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   831篇
  免费   42篇
  国内免费   3篇
林业   47篇
农学   23篇
基础科学   2篇
  150篇
综合类   59篇
农作物   42篇
水产渔业   55篇
畜牧兽医   441篇
园艺   19篇
植物保护   38篇
  2023年   14篇
  2022年   15篇
  2021年   40篇
  2020年   36篇
  2019年   33篇
  2018年   36篇
  2017年   28篇
  2016年   15篇
  2015年   26篇
  2014年   37篇
  2013年   58篇
  2012年   61篇
  2011年   73篇
  2010年   38篇
  2009年   48篇
  2008年   50篇
  2007年   62篇
  2006年   53篇
  2005年   30篇
  2004年   33篇
  2003年   23篇
  2002年   27篇
  2001年   4篇
  2000年   3篇
  1999年   9篇
  1998年   2篇
  1997年   5篇
  1996年   1篇
  1995年   3篇
  1993年   2篇
  1992年   1篇
  1991年   3篇
  1990年   2篇
  1989年   2篇
  1985年   1篇
  1976年   1篇
  1955年   1篇
排序方式: 共有876条查询结果,搜索用时 15 毫秒
871.
An excess of available nitrogen (N) in vineyard soil is considered detrimental for vine growth, making a thorough assessment of N mineralization dynamics in vineyard soils before the addition of winery waste necessary. This study assesses the changes in N mineralization in acid vineyard soils amended with bentonite winery waste (BW). Non-amended soil (control), BW and soil-waste mixtures (SBWM) with a low (+L) or high (+H) dose of BW were incubated for six weeks. After 7, 14, 21 and 42 days of incubation, the control soils, BW and SBWM were analysed for net ammonified N, net nitrified N and net mineralized N. Parameters related to the kinetics of N mineralization were also determined. The addition of BW increased the potentially mineralizable N (N0) in the amended soils (58–144% for the highest BW dose), although the mineralization rate was governed by the soil characteristics. Mineralizable N was only a small fraction (<4%) of the total organic nitrogen added to the soil through the BW addition, mainly due to the dominance of the nitrification process in the BW amended soils. These experimental results suggest that the addition of BW may be a suitable amendment for nitrogen fertilization in acid vineyard soil.  相似文献   
872.
The intensive use of pesticides has increased exponentially in Brazil and worldwide due to the need to meet the food demands of a growing population. If the management/monitoring of the use of pesticides is adequately performed, it would not compromise the expected benefits or have negative effects on the environment as a whole. In order to examine the information available on herbicide use in Brazil and worldwide, this paper presents a review of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and its chemical properties, action on target organisms, environmental fate, and toxicity to non-target organisms. This herbicide is a synthetic auxin used to control broad-leaved weeds, and the action in target organisms is well known. Although 2,4-D has been widely used worldwide, many studies have shown that this herbicide induces alterations in non-target organisms. Therefore, ecotoxicology studies are important to assess the risk the herbicides can be to different ecosystems. Thus, it is advised to use this herbicide and other pesticides with caution.  相似文献   
873.
In this study, the impact of rose chafer (Cetonia aurata L.) larvae on net and gross methane (CH4) fluxes in soil from an old permanent grassland site (Giessen, Germany) was investigated. Previous studies at this site suggested the existence of Scarabaeidae larvae-induced “CH4-emitting hot spots” within the soil profile which may subsequently lead to increased CH4 oxidation. The net (soil + larvae) and gross (soil and larvae separated) CH4 fluxes were studied in a 3-month laboratory incubation. Addition of larvae changed the soil from a net sink (?330 ± 11 ng CH4 kg?1 h?1) to a net source (637 ± 205 ng CH4 kg?1 h?1). Supply of plant litter to the soil + larvae incubation jars tended to increase CH4 emissions which was not significant due to large variability. After 11–13 weeks of incubation, the net soil CH4 oxidation was significantly stimulated by 13–21% in the treatments containing larvae when these were taken out. Analysis of archaeal 16S rRNA genes revealed that the majority of the obtained clones were closely related to uncultured methanogens from guts of insects and other animals. Other sequences were relative to cultivated species of Methanobrevibacter, Methanoculleus, and Methanosarcina. Hence, Scarabaeidae larvae in soils (i) may represent an underestimated source of CH4 emissions in aerobic upland soils, (ii) may stimulate gross CH4 consumption in their direct soil environment, and, thus, (iii) contribute to the spatial heterogeneity often observed in the field with closed-chamber measurements. Long-term CH4-flux balances may be wrongly assessed when “exceptional” net CH4 flux rates (due to larvae hot spots) are excluded from data sets.  相似文献   
874.
