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   410篇
  免费   35篇
  国内免费   1篇
林业   40篇
农学   9篇
基础科学   4篇
  89篇
综合类   32篇
农作物   14篇
水产渔业   25篇
畜牧兽医   190篇
园艺   10篇
植物保护   33篇
  2023年   6篇
  2022年   5篇
  2021年   19篇
  2020年   20篇
  2019年   16篇
  2018年   21篇
  2017年   20篇
  2016年   15篇
  2015年   19篇
  2014年   27篇
  2013年   25篇
  2012年   33篇
  2011年   49篇
  2010年   25篇
  2009年   17篇
  2008年   26篇
  2007年   22篇
  2006年   12篇
  2005年   20篇
  2004年   15篇
  2003年   9篇
  2002年   10篇
  2001年   2篇
  2000年   2篇
  1999年   3篇
  1998年   1篇
  1994年   1篇
  1993年   1篇
  1991年   1篇
  1988年   1篇
  1984年   1篇
  1972年   1篇
  1969年   1篇
排序方式: 共有446条查询结果,搜索用时 15 毫秒
441.
The removal efficiency and tolerance of Typha domingensis to Cr(VI) in treatments with and without organic matter (OM) addition were evaluated in microcosm-scale wetlands. Studied Cr(VI) concentrations were 15 mg L?1, 30 mg L?1, and 100 mg L?1, in treatments with and without OM addition, arranged in triplicate. Controls (without neither metal nor OM addition—without metal with OM addition) were disposed. Cr(VI) was removed efficiently from water in all treatments. OM addition enhanced significantly Cr(VI) and total Cr removals from water. In the treatments with OM addition, significantly higher Cr concentrations were found in sediment than the treatments without OM addition. Plants of the treatments without OM addition showed significantly higher Cr concentrations in tissues but lower biomass increase than the treatments with OM addition. The highest Cr concentrations in tissues were observed in submerged parts of leaves, followed by roots. According to SEM analysis, in the 100 mg L?1 treatments, the highest Cr accumulation was observed in the epidermis of old leaves. Although Cr(VI) produced changes in root morphology, the OM addition favored the plant growth. In T. domingensis, root morphological plasticity is an important mechanism to improve metal tolerance and Cr uptake in wetland systems minimizing the environmental impact.  相似文献   
442.
The worldwide increase of human pressure in rural areas has resulted in a progressive fragmentation of agro-forest landscapes. Relatively few studies have identified processes of landscape modification and patch fragmentation reflecting long-term human activity from those caused by recent urbanization stimulated by economic development, population growth and improved accessibility of rural areas. In rural districts dominated by high-quality crop mosaics and exposed to increasing levels of human pressure, urbanization-driven changes in the use of land have determined complex and non-linear processes of landscape fragmentation which deserve further investigation. Multidimensional procedures and data mining are appropriate to explore such transformations, basing on a comprehensive assessment of landscape metrics. The present study investigates changes in land-use and landscape structure (2000–2008) in three districts of Latium (central Italy) featuring different cropping systems, physiographic attributes and level of human pressure. A principal component analysis was run to identify a core set of metrics aimed at (i) evaluating the role of traditional cropping systems in the preservation of traditional agro-forest systems and (ii) inferring the relationship between spatial heterogeneity in rural landscapes and socio-ecological processes of change. Our results indicate that crop intensification has contributed to landscape homogeneity and simplification in all areas studied, in contrast with the effects of urban expansion resulting in a more fragmented and diversified landscape with relict tree crop patches. The findings of this study clarify the contribution of traditional tree crop systems to sustainable structures and functions of rural landscapes by preserving the place-specific eco-mosaic complexity.  相似文献   
443.
