全文获取类型
收费全文 | 1418篇 |
免费 | 110篇 |
国内免费 | 152篇 |
专业分类
林业 | 154篇 |
农学 | 165篇 |
基础科学 | 158篇 |
183篇 | |
综合类 | 466篇 |
农作物 | 70篇 |
水产渔业 | 126篇 |
畜牧兽医 | 204篇 |
园艺 | 90篇 |
植物保护 | 64篇 |
出版年
2024年 | 13篇 |
2023年 | 50篇 |
2022年 | 77篇 |
2021年 | 70篇 |
2020年 | 75篇 |
2019年 | 77篇 |
2018年 | 58篇 |
2017年 | 74篇 |
2016年 | 50篇 |
2015年 | 78篇 |
2014年 | 101篇 |
2013年 | 60篇 |
2012年 | 98篇 |
2011年 | 119篇 |
2010年 | 103篇 |
2009年 | 81篇 |
2008年 | 92篇 |
2007年 | 89篇 |
2006年 | 76篇 |
2005年 | 34篇 |
2004年 | 22篇 |
2003年 | 23篇 |
2002年 | 16篇 |
2001年 | 21篇 |
2000年 | 18篇 |
1999年 | 18篇 |
1998年 | 25篇 |
1997年 | 6篇 |
1996年 | 12篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1956年 | 2篇 |
排序方式: 共有1680条查询结果,搜索用时 828 毫秒
111.
112.
区域经济发展的土地利用及生态安全管理——以宝鸡地区为例 总被引:1,自引:0,他引:1
区域社会经济发展与土地利用结构关系密切。文中采用耕地动态度、相对变化率、转移率、新增耕地率、线性回归分析以及洛伦兹曲线等方法,以宝鸡地区为例分析了土地动态变化以及与区域经济发展关系、耕地与粮食匹配关系。结果表明:市区和山区县土地变化率大,川原县区相对较小,耕地变化与经济发展呈高度负相关关系。面对区域经济发展中土地资源出... 相似文献
113.
在不同放牧制度下,以荒漠草原短花针茅植物种群为研究对象,采用GS+软件和地统计学分析方法对其空间异质性分布进行了研究。结果显示:对照区和划区轮牧区或自由放牧区都表现出很强的空间自相关性,且结构是导致短花针茅空间变异的主要因素。对照区、划区轮牧区和自由放牧区,短花针茅空间格局的分形维数相近,都接近于2。不同放牧制度下短花... 相似文献
114.
115.
本文对准噶尔盆地北部天然分布的尖果沙枣林的土壤特征进行了分析。结果表明:林下土壤质地多为壤质砂土或砂土;林下土壤特征表现为:土壤表层养分含量高,自上而下有机质、全N、速效N、速效P及速效K含量呈下降趋势;与裸地相比,尖果沙枣林明显改善土壤肥力,有机质、全N、速效N、速效P、速效K含量明显高于裸地,尤其土壤表层养分富集特征最为明显;针对具体样地的分析表明:受土壤结构及其他条件(土壤发育程度与地表植被生长状况及人为活动)的影响,少数样地表现出特殊的土壤特征。 相似文献
116.
通过对河北省张家口市黄羊滩防护林带内封育草场和防护林带外未封育保护沙化草场的理化性质对比,分析封育和防护林带对沙化草场土壤理化性质影响。结果表明,封育和防护林带可以显著地改善沙化草场土壤机械组成,提高土壤养分含量,其中细沙、粗粉沙和物理黏粒的平均百分含量较未封育沙化草场分别增加了4.26%、10.20%和12.08%,有机质、全氮、全磷、全钾、碱解氮、速效磷、速效钾的平均含量总体提高了203.65%。研究成果为采用植被建设措施改良沙化草场提供了科学依据。 相似文献
117.
玉米二元覆盖农田水分动态及水分利用效率研究 总被引:2,自引:2,他引:2
旱地玉米采用垄面覆膜、垄沟覆不同用量麦草的二元覆盖试验结果表明:随麦草覆盖量的增加土壤保墒效果明显;其中0~100cm土层土壤玉米全生育期累积贮水量5250kg/hm2和9750kg/hm2的麦草覆盖处理比垄面覆膜垄沟未覆麦草处理平均增加146.2mm,水分利用效率提高16.67%~28.65%,降水利用率提高12.39%~18.29%,玉米增产12.10%~18.15%。 相似文献
118.
麦草与地膜覆盖对玉米田间生态环境及产量的影响 总被引:6,自引:0,他引:6
通过田间试验研究了麦草与地膜覆盖栽培措施对玉米田间生态环境及产量的影响。结果表明,麦草与地膜二元覆盖可降低作物冠层温度1~2℃,显著减少玉米田间土壤水分蒸发,对土壤具有一定的保温作用,并能明显提高玉米产量和水分利用效率。 相似文献
119.
Wang Zhijun 《Biological conservation》2003,110(2):231-243
This study took a comparative look at the relationship between human-altered landscapes and bird diversity in two mountainous swidden (slash and burn) agricultural sites in Xishuangbanna, Yunnan, China. Over the course of 3 years (1998-2000) the authors used line-transect identification surveys in the two regions, Mengsong and Jinuo, where four different habitats were surveyed: 6-year fallow fields; traditional economic forests; monsoon evergreen broadleaf forests; and montane rain forests. Data were analyzed for species diversity, richness (family, genera, species), co-occurrence (family, genera, species), characterization of migratory status and feeding habits. Both of these study areas are inhabited by minority groups (Hani and Jinuo) that employ swidden agriculture as their main form of economic activity. The forest landscape in the Jinuo ethnic region is quickly transforming due to changing agricultural practices, and as a result bird diversity and richness are declining, while the Hani ethnic region (Mengsong), with a stable form of traditional swidden agriculture, is maintaining a high diversity and richness of birds. The greatest differences in bird diversity between the two sites occurred in the traditional economic forests and the 6-year fallow fields where the Hani region had a much greater richness and diversity. The two natural forest types (monsoon evergreen broadleaf forests and montane rain forests) were more similar in richness and diversity and also maintained the greatest richness and diversity of the four forest types analyzed. The transformation of healthy forests into degraded landscapes is one of the major impacts of human activity on the natural environment, which is posing a great threat to biodiversity. This study looks at the bird diversity of the two regions and tries to take into account how humans can extract economic benefits while preserving ecological benefits, such as a landscape that allows biodiversity to thrive. 相似文献
120.
Using segmented regression models to fit soil nutrient and soybean grain yield changes due to liming
Xiufu?ShuaiEmail author Zhijun?Zhou Russell?S.?Yost 《Journal of Agricultural, Biological & Environmental Statistics》2003,8(2):240-252
Frequently soil-plant relationships and responses are complex combinations of increases-level-decreases consisting of linear
segments of differing slope. Segmented regression is very useful to express and quantify such relationships and responses.
Fitting segmented regression models to such data, however, remains a challenge. The problem is in estimating the join points
and coefficients. We use median functions to express segmented regression models, and estimate the join points by standard
estimation routines such as Marquardt, Newton, and doesn’t use derivatives (DUD) methods that are available in statistical
software such as SAS. Segmented straight-line models are fit to data reflecting soil Manganese (Mn), Calcium (Ca), Phosphorus
(P), and soybean yield changes under different soil pH conditions due to liming. A systematic comparison of the slopes and
join points suggests that different mechanisms are limiting soybean yield at different intervals as soil pH increased. 相似文献