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   22431篇
  免费   1215篇
  国内免费   776篇
林业   6685篇
农学   625篇
基础科学   1376篇
  2878篇
综合类   8070篇
农作物   694篇
水产渔业   1051篇
畜牧兽医   1285篇
园艺   437篇
植物保护   1321篇
  2024年   77篇
  2023年   333篇
  2022年   384篇
  2021年   537篇
  2020年   617篇
  2019年   746篇
  2018年   454篇
  2017年   606篇
  2016年   794篇
  2015年   677篇
  2014年   1176篇
  2013年   1306篇
  2012年   1813篇
  2011年   1883篇
  2010年   1446篇
  2009年   1579篇
  2008年   1399篇
  2007年   1428篇
  2006年   1142篇
  2005年   1099篇
  2004年   877篇
  2003年   762篇
  2002年   567篇
  2001年   506篇
  2000年   439篇
  1999年   334篇
  1998年   223篇
  1997年   218篇
  1996年   192篇
  1995年   165篇
  1994年   138篇
  1993年   104篇
  1992年   114篇
  1991年   85篇
  1990年   63篇
  1989年   65篇
  1988年   45篇
  1987年   20篇
  1986年   5篇
  1985年   4篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A two-year field trial was conducted to determine the growth response, and root emergence pattern of interior lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) seedlings in response to container type and forest floor removal. Seedlings were grown in StyroblocksTM, CopperblocksTM, or AirBlocksTM, and were planted directly into the undisturbed forest floor or into manually prepared planting spots where the forest floor had been scraped away to expose the mineral soil. Seedlings planted into scalped planting spots exhibited marginally but significantly (7%) greater above-ground growth rates (seedling stem volume); whereas seedlings planted into the forest floor produced significantly more (11%) new roots. There were no differences in above- or below-ground biomass. Seedlings grown in CopperblockTM containers produced a higher proportion of roots near the top of the plug when tested at lifting, however this pattern was not observed in the field. Given that scalping is more costly than forest floor planting, and that the increased shoot growth was relatively small, we recommend that forest floor planting be considered as an alternative to manual spot scalping for sites, such as the site tested here: those with cold, but well-drained soils and where competition from other plants is not a serious problem.  相似文献   
992.
介绍了森林资源管理目前发展的状况,阐述了研建林业局级森林资源管理系统的必要性,提出了该系统的设计方案。  相似文献   
993.
以麻竹笋用林高效栽培为目标,可持续经营为原则,总结了竹林林分结构调控、土壤管理、散生状培育、地理种源和无性系选优等操作简便、应用效果好的系列实用技术,分析了目前麻竹林立地生产力退化、产品产量和质量下降等的主要原由,提出了无害化、品种化和散生状培育的经营方向。  相似文献   
994.
通过对新植林采取两种经营管理模式的调查研究,获得两种不同效果,得出:实行农林间种,所形成的立体林分结构,不仅对林木生长无抑制作用,而且还能促进林木的生长发育,出现林茂粮丰的效果;实行单一营林,封闭经营管理林分,林木长势差,经济效益低;大枣经济林造林后可连续间种5~7a,杨树速生丰产林造林后可连续间种3~4a。  相似文献   
995.
四川天然林资源丰富,珍稀物种繁多,生态效能巨大,具有景观资源和生态系统多样性,针对这些特点,文章提出要切实保护好现有天然林,使之成为建设长江上游生态屏障的主体;采用近自然林业经营技术,对天然林实施合理经营;建立与可持续经营要求相适应的天然林资源调查监测体系和政策、法律法规保障体系;充分利用林区丰富的景观资源和野生经济资源,合理开发生态旅游、野生土特产品;鼓励和支持社会、公众广泛参与这项工作,加强自然保护区建设等方面,对天然林实行可持续经营。  相似文献   
996.
Relatively little is known about soil organic carbon (SOC) dynamics in montane ecosystems of the semi-arid western U.S. or the stability of current SOC pools under future climate change scenarios. We measured the distribution and quality of SOC in a mosaic of rangeland-forest vegetation types that occurs under similar climatic conditions on non-calcareous soils at Utah State University's T.W. Daniel Experimental Forest in northern Utah: the forest types were aspen [Populus tremuloides] and conifer (mixture of fir [Abies lasiocarpa] and spruce [Picea engelmannii]); the rangeland types were sagebrush steppe [Artemisia tridentata], grass-forb meadow, and a meadow-conifer ecotone. Total SOC was calculated from OC concentrations, estimates of bulk density by texture and rock-free soil volume in five pedons. The SOC quality was expressed in terms of leaching potential and decomposability. Amount and aromaticity of water-soluble organic carbon (DOC) was determined by water extraction and specific ultra violet absorbance at 254 nm (SUVA) of leached DOC. Decomposability of SOC and DOC was derived from laboratory incubation of soil samples and water extracts, respectively.

