1
A.H. van Loon H. Soomers P.P. Schot M.F.P. Bierkens J. Griffioen M.J. Wassen 《Biological conservation》2011,(3):1025-1035
The effectiveness of measures targeted at the restoration of populations of endangered species in anthropogenically dominated regions is often limited by a combination of insufficient restoration of habitat quality and dispersal failure. Therefore, the joint prediction of suitable habitat and seed dispersal in dependency of management actions is required for effective nature management. Here we demonstrate an approach, which links a habitat suitability and a seed dispersal model. The linked model describes potential species distribution as a function of current species distribution, species-specific dispersal traits, the number of successful dispersal events, dispersal infrastructure and habitat configuration. The last two variables were related to water management actions. We demonstrate the applicability of the model in a strategy analysis of hydrological restoration measures for a large fen area in which still numerous endangered plant species grow.With the aid of the linked model, we were able to optimise the spatial planning of restoration measures, taking into account both the constraints of water management practices on abiotic restoration and the effects of habitat fragmentation on dispersal. Moreover, we could demonstrate that stand-alone habitat suitability models, which assume unlimited dispersal, may considerably overestimate restoration prospects. For these reasons, we conclude that linked habitat suitability and dispersal models can provide useful insights into spatially differentiated potentials and constraints of nature restoration measures targeted at the sustainable conservation of endangered plant populations whose habitats have been deteriorated due to undesirable effects of land and water management on abiotic conditions. These insights may contribute to the design of cost-effective nature restoration and conservation measures. 相似文献
137.
Here we report on the frequency of melanized fungal hyphae in 323 soils, covering different land use types. The proportion of total hyphae that was melanized averaged 61%. Arable fields with loamy sand, heathlands and city parks on sandy soils had the highest percentage of melanized hyphae. In addition to the frequency determinations, a microcosm study was performed on the role of melanized hyphae in two different ecosystems: an ex-arable field and a forest. Melanized hyphae appeared to be part of the active hyphae in the forest soil but not in the ex-arable soil. In conclusion, our results indicate that (1) melanized hyphae represent a large proportion of the total fungal biomass in soils and that (2) their function might differ between ecosystems. 相似文献
138.
Johansson L Rytkonen A Bergman P Albiger B Källström H Hökfelt T Agerberth B Cattaneo R Jonsson AB 《Science (New York, N.Y.)》2003,301(5631):373-375
The human-specific bacterial pathogen Neisseria meningitidis is a major cause of sepsis and/or meningitis. The pili of N. meningitidis interact with CD46, a human cell-surface protein involved in regulation of complement activation. Transgenic mice expressing human CD46 were susceptible to meningococcal disease, because bacteria crossed the blood-brain barrier in these mice. Development of disease was more efficient with piliated bacteria after intranasal, but not intraperitoneal, challenge of CD46 transgenic mice, suggesting that human CD46 facilitates pilus-dependent interactions at the epithelial mucosa. Hence, the human CD46 transgenic mice model is a potentially useful tool for studying pathogenesis and for vaccine development against meningococcal disease. 相似文献
139.
Lahav O Naim A Buta RJ Corwin HG de Vaucouleurs G Dressler A Huchra JP van den Bergh S Raychaudhury S Sodré L Storrie-Lombardi MC 《Science (New York, N.Y.)》1995,267(5199):859-862
The quantitative morphological classification of galaxies is important for understanding the origin of type frequency and correlations with environment. However, galaxy morphological classification is still mainly done visually by dedicated individuals, in the spirit of Hubble's original scheme and its modifications. The rapid increase in data on galaxy images at low and high redshift calls for a re-examination of the classification schemes and for automatic methods. Here are shown results from a systematic comparison of the dispersion among human experts classifying a uniformly selected sample of more than 800 digitized galaxy images. These galaxy images were then classified by six of the authors independently. The human classifications are compared with each other and with an automatic classification by an artificial neural network, which replicates the classification by a human expert to the same degree of agreement as that between two human experts. 相似文献
140.
3S技术在中国湿地研究、调查与管理中的应用 总被引:10,自引:0,他引:10
CUI Li-juan Anna van Paddenburg ZHANG Man-yin 《林业研究》2005,16(4):317-322
This paper introduces the state of waterlands in China and discribes the applications of Remote Sensing (RS), Geographical Information System (G1S) and Global Positioning System (GPS) in wetland research, including land-coverclassification and change detection, wetland evolutionary processes, landscape-change analyses, channel migration, flood and wetlands resource monitoring and spatial quantitative analyses/modeling, ecosystem service evaluation, ecological processes and risk assessments, disease control, water quality monitoring/modeling, pollution monitoring/modeling, wetlands hydrology, wetland information systems and WebGIS. The limitations and needs for optimal use of these technologies are discussed, such as the limited advanced technical knowledge and skills, low awareness and capacity, unclear link between GIS output and policy making, lack of supporting policies and standards, lack of a wetlands geo-information networklimite, and the use of these techniques in wetland research. It is suggested that for realising true applications of RS, GIS and GPS technologies, the availability, accessibility, reliability, homogeneity, and continuity of wetlands-related geo-information enabling environment, policies and standards, and funding are needed. 相似文献