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   912篇
  免费   48篇
  国内免费   34篇
林业   123篇
农学   71篇
基础科学   27篇
  167篇
综合类   290篇
农作物   41篇
水产渔业   71篇
畜牧兽医   100篇
园艺   33篇
植物保护   71篇
  2024年   8篇
  2023年   16篇
  2022年   18篇
  2021年   32篇
  2020年   21篇
  2019年   34篇
  2018年   25篇
  2017年   32篇
  2016年   26篇
  2015年   43篇
  2014年   55篇
  2013年   77篇
  2012年   43篇
  2011年   74篇
  2010年   55篇
  2009年   68篇
  2008年   66篇
  2007年   61篇
  2006年   39篇
  2005年   37篇
  2004年   32篇
  2003年   12篇
  2002年   24篇
  2001年   10篇
  2000年   9篇
  1999年   6篇
  1998年   15篇
  1997年   8篇
  1996年   15篇
  1995年   11篇
  1994年   2篇
  1993年   4篇
  1992年   1篇
  1991年   2篇
  1990年   3篇
  1989年   4篇
  1988年   4篇
  1985年   1篇
  1984年   1篇
排序方式: 共有994条查询结果,搜索用时 328 毫秒
101.
Expert opinions have been used in a variety of fields to identify relevant issues and courses of action. This study surveys experts in forestry and climate change from the Asia–Pacific region to gauge their perspectives on the impacts of climate change and on the challenges faced by forest adaptation in the region, and explores recommendations and initiatives for adapting forests to climate change. There was consensus regarding the impacts of climate change on forest ecosystems and on economic sectors such as agriculture and forestry. Respondents also indicated a lack of public awareness and policy and legislation as challenges to addressing climate change. However, the results indicate differences in opinion between regions on the negative impacts of climate change and in satisfaction with actions taken to address climate change,highlighting the need for locally specific policies and research. The study presents specific recommendations to address issues of most concern, based on subregion and professional affiliation throughout the Asia–Pacific region.The results can be used to improve policy and forest management throughout the region. This research will also provide valuable suggestions on how to apply research findings and management recommendations outside of the AP region. The conclusions should be communicated relative to the level of the research and the target audience,ensuring that scientific findings and management recommendations are effectively communicated to ensure successful implementation of forest adaptation strategies.  相似文献   
102.
为研究不同牡丹品种引种的适应性,对初步筛选出的8个名优牡丹品种的生长特性和开花性状进行了研究。结果表明:筛选出的8个牡丹引种品种适生性良好,能够正常地分蘖,各引进品种的花型和花色等表型表现基本与原产地相当,均能维持较高的观赏价值。通过进一步对引种适应性进行综合评价,筛选出的8个牡丹引种品种的各项得分均在10分以上,且综合评分均较高(≥70分),说明引进品种的适应性均较好,开花品质均较高,可从中筛选出引种适应性综合评分较高的品种进行进一步的引种驯化研究。  相似文献   
103.
《水土保持通报》2019,39(3):307-312
[目的]回顾和总结近年来东南亚地区森林植被变化与水土流失演变领域研究项目的成果,并探讨东南亚地区改良后的保育措施在流域尺度水土流失与产沙管理方面的成效。[方法]根据东南亚各国不同时空尺度的数据,分析了该区森林植被的变化及采取的对策。选择泰国实施流域管理的地区为研究区,阐述了被动和主动侵蚀控制策略的有效性和实用性。[结果]①在2000—2005年期间,东南亚地区的森林面积大幅减少,累计年度森林砍伐率达到2.76×10~6 hm~2。然而,由于森林种植园的建立,该区森林覆盖率近期已经增加。②东南亚雾霾是一种定期发生的大范围空气污染问题,它在很大程度上起因于农业上的非法放火烧荒。③东南亚地区土壤侵蚀管理措施包括带状种植、等高耕作、条状种植、草障、保护性耕作、免耕种植和植物篱间作。尽管在小区尺度上,保护措施在减少径流方面发挥了良好的作用,但对大尺度而言,其作用还有待进一步的研究和验证。水土保持对产量的潜在影响可能是影响水土保持投资价值的一个关键因素。[结论]东南亚地区森林覆盖变化的挑战既真实又迫切。我们应该建立区域防控机制减少森林火灾和雾霾,并通过保持一个可持续的林业,维护陆地生物多样性减少土壤侵蚀。为低收入小户型农民选择适当的水土保持方法,对于降低水土流失速度和提高作物产量至关重要。  相似文献   
104.
