全文获取类型
收费全文 | 4893篇 |
免费 | 290篇 |
国内免费 | 394篇 |
专业分类
林业 | 282篇 |
农学 | 253篇 |
基础科学 | 153篇 |
1475篇 | |
综合类 | 2201篇 |
农作物 | 286篇 |
水产渔业 | 48篇 |
畜牧兽医 | 379篇 |
园艺 | 379篇 |
植物保护 | 121篇 |
出版年
2024年 | 41篇 |
2023年 | 164篇 |
2022年 | 221篇 |
2021年 | 209篇 |
2020年 | 194篇 |
2019年 | 190篇 |
2018年 | 156篇 |
2017年 | 247篇 |
2016年 | 280篇 |
2015年 | 183篇 |
2014年 | 290篇 |
2013年 | 376篇 |
2012年 | 417篇 |
2011年 | 369篇 |
2010年 | 282篇 |
2009年 | 329篇 |
2008年 | 240篇 |
2007年 | 262篇 |
2006年 | 218篇 |
2005年 | 183篇 |
2004年 | 101篇 |
2003年 | 83篇 |
2002年 | 60篇 |
2001年 | 71篇 |
2000年 | 54篇 |
1999年 | 43篇 |
1998年 | 33篇 |
1997年 | 40篇 |
1996年 | 43篇 |
1995年 | 31篇 |
1994年 | 28篇 |
1993年 | 45篇 |
1992年 | 17篇 |
1991年 | 20篇 |
1990年 | 17篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1984年 | 1篇 |
1981年 | 3篇 |
1978年 | 3篇 |
1975年 | 1篇 |
排序方式: 共有5577条查询结果,搜索用时 15 毫秒
91.
92.
The potential contribution of separation technologies to the management of livestock manure 总被引:1,自引:0,他引:1
Separation processes have a distinct role in the management of livestock slurries, but it is important to recognise their limitations. Such technology can direct certain manure components into a small volume of a concentrated stream which is useful for the production of organic products in subsequent processes. Equipment generally falls into systems based either on mechanical screening (which can produce a fibrous and seemingly dry product), filtration processes (producing a cake), or sedimentation leading to a sludge product. Although physical separation can remove up to 80% of the total solids content from livestock manures, this will only include a relatively small part of the soluble nutrient and of the reactive organic matter; this is particularly so where separation is based on screens. The complete removal of all suspended matter (total clarification) of an effluent is theoretically possible by settling — a decanter centrifuge will accelerate the process. This can remove most of the phosphorous, especially if coupled with chemical pre-treatment to raise the pH. However, the clarified stream still retains a significant polluting potential in terms of the residual nitrogen content (as ammonia) and potassium. Only membrane separation can greatly reduce the potassium concentration, but such technology is rarely suitable for the farm situation. 相似文献
93.
目的:应用SYBR G reen I染料法建立检测Mad2 mRNA表达的实时荧光定量PCR的方法,并探讨小鼠卵母细胞减数分裂成熟中Mad2 mRNA的表达。方法:采用实时荧光定量PCR的方法,以SYBR G reen I为荧光染料,梯度稀释的重组质粒为模板制作标准曲线,并以此方法分析了小鼠卵母细胞减数分裂成熟过程中Mad2 mRNA表达的动态变化。结果:建立了实时荧光定量检测方法分析小鼠卵母细胞中Mad2 mRNA的表达,该方法灵敏、特异,扩增效率接近100%。进行了标准曲线的制作,以及对目的基因Mad2转录水平的绝对定量。结论:实时荧光定量PCR是检测基因表达水平的理想选择,在小鼠卵母细胞减数分裂成熟过程中Mad2 mRNA表达存在动态变化。 相似文献
94.
Treatment of cow-waste slurry by a microbial fuel cell and the properties of the treated slurry as a liquid manure 总被引:1,自引:0,他引:1
Hiroshi YOKOYAMA Hideyuki OHMORI Mitsuyoshi ISHIDA Miyoko WAKI Yasuo TANAKA 《Animal Science Journal》2006,77(6):634-638
Resource recycling and the proper treatment of animal waste to reduce its environmental impact are currently important issues for the livestock industry. A microbial fuel cell (MFC), a new type of bioreactor, is expected to play roles in both waste‐water purification and energy recovery. However, the generation of electricity from cow waste has not yet been examined. In this study, using an MFC, we examined the possibility of generating electricity from dairy‐cow waste slurry, and analyzed the properties of the treated slurry as liquid manure for resource recycling. The MFC treatment of the slurry generated electricity in a dose‐dependent manner, and the maximum power output by the MFC from a 1 g of chemical oxygen demand/L slurry was 0.34 mW/m2. After the MFC treatment, 84% of the biological oxygen demand in the slurry was removed and three essential fertilizer elements (nitrogen, phosphorus, and potassium) were retained at 84, 70, and 91% levels, respectively. The amount of ammonia nitrogen in the slurry, as an element of fast‐release fertilizer, was increased by 1.9‐fold. Although the treated slurry displayed properties that made it preferable as liquid manure, further studies to improve the electrical power output by the MFC are required for practical use. 相似文献
95.