Although there was little difference in total SOC between soils sampled under different vegetation types, vertical distribution, and quality of SOC appeared to be influenced by vegetation. Forest soils had a distinct O horizon and higher SOC concentration in near-surface mineral horizons that declined sharply with depth. Rangeland soils lacked O horizons and SOC concentration declined more gradually. Quality of SOC under rangelands was more uniform with depth and SOC was less soluble and less decomposable (i.e., more stable) than under forests. However, DOC in grass-forb meadow soils was less aromatic and more bioavailable, likely promoting C retention through cycling. The SOC in forest soils was notably more leachable and decomposable, especially near the soil surface, with stability increasing with soil depth. Across the entire dataset, there was a weak inverse relationship between the decomposability and the aromaticity of DOC. Our data indicate that despite similar SOC pools, vegetation type may affect SOC retention capacity under future climate projections by influencing potential SOC losses via leaching and decomposition.  相似文献   

997.
应用实验种群生命表数据分析毛竹林本土优势种竹盲走螨和引进种胡瓜钝绥螨对毛竹4种害螨南京裂爪螨、竹裂爪螨、竹缺爪螨、竹刺瘿螨的控制能力。结果表明①胡瓜钝绥螨取食上述4种害螨的世代存活率分别为90.4%、77.55%、87.93%、81.63%,每雌总产卵量分别为38.12粒、45.77粒、35.59粒、30.26粒,而乡土优势种竹盲走螨取食上述4种害螨的世代存活率分别为95.23%、100%、87.7%、80.48%,每雌总产卵量分别为44.5、46.8、41.15、20.1粒;②竹盲走螨以南京裂爪螨为猎物时其内禀增长率(0.155)与引进种胡瓜钝绥螨(0.154)相近,均明显高于南京裂爪螨(0.1089);③竹盲走螨以竹裂爪螨为猎物其内禀增长率(0.152)与胡瓜钝绥螨(0.152)相同但明显低于竹裂爪螨(0.192);④竹盲走螨以竹缺爪螨为猎物时其内禀增长率为(0.148)明显低于引进种胡瓜钝绥螨(0.175)和其猎物(0.185);⑤竹盲走螨取食竹刺瘿螨易大量逃跑,雌螨产卵量低,引进种胡瓜钝绥螨取食竹刺瘿螨能正常生长发育,但是其内禀增长率(0.144)明显低于其取食上述其余3种害螨的内禀增长率,产卵量高于当地种竹盲走螨,并描述1998年以来每年5—6月助迁人工繁殖的胡瓜钝绥螨控制毛竹害螨蔓延的效果。  相似文献   
998.
文心兰组织培养基激素选择及组培苗的移植管理技术   总被引:2,自引:0,他引:2  
以文心兰杂交新品种星战、达卡、爪哇的茎尖及花蕾为组培材料,对适合外植体的原球茎增殖、分化的培养基、培养方式进行试验研究。对培养基激素进行筛选试验结果表明:利于原球茎诱导增殖的培养基为1/2MS 6-BA5.0mg/L NAA0.5mg/L;利于不定芽增殖的培养基为1/2MS 6-BA2.0mg/L NAA0.1mg/L;利于生根的培养基为1/2MS NAA0.5mg/L。其试管苗移植的栽培基质以小石子 小树皮(1:1)为佳。并从控制温、湿度,采取合理的水肥管理措施,进行病害防治等方面,介绍了文心兰组培苗的移植管理技术。  相似文献   
999.
论述了宜昌公益林可持续经营数量化评价的指导思想和基本原则,并以公益林可持续经营的基本要素为依据,建立了综合评价指标变量集;通过主成分分析和层次分析,提取了累计贡献率大于81%的11个指标因子,确定了各指标因子的权重;从发展规模总量和结构协调度两方面对公益林可持续经营能力进行了评价,认为宜昌公益林的发展规模不断扩大,但其结构协调度DHT=0.722,协调能力较差,仍处于弱可持续发展阶段.  相似文献   
1000.
在简述林业分类经营背景的基础上,提出了林业分类经营的原则,重点提出了商品林和公益林分类指标体系和技术标准,最后探讨了国有林场的分类经营。  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