全文获取类型
收费全文 | 7161篇 |
免费 | 288篇 |
国内免费 | 781篇 |
专业分类
林业 | 501篇 |
农学 | 642篇 |
基础科学 | 746篇 |
1204篇 | |
综合类 | 3305篇 |
农作物 | 657篇 |
水产渔业 | 375篇 |
畜牧兽医 | 312篇 |
园艺 | 365篇 |
植物保护 | 123篇 |
出版年
2024年 | 24篇 |
2023年 | 58篇 |
2022年 | 98篇 |
2021年 | 109篇 |
2020年 | 127篇 |
2019年 | 135篇 |
2018年 | 94篇 |
2017年 | 214篇 |
2016年 | 271篇 |
2015年 | 328篇 |
2014年 | 346篇 |
2013年 | 417篇 |
2012年 | 491篇 |
2011年 | 721篇 |
2010年 | 666篇 |
2009年 | 649篇 |
2008年 | 537篇 |
2007年 | 562篇 |
2006年 | 421篇 |
2005年 | 311篇 |
2004年 | 208篇 |
2003年 | 249篇 |
2002年 | 143篇 |
2001年 | 122篇 |
2000年 | 127篇 |
1999年 | 131篇 |
1998年 | 94篇 |
1997年 | 97篇 |
1996年 | 88篇 |
1995年 | 84篇 |
1994年 | 69篇 |
1993年 | 64篇 |
1992年 | 56篇 |
1991年 | 47篇 |
1990年 | 29篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有8230条查询结果,搜索用时 15 毫秒
31.
首先,提出了大连国家生态工业示范园净水厂建设的必要性;然后,分析项目使用林地对当地森林资源、生物多样性、生态效能、景观风貌、环境质量及林业发展的影响;最后,结合园区供水安全现状、相关法规及其自然地理情况,综合分析该项目使用林地的可行性,为林业管理者科学管理提供参考依据。分析表明,为了保障大连国家生态工业示范园区供水安全,建设大连国家生态工业示范园净水厂使用林地是可行的。 相似文献
32.
33.
本试验以水蚯蚓作为饵料,研究了60日龄鳝苗不同密度下的增重、存活率及饵料转化率。试验分3个密度组,其对应的密度分别为2000尾/m2、3000尾/m2、4000尾/m2,每个密度组设4个平行;试验时间为30d。结果表明:在本试验条件下,3个密度组鳝苗的增重率、存活率随着密度的增大呈下降趋势,而其饵料系数随着密度增大而增大;其中在4000尾/m2的密度下,鳝苗的增重率最低,仅为1.12±0.13;饵料系数最高,为5.71±1.37。 相似文献
34.
基于信号博弈的阳澄湖大闸蟹绿色标签市场应用分析 总被引:2,自引:0,他引:2
水产品绿色标签对消费者来说是高质量的信号,对卖家来说则是卖出高价的资本.但是欺诈行为的存在损害了消费者和其他卖家的利益,扰乱了市场秩序.以阳澄湖大闸蟹为例,通过信号博彝理论,对螃蟹市场中关卖双方的消费行为进行博彝分析,得到了精炼贝叶斯Nash均衡. 相似文献
35.
蒸腾能使木质部产生不超过-0.1MPa的有限负压,这个负压在木质部内自上而下依次向下传递。水分在导管或管胞内所受的吸附力、毛管力、真空力等综合作用力要大于或等于所受的重力,这个综合作用力可以将水分托住而不受重力作用而下移,使水分在负压作用下以分段移动的方式从根部上升到植物顶端。空穴和栓塞是两个不同的概念,其形成过程、恢复机制都有其本质的不同。空穴的发生与负压和水分的分段移动有密切的关系。 相似文献
36.
[目的]研制本质快速检测设备,实现水质的现场多指标快速检测.[方法]对设备的硬件及软件进行了设计.硬件设计包含了1P2303芯片、锂电,Cortex-M3型ARM微控制器、树莓派显示系统、光频转换器等的设计;软件设计则包含了软件设置、项目选项、项目测量结果以及测量数据表格输出等模块的设计;并对水质快速检测设备的可靠性进行了试验检验.[结果]通过对该设计的设备硬件和软件合理搭配连接,从而研发出了水质快速检测设备.用该设备与国家标准要求的设备同时测定某一河道水样的5项指标结果发现,3个样点水样中铁、氨态氮、氟化物、硝酸盐和6价铬含量的两组数据值接近,无明显差异.虽然国家标准法检测下限比快测法高一个数量级,但快速设备的测定结果不影响操作员对水质的判断,说明该水质快速检测设备精密度高;两种测定方法的稳定性比较,结果快速测定法的5个指标数据的相对偏差略高于国家标准法(3.92%),但其测定结果能够较好地满足现场测定要求,说明该水质快速检测设备稳定性良好.[结论]该水质快速检测设备的检测精度和稳定性较高,可满足水质的现场快速检测要求. 相似文献
37.