中部地区生态经济系统协调度的时空演化   总被引:1,自引:1,他引:0  
[目的]对中部地区生态经济系统协调度进行评估,为该地区生态经济系统治理提供理论支持。[方法]基于1992,2000,2008和2016年中部6省GDP数据和Landsat TM遥感影像土地利用数据,计算生态系统服务价值(ESV)以及生态经济系统协调度(EEH),评估其生态经济系统协调度及时空演化特征。[结果]①1992年以来,中部地区GDP呈现快速增长模式,但增长速度具有明显的时空差异。安徽和江西2省快于其他地区,湖南省增长最为缓慢;②中部地区生态系统服务价值(ESV)波动上升,呈现良性发展态势,空间分布特征是南部高于北部,西部高于东部;③1992—2016年中部地区EEH值在各省变化较为明显,安徽和湖北省EEH呈现改善趋势,河南省EEH值最高,其余省份改善不明显或有下降趋势。[结论]由于各地区自然条件和社会经济等方面的差异,生态经济系统协调度(EEH)存在一定差异,可持续发展水平也表现出一定的空间分异。  相似文献   
105.
Soundly-based conservation plans for Anatidae require abundance and distributional data to provide accurate estimation of population sizes and trends and to identify key sites for protection. Here, we report the first ever extensive surveys throughout eastern China’s wetlands for Anatidae, 80% of which occurred in the Yangtze floodplain. Population estimates for 24 species with sufficient data are provided, allowing assessment of the accuracy of current flyway population estimates; fourteen species are far less numerous than previously believed and three far more numerous. Further improvement of the population estimates for eastern China will need coordinated counts across the region requiring a large, skilled counter network.  相似文献   
106.
We provide estimates of glacier mass changes in the High Mountain Asia (HMA) area from April2002 to August 2016 by employing a new version of gravity solutions of the Gravity Recovery and ClimateExperiment (GRACE) twin-satellite mission. We find a total mass loss trend of the HMA glaciers at a rateof –22.17 (±1.96) Gt/a. The largest mass loss rates of –7.02 (±0.94) and –6.73 (±0.78) Gt/a are found forthe glaciers in Nyainqentanglha Mountains and Eastern Himalayas, respectively. Although most glaciers inthe HMA area show a mass loss, we find a small glacier mass gain of 1.19 (±0.55) and 0.77 (±0.37) Gt/a inKarakoram Mountains and Western Kunlun Mountains, respectively. There is also a nearly zero massbalance in Pamirs. Our estimates of glacier mass change trends confirm previous results from the analysisof altimetry data of the ICESat (ICE, Cloud and Land Elevation Satellite) and ASTER (AdvancedSpaceborne Thermal Emission and Reflection Radiometer) DEM (Digital Elevation Model) satellites inmost of the selected glacier areas. However, they largely differ to previous GRACE-based studies which weattribute to our different post-processing techniques of the newer GRACE data. In addition, we explicitlyshow regional mass change features for both the interannual glacier mass changes and the 14-a averagedseasonal glacier mass changes. These changes can be explained in parts by total net precipitation (netsnowfall and net rainfall) and net snowfall, but mostly by total net radiation energy when compared to datafrom the ERA5-Land meteorological reanalysis. Moreover, nearly all the non-trend interannual masschanges and most seasonal mass changes can be explained by the total net radiation energy data. The massloss trends could be partly related to a heat effect due to increased net rainfall in Tianshan Mountains, QilianMountains, Nyainqentanglha Mountains and Eastern Himalayas. Our new results for the glacier mass changein this study could help improve the understanding of glacier variation in the HMA area and contribute tothe study of global change. They could also serve the utilization of water resources there and in neighboringareas.  相似文献   
107.
LIU Pengde 《干旱区科学》2021,13(11):1163-1182
West Junggar is a key area for understanding intra-oceanic plate subduction and the final closure of the Junggar Ocean. Knowledge of the Carboniferous tectonic evolution of the Junggar Ocean region is required for understanding the tectonic framework and accretionary processes in West Junggar, Central Asian Orogenic Belt. A series of Early Carboniferous volcanic and intrusive rocks, namely, basaltic andesite, andesite, dacite, and diorite, occur in the Mayile area of southern West Junggar, northwestern China. Our new LA-ICPMS zircon U-Pb geochronological data reveal that diorite intruded at 334 (±1) Ma, and that basaltic andesite was erupted at 334 (±4) Ma. These intrusive and volcanic rocks are calc-alkaline, display moderate MgO (1.62%-4.18%) contents and Mg# values (40-59), and low Cr (14.5×10-6-47.2×10-6) and Ni (7.5×10-6-34.6×10-6) contents, and are characterized by enrichment in light rare-earth elements and large-ion lithophile elements and depletion in heavy rare-earth elements and high-field-strength elements, meaning that they belong to typical subduction-zone island-arc magma. The samples show low initial 87Sr/86Sr ratios (range of 0.703649-0.705008), positive εNd(t) values (range of 4.8-6.2 and mean of 5.4), and young TDM Nd model ages ranging from 1016 to 616 Ma, indicating a magmatic origin from depleted mantle involving partial melting of 10%-25% garnet and spinel lherzolite. Combining our results with those of previous studies, we suggest that these rocks were formed as a result of northwestward subduction of the Junggar oceanic plate, which caused partial melting of sub-arc mantle. We conclude that intra-oceanic arc magmatism was extensive in West Junggar during the Early Carboniferous.  相似文献   
108.
