全文获取类型
收费全文 | 87483篇 |
免费 | 4043篇 |
国内免费 | 9560篇 |
专业分类
林业 | 9523篇 |
农学 | 12456篇 |
基础科学 | 7288篇 |
13492篇 | |
综合类 | 29067篇 |
农作物 | 4775篇 |
水产渔业 | 3113篇 |
畜牧兽医 | 12184篇 |
园艺 | 3678篇 |
植物保护 | 5510篇 |
出版年
2024年 | 280篇 |
2023年 | 1039篇 |
2022年 | 2530篇 |
2021年 | 3454篇 |
2020年 | 3028篇 |
2019年 | 3060篇 |
2018年 | 2211篇 |
2017年 | 3141篇 |
2016年 | 2896篇 |
2015年 | 4037篇 |
2014年 | 3941篇 |
2013年 | 4664篇 |
2012年 | 5781篇 |
2011年 | 6164篇 |
2010年 | 6099篇 |
2009年 | 5566篇 |
2008年 | 5126篇 |
2007年 | 5281篇 |
2006年 | 4919篇 |
2005年 | 4339篇 |
2004年 | 2111篇 |
2003年 | 1995篇 |
2002年 | 1731篇 |
2001年 | 1713篇 |
2000年 | 1792篇 |
1999年 | 1993篇 |
1998年 | 1738篇 |
1997年 | 1570篇 |
1996年 | 1433篇 |
1995年 | 1281篇 |
1994年 | 1100篇 |
1993年 | 1131篇 |
1992年 | 892篇 |
1991年 | 749篇 |
1990年 | 643篇 |
1989年 | 511篇 |
1988年 | 390篇 |
1987年 | 234篇 |
1986年 | 128篇 |
1985年 | 68篇 |
1984年 | 45篇 |
1983年 | 42篇 |
1982年 | 49篇 |
1981年 | 49篇 |
1980年 | 32篇 |
1979年 | 14篇 |
1965年 | 15篇 |
1964年 | 11篇 |
1963年 | 12篇 |
1956年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Impact of grassland contract policy on soil organic carbon losses from alpine grassland on the Qinghai–Tibetan Plateau 下载免费PDF全文
J. Cao Y. Gong E. T. Yeh N. M. Holden J. F. Adamowski R. C. Deo M. Liu J. Zhou J. Zhang W. Zhang S. Zhang D. Sheng S. Yang X. Xu M. Li Q. Feng 《Soil Use and Management》2017,33(4):663-671
Carbon storage in the soils on the Qinghai–Tibetan Plateau plays a very important role in the global carbon budget. In the 1990s, a policy of contracting collective grasslands to smaller units was implemented, resulting in a change from the traditional collective grassland management to two new management patterns: a multi‐household management pattern (MMP: grassland shared by several households without enclosures) and a single‐household management pattern (SMP: grassland enclosed and used by only one household). In 2016, 50 MMP and 54 SMP winter pastures on the Qinghai–Tibetan Plateau were sampled to assess the differences in soil organic carbon (SOC) between the two management patterns. Results showed that average SOC was significantly greater under MMP than under SMP, with an estimated 0.41 Mg C/ha/yr lost due to SMP following the new grassland contract. Based on the government's grassland policy, four grassland utilization scenarios were developed for both summer and winter pastures. We found that if the grassland were managed under SMP, likely C losses ranged between 0.31 × 107 and 6.15 × 107 Mg C/yr across the Qinghai–Tibetan Plateau relative to MMP, which more closely resembles pre‐1990s grassland management. Previous estimates of C losses have only considered land use change (with cover change) and ignored the impacts driven by land management pattern changes (without cover change). The new data suggest that C losses from the Qinghai–Tibetan Plateau are greater than previously estimated, and therefore that the grassland contract policy should be reviewed and SMP households should be encouraged to reunite into the MMP. These findings have potential implications for land management strategies not only on the Qinghai–Tibetan Plateau but also other grazing regions globally where such practices may exist. 相似文献
2.
Effects of Feeding Frequency on Apparent Energy and Nutrient Digestibility/Availability of Channel Catfish,Ictalurus punctatus,Reared at Optimal and Suboptimal Temperatures 下载免费PDF全文
This study examined the effects of feeding frequency (daily versus every other day [EOD]) on nutrient digestibility/availability of channel catfish, Ictalurus punctatus, reared at optimal (30 C) and suboptimal (24 C) temperatures. A 28% protein practical diet was used as the test diet, and chromic oxide was used at 0.75% of the diet as a maker. Twenty channel catfish of a mean weight of 141 g/fish were stocked into 12 flow‐through aquaria (110 L). Fish were fed daily or EOD to apparent satiation for 7 d before fecal samples were collected by the dissection method. Fish fed EOD to apparent satiation consumed more diet on days fed than those fed daily, and fish reared at 30 C consumed more diet than those reared at 24 C. Neither feeding frequency nor water temperature significantly affected apparent digestibility coefficients for energy and protein and apparent availability coefficients for total amino acids. Results show no advantage by feeding EOD over feeding daily on nutrient digestibility and diet utilization efficiency. 相似文献
3.