96.
以豆瓣菜葡匐茎的上,中、下三段的试材,进行不同茎段的扦插繁殖试验结果表明,中段成活率高,分枝快,适于做扦插材料。 相似文献
97.
Recent studies have emphasized the presence of microclimates in urban settings, but most do not have the high resolution observations necessary to understand the interactions taking place at a neighborhood scale. This study used a network of 10 identical weather stations and high resolution land cover data in Knoxville, Tennessee, to analyze the microclimates of a medium-sized city with a temperate climate. Two stations were installed in each of four urban neighborhoods in locations with varying localized tree cover, and two additional stations were installed in the center of downtown and in a nearby urban nature center. The intra-neighborhood results suggested that there is significant temperature variability within a single neighborhood based on the tree canopy density immediately surrounding a given weather station. However, the inter-neighborhood variability (differences between neighborhoods) was similar in magnitude, which suggests that the overall differences in neighborhood characteristics also have an effect on climate. Land cover at the neighborhood scale (in particular tree canopy percentages at the 500-m radii) had the highest correlation with the minimum daily temperature (Tmin) during the summer season. Maximum daily temperature (Tmax) relied most on the distance of each station from Downtown and the amount of impervious area in the 50 m surrounding each station. Tmax was also most influenced by surrounding land cover during dry conditions (a Dry Moderate air mass). Overall, highly localized impervious land cover percentages and larger-scale forested canopy were important in explaining temperature fluctuation, pointing to the importance of scale in microclimate assessments. Dry air masses enhanced the relationship between land cover and temperature during the day, while moist air masses did the same overnight. These data can be used to better inform planning strategies to build resiliency to extreme heat into urban environments by considering the influence of tree canopy. 相似文献
98.
农业院校开展绿色化学教育研究 总被引:2,自引:1,他引:1
简要介绍了绿色化学的内涵,阐述了开展绿色化学教育的必要性,提出了开展绿色化学教育的设想与措施。 相似文献
99.
堆沤肥是中国畜禽粪便处理利用的最主要方式。为探究不同堆沤肥方式对温室气体和氨气排放规律和温室效应,在北京市密云区开展了为期71d的堆沤肥试验,以牛粪和玉米秸秆为原料,分析了静态堆沤 (T1)、堆沤+翻抛 (T2)、堆沤+覆膜 (T3)、翻抛+覆膜 (T4)等4种方式对堆沤肥过程中腐熟度、温室气体和氨气等指标的影响。结果表明,4个处理的种子发芽指数分别达到131.33%、134.49%、108.76%和136.24%,均满足堆肥产品的腐熟度要求(≥70%)。翻抛有利于CO2、N2O和NH3的排放,而覆膜抑制了N2O的生成,T2处理的增温潜势最高,T3处理的增温潜势最低,较对照组T1处理减排20.14%。因此,T3处理在能够保证堆肥产品的腐熟度要求的前提下,还可有效减少增温潜势。该研究可为中小养殖场户堆沤肥的工艺优化实践提供技术指导。 相似文献
100.
Water use of spring wheat to raise water productivity 总被引:1,自引:0,他引:1
In semi-arid environments with a shortage of water resources and a risk of overexplotation of water supplies, spring wheat (Triticum aestivum L.) is a crop that can reduce water use and increase water productivity, because it takes advantage of spring rainfall and is harvested before the evaporative demands of summer. We carried out an experiment in 2003 at “Las Tiesas” farm, located between Barrax and Albacete (Central Spain), to improve accuracy in the estimation of wheat evapotranspiration (ETc) by using a weighing lysimeter. The measured seasonal ETc averages (5.63 mm day−1) measured in the lysimeter was 417 mm compared to the calculated ETc values (5.31 mm day−1) calculated with the standard FAO methodology of 393 mm. The evapotranspiration crop coefficient (Kc) derived from lysimetric measurements was Kc-mid: 1.20 and Kc-end: 0.15. The daily lysimeter Kc values were fit to the evolution linearly related to the green cover fraction (fc), which follows the crop development pattern. Seasonal soil evaporation was estimated as 135 mm and the basal crop coefficient approach was calculated in this study, Kcb which separates crop transpiration from soil evaporation (evaporation coefficient, Ke) was calculated and related to the green cover fraction (fc) and the Normalized Difference Vegetation Index (NDVI) obtained by field radiometry in case of wheat. The results obtained by this research will permit the reduction of water use and improvement of water productivity for wheat, which is of vital importance in areas of limited water resources. 相似文献