首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   778篇
  免费   47篇
林业   92篇
农学   30篇
基础科学   4篇
  182篇
综合类   32篇
农作物   40篇
水产渔业   133篇
畜牧兽医   247篇
园艺   17篇
植物保护   48篇
  2024年   1篇
  2023年   9篇
  2022年   13篇
  2021年   35篇
  2020年   24篇
  2019年   36篇
  2018年   31篇
  2017年   30篇
  2016年   29篇
  2015年   21篇
  2014年   47篇
  2013年   47篇
  2012年   75篇
  2011年   84篇
  2010年   37篇
  2009年   42篇
  2008年   52篇
  2007年   46篇
  2006年   38篇
  2005年   31篇
  2004年   28篇
  2003年   23篇
  2002年   27篇
  2001年   3篇
  2000年   4篇
  1999年   1篇
  1998年   1篇
  1997年   3篇
  1996年   1篇
  1994年   1篇
  1992年   1篇
  1990年   1篇
  1986年   1篇
  1977年   1篇
  1954年   1篇
排序方式: 共有825条查询结果,搜索用时 15 毫秒
741.
Senegalese sole is one of the most promising fish species cultivated in the Southern European countries. This study was aimed at assessing the effects of microalgae biomass added to diets for Senegalese sole juveniles on fish growing and condition status. Three isoproteic (52%) and isolipidic (10%) were formulated containing 15% Tisochrysis lutea (TISO), Nannochloropsis gaditana (NAN), or Scenedesmus almeriensis (SCE) biomass, respectively. An experimental microalgae-free diet (CT) and a commercial diet (COM) were used as controls. Fish were fed at 3% of their body weight for 85 days. Final body weight of fish fed microalgae-supplemented diets did not differ from group fed CT diet. Fish-fed CT, TISO, NAN, and SCE showed higher growth performance and nutrient utilization figures than specimen-fed COM diet. The highest carcass lipid content was found in COM group (141 g kg?1), and no differences were observed in body protein content. Ash was significantly higher in TISO, NAN, and SCE groups compared to fish-fed CT. Muscle EPA and DHA contents were not modified owing to the different dietary treatments. The n3/n6 and EPA/DHA ratios in muscle were similar in all the experimental groups. The quantification of digestive proteolytic activities did not differ among experimental groups, although differences in the protease pattern in digestive extracts by zymography were revealed in those fish fed on COM diet. Both α-amylase activity in the intestinal lumen and leucine aminopeptidase in the intestinal tissue were significantly lower in COM fish. Specimens fed on SCE diet showed a higher leucine aminopeptidase activity associated to the intestinal tissue compared to NAN-fed fish (0.40 and 0.25 U g tissue?1, respectively). The ultrastructural study revealed that the dietary inclusion of algal biomass, especially T. lutea and N. gaditana, had a positive impact on the absorptive capacity of the intestinal mucosa. The highest values for the parameters microvilli length and microvilli absorption surface were observed in fish fed on NAN diet (1.99 μm and 45.93 μm2, respectively). Even though further studies aimed at optimizing commercial formulas for Senegalese sole are required prior to any large-scale practical utilization, the results obtained clearly suggest the potential of microalgae as dietary ingredients for this fish species.  相似文献   
742.
743.
The lack of specific compound diets for cephalopods is limiting the industrial development of some species. In this study, four agglutinated moist diets were tested in individually reared Octopus vulgaris (979 ± 151 g) for 8 weeks. All diets were based on bogue Boops boops, accidentally reared in fish farms (aquaculture by‐product), and agglutinated with alginate and calcium. One diet was based exclusively on bogue fillets, two on bogue fillets complemented with meat from two crab species (Portunus pelagicus, Grapsus grapsus) and the last one on bogue and G. grapsus meals. As a control diet, bogue and P. pelagicus were supplied fresh on alternate days. All diets induced similar feed intake (2.1–2.6% day?1). However, the meal‐based diet induced negative growth in comparison with the control and the other agglutinated diets (0.80–0.85% day?1). Higher lipid content in agglutinated diets (28–30% dw) in comparison with the control diet (16% dw) led to higher protein retention in muscle. These results underline the inadequacy of traditional meals in diets for cephalopods and that the inclusion of crab meat did not increase feeding rates and growth in O. vulgaris. The dietary fatty acid, with high levels of oleic and linoleic acid and low levels of ARA, clearly reflected in digestive gland, while only decreasing ARA and increasing EPA levels were observed in muscle, with no apparent negative effect on growth.  相似文献   
744.