对5个无核葡萄品种的试验表明,不同培养方式对离体胚发育的影响是很明显的,其中4个品种胚发育率液体培养高于固体培养,‘京早晶’在液体培养中胚发育率高达73.7%,是目前国内外胚发育率最高的报道;液体培养比固体培养胚的发育程度高,发育整齐,无畸形胚,易成苗. 相似文献
4.
石灰性土壤的电超滤(EUF)特性主要是浸提时因需控制电流而必须大大降低电压。在一定范围内,降压程度与土壤 CaCO_3%有密切关系。控流限额愈低,电压需成比例地下降,但土壤EUF-值减小不多。电流控制在一定值时,离子的解吸总量亦为定值。控流主要控制了各石灰性土壤解吸/溶解份额特大的 Ca+Mg 量近于定值,对于含量小的其他养分离子则并无多大影响,仍可区辨各土壤间的差别。通过冬小麦盆栽试验和有关化学测试法等方面的相关研究,表明采用国际通用的5g 土样和控电流方法测定 EUF-K 和-P 等有效养分,对于石灰性土壤也是可行的,但其EUF-(Ca+Mg)值则毫无植物营养有效性的意义。土壤 EUF 值只是一种指数,并非真正的解吸离子量。石灰性土壤 EUF-K 值因浸提时降压控流而略低,80℃时5min 内解吸的钾不一定是非交换性钾。由于 Ca-P 和 CaCO_3在高温高电压时浸出较多,故石灰性土壤 EUF-P 和-(Ca+Mg)的80℃值/20℃值之比,与酸性土壤或非石灰性土壤相比要大得多。 相似文献
5.
6.
李燕凌 《绿色中国(A版)》2001,(8):40-42
发展证券资本市场可有效地推动经济快速发展 ,但林业产业上市融资明显处于低水平状态。湖南省林业产业化发展 ,客观上要求在证券资本市场上拓宽发展空间 ,为此提出林业产业发展和上市融资需采取的 6项对策与措施。 相似文献
7.
8.
Big bluestem (Andropogon gerardii Vitman) and little bluestem [Schizachyrium scoparium (Michaux) Nash.] are native to the North America and are important forage grasses and ornamental grasses. Both grasses are proposed as ideal biomass producers for cellulosic ethanol production. To apply genetic transformation, which is an important tool for incorporating desirable agronomic traits into plants to both species, however requires an efficient and reproducible regeneration protocol. We used mature caryopses from big and little bluestem as explants and tested the effect of various combinations of 2, 4-dichlorophenoxyacetic acid (2, 4-D) (1, 2, 3, 4 or 5 mg l−1) and kinetin (KT) (0, 0.1 or 0.2 mg l−1) on embryogenic callus induction with LS as the basal medium. The highest percentage of embryogenic calli induction occurred on medium containing 2, 4-D alone at 2 mg l−1 for ‘Bison’ and on medium containing 4 mg l−1 2, 4-D alone for ‘Bonilla’ big bluestem. For little bluestem, the highest percentage of embryogenic callus induction occurred on medium containing 3 mg l−1 2, 4-D plus 0.1 mg l−1 kinetin, suggesting that addition of KT is beneficial. Shoot regeneration took place on LS basal medium without any plant growth regulator for both species, although the addition of KT increased both regeneration frequency and the number of shoots produced per callus. Rooting of shoots reaching about 2 cm long occurred readily with or without α-naphthaleneacetic acid (NAA). Rooted plantlets were all successfully established in the soil. 相似文献
9.
中国学术文献总库(CNKI)的主要功能和使用技巧 总被引:3,自引:1,他引:2
结合在线咨询服务和用户培训中积累的经验,就升级后新版的CNKI的主要功能和使用技巧进行了整理和总结. 相似文献
10.
从师生自身反馈角度,结合成立于2003年的中国地质大学工程学院土木工程试验班的具体情况,针对其本硕贯通式的培养教学计划,提出了包括增加专业软件学习、互动式外语教学、高低阶力学课程合并、压缩人文社科类课程等具体改进建议。 相似文献