Greater amberjack is one of the major candidate species for aquaculture diversification; however, with the exception of sporadic data on the proximate (PC) and fatty acid (FA) composition, limited knowledge exists regarding the quality variations occurring between wild and reared Greater amberjack. To expand this knowledge, the current study examined, besides the PC and FA composition of raw tissue, the volatile compounds (VC) and sensory profiles of cooked tissue. The studied specimens originated both from the wild and from aquaculture and were of small commercial size (0.5–1 kg). The FA profiles varied with most profound differences being the lower n‐6/n‐3 ratio and the higher arachidonic (ARA) and docohexaenoic acid (DHA) proportions of wild counterparts. A superior lipid quality was found for reared fish, which was attributed, among other reasons to their significantly (p < .001) higher fillet lipid (wild: 0.27%; reared: 3.92%). The VC profiles differed with rearing origin. Reared Greater amberjack exhibited a significantly higher content in the majority of aromatic hydrocarbons and terpenes, while wild exhibited higher contents for the majority of compounds belonging to the alcohol, aldehyde and ketone groups. Minor sensory profile differences were observed, with reared and wild counterparts exhibiting higher butter and sardine flavour, respectively.  相似文献   
444.
A mortality event among recently captured feral donkeys (Equus asinus) occurred in south-central Utah in 2016. The deaths were sporadic, and clinical signs were indicative of respiratory disease, likely associated with an infectious etiology. Ten of 13 donkeys autopsied had moderate-to-severe interstitial fibrosing pneumonia, and one had pyogranulomatous pneumonia. Consensus PCRs directed toward the DNA polymerase and DNA packaging terminase subunit 1 for herpesviruses were performed followed by sequencing of the PCR amplicons and phylogenetic analysis. Asinine herpesvirus 4 (AsHV4) and 5 (AsHV5) were consistently identified in lung tissues of affected donkeys. No other herpesviruses were identified, and herpesviral DNA was not detected in lung tissues of 2 donkeys without evidence of respiratory disease. The detection of asinine gammaherpesviruses may have been associated with the lesions described. AsHV4 and AsHV5 have been reported in previous studies as novel gammaherpesviruses based on sequences obtained from donkeys with interstitial pneumonia and marked syncytial cell formation. Our findings suggest that the association of asinine gammaherpesviruses with respiratory conditions in equids deserves further attention.  相似文献   
445.
In February 2015, an unexpected windstorm downed five hectares of a European black pine Pinus nigra subsp. laricio forest formation located close to Vallombrosa, Florence (Central Italy). In the following spring, an extensive survey was conducted in the area. Felled trees, stumps and all the suitable plant material were screened for the presence of the pinewood nematode (PWN), Bursaphelenchus xylophilus, by sampling wood and bark. Bark beetles were then collected from the gallery systems on the inner side of bark samples and observed in the laboratory. The following bark beetles were morphologically identified: Ips sexdentatus, Orthotomicus erosus, O. laricis and Pityogenes bidentatusa. The dissection of Ips sexdentatus allowed the extraction of numerous nematodes that were morphologically and molecularly identified as Ektaphelenchoides pini. Conversely, only few nematode specimens were isolated from either pine bark or wood. These individuals could be only molecularly identified and belonged to an undescribed nematode taxon. Even though no PWN was recorded in the investigated sites, our survey allowed the detection of a new association between E. pini and I. sexdentatus on P. nigra.  相似文献   
446.
The Mediterranean region has been regarded as a critical hotspot for desertification due to the impact of soil degradation, the land‐use changes and the climate variations. Few large‐scale studies have been devoted to analyse trends in land sensitivity to desertification in the northern Mediterranean basin. The present paper contributes to this deserving issue by quantifying the level of land sensitivity to desertification in Italy at seven points between 1960 and 2010 at a fine spatial scale. The approach used followed the Environmentally Sensitive Area scheme that assesses changes in four key themes (climate, soil, vegetation and land management) related to land degradation processes. Italian land was classified into four levels of sensitivity to desertification (non‐affected, potentially affected, fragile and critical) according to the Environmentally Sensitive Area framework. Interestingly, although land surface area classified as ‘fragile’ and ‘critical’ grew homogeneously in Italy between 1960 and 1990, the increase observed in the most recent time period was spatially clustered and contributed to reverse the polarisation in ‘structurally vulnerable’ and ‘non‐affected’ regions observed in Italy. The paper discussed these trends in the light of socioeconomic changes that occurred in Italy after World War II. Copyright © 2013 John Wiley & Sons, Ltd.  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