The goal of this study was to assess the water status variability of a commercial rain-fed Tempranillo vineyard (Vitis vinifera L.) by thermal and multispectral imagery using an unmanned aerial vehicle (UAV). The relationships between aerial temperatures or indices derived from the imagery and leaf stomatal conductance (g s) and stem water potential (Ψstem) were determined. Aerial temperature was significantly correlated with g s (R 2?=?0.68, p?<?0.01) and Ψstem (R 2?=?0.50, p?<?0.05). Furthermore, the thermal indices derived from aerial imagery were also strongly correlated with Ψstem and g s. Moreover, different spectral indices were related to vineyard water status, although NDVI (normalized difference vegetation index) and TCARI/OSAVI (ratio between transformed chlorophyll absorption in reflectance and optimized soil-adjusted vegetation index) showed the highest coefficient of determination with Ψstem (R 2?=?0.68, p?<?0.05) and g s (R 2?=?0.84, p?<?0.05), respectively. While the relationship with thermal imagery and water status parameters could be considered as a short-term response, NDVI and TCARI/OSAVI indices were probably reflecting the result of cumulative water deficits, hence a long-term response. In conclusion, thermal and multispectral imagery using an UAV allowed assessing and mapping spatial variability of water status within the vineyard.  相似文献   
745.
This study evaluated the probiotic potential of the biofilm formed by the strain Pseudomonas sp. RGM2144 on rainbow trout survival. When challenged with the fish pathogen Flavobacterium psychrophilum, Pseudomonas sp. RGM2144 increased rainbow trout survival to 92.7 ± 1.2% (control: 35.3 ± 9.5%, p < .0001). The draft genome of Pseudomonas sp. RGM2144 is 6.8 Mbp long, with a completeness 100% and a contamination of 0.4%. The genome contains 6122 protein-coding genes of which 3564 (~60%) have known functions. The genome and phylogeny indicate that Pseudomonas sp. RGM2144 is a new species in the Pseudomonas genus, with few virulence factors, plasmids, and genes associated with antimicrobial resistance, suggesting a non-pathogenic bacterium with protective potential. In addition, the genome encodes for 11 secondary metabolite biosynthetic gene clusters that could be involved in the inhibition of F. psychrophilum. We suggest that Pseudomonas sp. RGM2144 may be applied as a probiotic in salmonid fish farming.  相似文献   
746.
The altitudinal pattern of genetic variation in Pinus hartwegii Lindl. populations was explored for seedling height growth, frost damage, grass-stage, and phenological stage of the terminal shoot. A provenance test was conducted with open-pollinated seed from 13 populations collected along an altitudinal transect (3000–3600 m) at the National Park Pico de Tancítaro, Michoacán, center-west Mexico. Height growth of seedlings in a nursery was assessed at seven and 18 months of age. Frost damage at −15 °C was evaluated in laboratory at 18 months of age; proportion of plants that had left grass-stage and stage of shoot development was assessed at the age of 22 months. Significant differences among provenances (P < 0.0001) were detected for all of the evaluated characters. The variation among populations was structured as a moderate altitudinal cline, with populations from lower altitudes showing larger height growth in seedlings, larger proportion of frost damages, fewer seedlings in grass-stage and more seedlings with developed shoot, whereas in populations from higher altitudes, seedlings exhibiting shorter plant height, lower percentages of frost damage, more seedlings with unbroken grass-stage, and fewer seedlings with advanced shoot development were displayed. Options for seed and seedling movement along the altitudinal gradient are discussed under the scope of reforestation, aiming at ecological restoration, conservation of forest genetic resources, and assisted migration considering global warming. We suggest delineation of two altitudinal seed zones (Zone I: 3000–3350 m; Zone II: 3350–3700 m).  相似文献   
747.
Reference evapotranspiration (ETo) determination is a key factor for water balance and irrigation scheduling. Evapotranspiration can be measured directly by high-cost micrometeorological techniques, or estimated by mathematical models. The combination equation of Penman–Monteith, modified by Allen et al. [Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. Guidelines for computing crop water requirements. FAO Irrigation and Drainage, Paper no. 56. FAO, Rome] (PM56), is the reference equation for ETo estimation. This method is also appropriate for the calibration of other ETo estimation equations. The utilization of these calibrated ETo equations is recommended in the absence of data of any of the meteorological parameters necessary for the application of PM56. In addition to the use of classic ETo equations, the adoption of artificial neural network (ANN) models for the estimation of daily ETo has been evaluated in this study. ANNs are mathematical models, whose architecture has been inspired by biological neural networks. They are highly appropriate for the modelling of non-linear processes, which is the case of the evapotranspiration process. Seven ANNs (with different input combinations) have been implemented and compared with ten locally calibrated empirical and semi-empirical ETo equations and variants of these equations (with estimated meteorological parameters as inputs). The comparisons have been based on statistical error techniques, using PM56 daily ETo values as a reference. ANNs have obtained better results than the locally calibrated ETo equations in the three groups of evaluated methods: temperature and/or relative humidity-based methods (0.385 mm d−1 of root mean square error (RMSE)), solar radiation-based methods (0.238 mm d−1 of RMSE), and methods based on similar requirements to those of PM56 except for the estimation of solar radiation and/or relative humidity (0.285 mm d−1 of RMSE).  相似文献   
748.