水肥耦合对小麦/玉米带田产量及构成因素的影响 总被引:5,自引:0,他引:5
采用"3414"最优回归设计方案,于2007-2008年在甘肃省农业科学院张掖节水试验站,开展不同水肥耦合处理小麦/玉米带田产量效应研究。结果表明,不同水肥耦合模式对小麦/玉米带田产量的影响较大,其差异达极显著水平。其中,氮肥对产量的贡献最大,水分次之,磷肥最小,氮肥对带田混合产量的绝对贡献率达79.4%,而水分对带田混合产量的绝对贡献率达为52.9%;水肥耦合效应为:水氮耦合水磷耦合氮磷耦合;获得高产量12 952.5~13 880.0kg/hm2的施氮量为420~630kg/hm2、灌水量为5 550~6 750m3/hm2、施磷量为120kg/hm2。相关分析表明,施氮量和灌水量与间作玉米的穗粒数、千粒质量、穗粒质量、株高均呈极显著正相关,施氮量与间作小麦的穗粒质量、穗粒数呈极显著正相关。 相似文献
38.
Design and performance of recirculating aquaculture system for marine finfish broodstock development
Tropical and subtropical climatic conditions in India present an ideal and unique opportunity for being the leader in tropical marine finfish aquaculture. However, the problem persist due to non-availability of marine finfish seed for the culture. In response to this problem, broodstock development of different tropical marine finfishes for seed production was started. The present study was undertaken to design a recirculating aquaculture system (RAS) and studying their performance in managing the various water quality required for the marine finfish broodstock development and breeding. The design of RAS, developed in the present study, included a broodstock tank, egg collection chamber, electrical pump, rapid sand filter, venturi type protein skimmer and biological filter. Two RAS were designed, one was stocked with a demersal fish species, orange spotted grouper (Epinephelus coioides) and the other was stocked with a pelagic fish species, Indian pompano (Trachinotus mookalee) at the rate 1 and 0.5 kg/m3 with a sex ratio of 1:1 and 1:2 (female: male) respectively. Various physio-chemical parameters, viz, total ammonia nitrogen (TAN), nitrite, nitrate, pH, alkalinity, temperature, free carbon dioxide (CO2) and dissolved oxygen (DO) of both tank water were analyzed to assess the performance of recirculating aquaculture system in maintaining the water quality. Gonadal development of the fishes was assessed and the spawning performance was recorded and finally, economic performance of the system was also evaluated. During the entire experimental period, mean monthly total ammonia nitrogen was less than 0.07 and 0.06 mg L−1 and mean monthly nitrite was less than 0.02 and 0.01 mg L−1 in orange spotted grouper and Indian pompano RAS tanks respectively. The pH (7.8–8.2), DO (>4 mg/L) and alkalinity (100–120 mg/L) were found to be in optimum range in both recirculating aquaculture systems. Carbon dioxide was found to be nil during the entire experimental period in both the systems. In fact these levels were comparable or less than that is reported as the permissible limits for broodstock development. Indian pompano and Orange spotted grouper matured and spawning was obtained with production of fertilized eggs round the year. Economic evaluation showed the price of 10,000 fertilized eggs of orange spotted grouper to be US $ 1.33. The design of RAS devised in the present study is efficient in controlling and maintaining optimum water quality for broodstock development of both demersal and pelagic finfishes. The fishes stocked in RAS attained final maturation and round the year spawning was obtained. 相似文献
39.
节水农业是水资源高效利用和农业可持续发展的重大问题,对于节约水资源具有极其重要的意义。为了解当前绿色农业高效节水领域的最新研究热点和发展趋势,本文梳理总结了我国现代化节水农业领域的基础背景、研究现状以及发展态势。结果表明,“灌溉设备与技术开发”、“农业用水精量调配与控制”、“作物高效用水机理”及“农业用水对环境变化的响应”等是近年来的研究热点,并建议开展水肥一体化智能技术与管理及其对作物、土壤、环境、社会等综合效应等的相关研究,以期为我国农业绿色高质量发展提供帮助。 相似文献
40.
Soil physical changes and maize growth in a structurally fragile tropical soil due to mulching and duration between irrigation intervals 下载免费PDF全文
E. G. Moura V. R. A. Macedo V. G. L. Sena L. S. Campos A. C. F. Aguiar 《Soil Use and Management》2017,33(4):631-638
Under tropical meteorological conditions, the volume of soil explored by plant roots is crucial for crop growth as it allows increased water and nutrient use efficiency. We hypothesized that, under different irrigation intervals, leguminous mulch can extend the duration between irrigation events but maintain crop performance, because decreased evaporative fluxes also reduce constraints to root exploration imposed by mechanical stress. We evaluated the combined effects of leguminous mulch and irrigation intervals on soil physical properties to determine whether the growth and productivity of maize were modified in a structurally fragile tropical soil. The experiment involved the following treatments: 4‐day irrigation intervals with soil mulched (4C) or bare (4S), 6‐day irrigation intervals with soil mulched (6C) or bare (6S), 8‐day irrigation intervals with soil mulched (8C) or bare (8S) and 10‐day irrigation intervals with soil mulched (10C) or bare (10S). Mulch decreased soil penetration resistance and increased to 4 days the favourable time for root development in drying soil. Relative to bare soil, mulch with a 6‐day irrigation interval almost doubled nitrogen uptake post‐tasselling, which decreased nitrogen remobilization and increased the crop growth rate during this stage. These conditions had a positive effect on the transpiration rate and stomatal conductance as well as on the growth and yield of maize. A 6‐day irrigation interval with mulch compared to 4 days with bare soil resulted in similar conditions for root development, but greater uptake of nitrogen (102.73–78.70 kg/ha) and better yield (6.2–5.3 t/ha), which means greater efficiency in nitrogen and water use. 相似文献