Recent evidence suggests a possible association of Felis catus papillomavirus type 2 (FcaPV-2) DNA with feline oral squamous cell carcinoma (FOSCC). In this study, type-specific PCR targeting two genes (L1/E6 or E1/E6) of FcaPV-1/-2/-3/-4/-5/-6 was performed to detect viral DNA in a large amount of FOSCC samples collected in Italy and Austria. FcaPV-1/-2/-3/-4/-5 were detected in 7/113 (6.2%), 7/93 (7.5%), 6/113 (5.3%), 1/113 (0.9%) and 2/113 (1.8%) specimens, respectively, with different prevalences in Italian vs. Austrian samples, whilst FcaPV-6 went undetected. Our results confirms that FcaPV-2 is the most prevalent in FOSCC, followed by FcaPV-1/-3 and suggest that FcaPVs have variable circulation rates in European countries.  相似文献   
875.
The anatomy of the sternum in birds varies according to their habitats and type of locomotion. In particular, the carina sterni manifest different shape variations. In this study, the shape of the carina sterni was investigated by means of geometric morphometrics. Birds of different types of locomotion were used in the study: flying, swimming, and terrestrial. Ducks and chickens show a wider variety of shapes. Pigeons are the species with the least differences. The margo cranialis carinae in a turkey is the flattest compared to other species. In chickens, the apex carinae is more caudally than the base of the carina sterni. The margo cranialis of the carina sterni in ducks is concave. The differences in centroid size and shape differences between species collectively are statistically significant (p < 0.0001). The most distinct shape contrast is between the duck and turkey (p: 0.0003). Form differences between the ducks and geese as well as between the chicken and turkey are statistically insignificant. There is less variation in the shape of the carina sterni among avian species representing the same type of locomotion. Although there are many comparative morphological and morphometric studies of birds, shape analysis studies revealing the interspecific differences and similarities of the sternum are very limited. Morphology of the carina sterni can be useful in taxonomic investigations.  相似文献   
876.
  1. Commodity-driven forest conversion represents one of the most severe threats to freshwater biodiversity in Southeast Asia, notably causing population declines and the extinction of freshwater fish species.
  2. Although a variety of freshwater taxa are likely to be adversely affected by forest conversion, little is known about the impact on ecologically and economically important invertebrates such as decapod crustaceans.
  3. This study evaluated the impact of forest conversion and land-use change on freshwater Macrobrachium shrimp species, using species richness, abundance, and environmental data collected from 20 streams across southern Peninsular Malaysia. Streams were located in three types of landscape: forest; oil palm plantation; and mixed land use, comprising young secondary forest, small-scale plantations, patches of open and sparsely vegetated areas, and agricultural fields and clearings.
  4. Generalized linear models showed that even incomplete change from forest habitats to mixed land use and oil palm plantation resulted in significantly lower Macrobrachium native species richness and higher non-native species abundance. Native species richness was positively correlated with canopy cover, leaf litter, substrate size, and dissolved oxygen, and was negatively correlated with water temperature and conductivity. Native species richness was also negatively correlated with non-native species abundance, with non-native species abundance increasing along the human disturbance gradient.
  5. These results highlight the need for riparian habitat protection to conserve native Macrobrachium and limit the spread of non-native species. A management priority should be to maintain or restore optimum instream habitat conditions for shrimps, which would also benefit fish and other benthic macroinvertebrates. Suitable riparian management requires substantial support and funding from multiple stakeholders, but it can be aligned with other catchment-based strategies to optimize the use of limited resources available for freshwater biodiversity conservation.
  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