Differences in soil properties between forests and pastures have been well documented in the literature, especially under coniferous forests. However, since nearly all of these reports have been time-point comparisons, utilizing long-term paired-sites, properties of transitional states and time of their appearance can only be inferred at present. In this study, a deciduous forest ecosystem was converted to a silvopasture ecosystem by tree thinning, fertilization, and sheep incorporation of seed and forest litter. After 2 years, topsoil (0–15 cm) physico-chemical properties, particularly P fractions, and phosphatases were monitored over the growing season in these ecosystems, and a nearby pasture ecosystem. Initially, before spring vegetative growth, differences were found for pH, exchangeable cations and soil moisture, most of which could be explained by management history. Compared to forest, organic-C (Co) and organic-N (No) concentrations were reduced in silvopasture by 17 and 9%, respectively, indicative of substantial litter decomposition. Most values for all these physico-chemical properties for silvopasture were intermediate between forest and pasture, and generally remained so throughout the growing season. Initial total P (TPt), organic-P (TPo) and inorganic-P (TPi) concentrations were generally as anticipated for the forest and pasture. Silvopasture, however, had 36 and 23% greater TPo than forest and pasture, respectively, presumably due to fertilizer-P immobilization induced by incorporation of forest litter. Total P components remained essentially constant over the growing season in all ecosystems, with the exception of pasture, likely due to high forage TPi uptake. Bray I-extractable-organic-P (BrPo) and bicarbonate-extractable-organic-P (BiPo) concentrations, although consistently highest in the forest and silvopasture soils, were not reflective of the increase in TPo under silvopasture. Acid phosphatase (PMEac) activities were highest in spring in all the ecosystems, then gradually declined to typically 25–50% initial activities. Alkaline phosphatase (PMEal) activities showed a broadly-similar pattern, with exception of forest and silvopasture soils, which exhibited low activities throughout the season. For the entire data set, PMEac and PMEal activities were poorly correlated with TPi, BrPi, and BiPi. These results demonstrate that conversion of forest to silvopasture results in soil changes indicative of its rapid transition to pasture and that an increased Po reservoir results that should be taken into account in fertilizer-P recommendations for temperate silvopastures.  相似文献   
749.
Large changes in land use have occurred in the Central Spanish Pyrenees during the twentieth century. This mountain area supported an intense human use since the Middle Ages, that greatly transformed the natural landscape. The land uses changed dramatically during the first half of the twentieth century, due to major socioeconomic forces which lead to depopulation and land abandonment. Since then, a process of natural vegetation recovery has taken place. The anthropogenic impact during centuries deteriorated the soil quality, threatening the sustainability of agroecosystems and the natural vegetation re-growth. In this study, a soil survey was performed to identify the soil types and the physico-chemical properties of the soil that are relevant for maintaining the soil function for the recovery of vegetation after land abandonment. The study was conducted in the Arnás River catchment, which is representative of the region. The highest cation exchange capacity (CEC), nutrients, water retention, water availability and infiltration rates, as well as the lowest pH and carbonates, occurred in Lithic Hapludolls and Calcic Haploxerolls at the shady aspect, under a forest cover. Lithic Ustochrepts and Lithic Xerorthents on the sunny aspect, which were covered by shrubs, were of lesser quality than were the deeper and better-developed soils on the shady aspect. This is seen as an evidence of different patterns of soil degradation, which conditioned the re-growth of natural vegetation after land abandonment. The results provide insights into the main soil factors that have affected the re-establishment of natural vegetation in recent years. Recommendations are given for designing effective strategies for soil conservation after land abandonment in similar mountain environments.  相似文献   
750.
Journal of Soils and Sediments - Biogeochemical processes occurring in lake sediments are crucial for element cycling. In acidic pit lakes, they play a major role in acidity neutralization and...  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号