本研究利用单向水平板淀粉凝胶电泳以我国5个固有地方绵羊品种包括湖羊(Hu)、同羊(Tong)、滩羊(Tan)、小尾寒羊(Han)、洼地羊(WD)为研究对象对X-蛋白遗传多样性进行了检测,并引用我国周边国家、地区绵羊品种为分析背景。结果表明以喜马拉雅山脉为界的亚洲东部和南部的北部群体和南部群体在编码的X(+)型的显性等位基因X频率上存在极显著差异(P〈0.01)。北部检测的群体中X等位基因频率范围为0~0.18,平均值为0.0878,包括属于藏羊和蒙古羊系统的小尾寒羊(Han)、不丹羊(Bhutan)、同羊(Tong)、湖羊(Hu)、滩羊(Tan)、洼地绵羊(WD)、Bhyanglung绵羊(Bhy)、Baruwal绵羊(Bar)、云南绵羊(Yunnan)、哈拉和林绵羊(Kh)和乌兰巴托绵羊(Ub)。南部检测的群体中X等位基因频率范围为0.2037~0.4655,平均值为0.3082,包括属于印度绵羊系统的孟加拉国绵羊(Ban)、Kagi绵羊(Kagi)、Lampuchhre绵羊(Lamp)、越南占部落绵羊(Cham)和缅甸绵羊(Mya)。本研究揭示X等位基因可作印度绵羊的标记,在绵羊群体尤其是亚洲东部和南部绵羊系统形成的研究中具有潜在的重要作用。  相似文献   
109.
BACKGROUND: Although the histopathologic features of necrotizing meningoencephalitis (NME) have been described previously, little information is available concerning the signalment, geographic distribution, seasonal onset, treatment, and survival of affected dogs. ANIMALS: Sixty Pugs with NME and 14 contemporaneous control Pugs with other intracranial diseases (non-NME group). METHODS: Pugs that were euthanized or died because of intracranial disease were prospectively obtained. All dogs had necropsy, histopathology, and testing for various infectious diseases and were subsequently divided into NME and non-NME groups. Signalment, geographic distribution, seasonal onset, treatment, and survival were compared between groups. RESULTS: In Pugs with NME, median age at onset of clinical signs was 18 months (range, 4-113 months). A greater proportion of female dogs were present in the NME group (40/60) compared with the control group (6/14). Pugs with NME had a significantly lower mean weight (7.81 kg) than control Pugs (9.79 kg) (P= .012). Mean survival in Pugs with NME was 93 days (range, 1-680 days), with dogs receiving any form of treatment living significantly longer than those that were not treated (P= .003). Anticonvulsive drugs were the only treatment significantly associated with longer survival (P= .003). CONCLUSIONS AND CLINICAL IMPORTANCE: NME appears to be a common cause of intracranial signs in Pugs, based on the high proportion of NME dogs reported in this population. Pugs with NME are most commonly young adult female dogs. Although further investigation is needed to determine the optimal treatment of NME, anticonvulsive drugs appear to beneficially affect duration of survival.  相似文献   
110.
BACKGROUND: Preclinical studies of peripheral blood mononuclear cell (PBMC) transplantation conducted in a well-established canine hematopoietic cell transplantation (HCT) model have been successfully translated to human patients over the past 5 decades. OBJECTIVE: We retrospectively investigated the safety and feasibility of PBMC apheresis in the canine model of HCT by analyzing apheresis parameters, cell yields, and the impacts of donor-related and apheresis-related variables on collection yields and donor stability. ANIMALS: One hundred and twenty dogs that underwent PBMC aphereses were evaluated. METHODS: Aphereses were performed with a COBE Spectra blood separator and a central dual-lumen catheter, with or without recombinant canine granulocyte colony-stimulating factor (rcG-CSF) stem cell mobilization. RESULTS: Aphereses from dogs not given rcG-CSF yielded an average volume of 280 +/- 42 mL containing an average of 15,086 +/- 9,834 leukocytes/mL. Aphereses from dogs given rcG-CSF yielded an average volume of 261 +/- 55 mL containing an average of 39,711 +/- 24,488 leukocytes/mL. Higher pre-apheresis white blood cell (WBC) counts correlated with higher apheresis WBC yields (R=0.50, P<.0001). The correlations of collection time, inlet volume, and collection flow rate on WBC yields were statistically significant but only weak to moderate in magnitude (R=0.34, P=.0001; R=0.38, P=.0006; R=0.26, P=.002, respectively) as were the correlations of collection time and inlet volume on collection volumes (R=0.30, P=.002; R=0.42, P<.0001, respectively). All dogs recovered promptly after PBMC aphereses and catheter removal, without complications. CONCLUSIONS AND CLINICAL IMPORTANCE: These data may be useful for translating PBMC apheresis technology to the field of veterinary oncology for the treatment of dogs with hematologic malignancies.  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