全文获取类型
收费全文 | 2938篇 |
免费 | 194篇 |
国内免费 | 310篇 |
专业分类
林业 | 335篇 |
农学 | 147篇 |
基础科学 | 109篇 |
261篇 | |
综合类 | 1479篇 |
农作物 | 218篇 |
水产渔业 | 91篇 |
畜牧兽医 | 415篇 |
园艺 | 261篇 |
植物保护 | 126篇 |
出版年
2024年 | 19篇 |
2023年 | 50篇 |
2022年 | 116篇 |
2021年 | 125篇 |
2020年 | 130篇 |
2019年 | 110篇 |
2018年 | 84篇 |
2017年 | 138篇 |
2016年 | 102篇 |
2015年 | 145篇 |
2014年 | 153篇 |
2013年 | 164篇 |
2012年 | 282篇 |
2011年 | 248篇 |
2010年 | 213篇 |
2009年 | 227篇 |
2008年 | 234篇 |
2007年 | 220篇 |
2006年 | 188篇 |
2005年 | 145篇 |
2004年 | 80篇 |
2003年 | 60篇 |
2002年 | 51篇 |
2001年 | 60篇 |
2000年 | 67篇 |
1999年 | 14篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1981年 | 1篇 |
1956年 | 2篇 |
排序方式: 共有3442条查询结果,搜索用时 15 毫秒
191.
四川丘陵区农林系统和农草系统流域的径流和土壤水分特征 总被引:1,自引:0,他引:1
A study was conducted in a hilly area of Sichuan Province, Southwestern China, to compare the streamflow and soil moisture in two upland watersheds with different land use patterns. One was an agroforestry watershed, which consisted mainly of trees with alder (Alnus cremastogyne Burkill) and cypress (Cupressus funebris Endl.) planted in belts or strips with a coverage of about 46%, and the other was a grassland primarily composed of lalang grass (Imperata cylindrica var. major (Nees) C. E. Hubb.), filamentary clematis (Clematis filamentosa Dunn) and common eulaliopsis (Eulaliopsis binata (Retz.) C. E. Hubb) with a coverage of about 44%. Streamflow measurement with a hydrograph established at the watershed outlet showed that the average annual streamflow per 100 mm rainfall from 1983 to 1992 was 0.36 and 1.08 L s-1 km-2 for the agroforestry watershed and the grass watershed, respectively. This showed that the streamflow of the agroforestry watershed was reduced by 67% when compared to that of the grass watershed. The peak average monthly streamflow in the agroforestry watershed was over 5 times lower than that of the grass watershed and lagged by one month. In addition, the peak streamflow during a typical rainfall event of 38.3 mm in August 1986 was 37% lower in the agroforestry watershed than in the grass watershed. Results of the moisture contents of the soil samples from 3 slope locations (upper, middle and lower slopes) indicated that the agroforestry watershed maintained generally higher soil moisture contents than the grass watershed within 0-20 and 20-80 cm soil depths for the upper slope, especially for the period from May through July. For the other (middle and lower) slopes, soil moisture contents within 20-80 cm depth in the agroforestry watershed was generally lower than those in the grass watershed, particularly in September, revealing that water consumption by trees took place mainly below the plow layer. Therefore, agroforestry land use types might offer a complimentary model for tree-annual crop water utilization. 相似文献
192.
降低畜禽养殖污水处理成本是促进养殖业健康发展的重要因素,介质过滤能截留污水中的悬浮性固体,是养殖污水预处理的有效途径之一。其中,以作物秸秆为滤料的自然过滤装置结构简单、能耗低,能有效去除养殖污水中的固体污染物,但仍存在易堵塞、过滤速度慢、秸秆利用率低等问题。对此,该研究提出负压抽滤方案并进行了相关试验:首先通过单因素试验确定了各试验参数的边界条件,再利用二次回归组合试验,优化了负压抽滤的运行参数,试验结果表明当滤层厚度、压实密度、过滤压差分别为10 cm、130 kg/m3、30 kPa时综合过滤效果最佳,此时总固体(total solid,TS)、挥发性固体(volatile solid,VS)、化学需氧量(chemical oxygen demand,COD)、氨氮(NH4+-N)的去除率分别为40.47%、45.07%、13.56%、11.75%;自然过滤和负压抽滤的对比试验表明,相同滤层条件负压抽滤的污水处理量是自然过滤的4倍,而用时仅占自然过滤的1/5,负压抽滤能有效提高过滤速率和秸秆利用率。最后利用厌氧发酵试... 相似文献
193.
194.
氮肥种类和用量对黄河三角洲玉米产量及氮肥利用率的影响 总被引:1,自引:1,他引:1
195.
196.
以水稻为主的种植系统对土壤有机质状况的影响 总被引:1,自引:1,他引:1
XIAO Yong-Lan ZHANG Yang-Zhu HU Rui-Zhi HUANG Yun-Xiang ZHOU Qing YUAN Zheng-Ping YANG Dong-Ping SHU Li 《土壤圈》1997,7(4):349-354
A long-term simulation experiment was carried out to study the effect of rice-based cropping system, green manure and ground-water level on soil organic matter. Soil organic matter content increased when upland soil was puddled and cropped under submerged conditions. Among all treatments, soil organic matter contents in the treatments of rice-rice-flooded fallow in winter (WF) were the highest, those in the treatments of rice-rice-astragalus (WG) came the second, and those in the treatments of rice-rice-rape (WR) were the lowest. At the same rate of green manure application, the degrees of organo-mineral complexing in soils of treatments WG and WR were higher than those of treatment WF. After 9-year rice cultivation, the state of combination of humus in heavy fraction varied with treatments. The relative content of loosely bonded humus decreased in the order of WR > WG > WF, and it correlated significantly with Se availability. 相似文献
197.
研究了水牛胎儿成纤维细胞(buffalo fetal fibroblasts,BFF)转染外源基因后的细胞生物学特性。纯化含有绿色荧光蛋白(green fluorescent protein,GFP)和新霉素抗性基因的线性化载体,用脂质体法转染体外培养3代的BFF细胞,应用抗生素G418筛选2周后挑取转基因抗性细胞集落扩大培养。结果发现:经过G418筛选后细胞增殖活力下降,胰蛋白酶消化法共分离获得162个转基因水牛阳性细胞集落,转移到24孔板扩大培养后,132个(85.2%)细胞集落可以存活,转移到4孔板中后仅有52个(32.1%)细胞集落能继续生长,最终能在35mm培养皿中大量培养的只有15个(〈9.3%)。抗性细胞由梭形、成簇生长变为多角形、不规则形弥漫生长,立体感减弱,细胞生长速度减慢,活力下降。染色体核型异常率增加,10个抗性细胞集落核型正常率平均为61%,显著低于对照组86.7%(P〈0.01)。研究结果为建立和优化BFF细胞转染外源基因平台奠定了工作基础。 相似文献
198.
199.
200.
在小尺度上研究植被对气候变化的响应,辨析起关键作用的气候因子,对于区域植被恢复与生态重建具有重要指导意义.利用NASA GIMMS 1982-2006年的NDVI数据集和黄南州4个气象站的气候资料,计算了NDVI和气候的变化趋势及其相关系数,探讨了影响NDVI的主要气候因子.结果表明,尖扎县NDVI略有下降,同仁县和泽库县下降趋势显著,河南县呈增加趋势;气候向于暖化方向发展,4县平均的气温倾向率高达0.67℃/10 a,同仁县降水量略有减少,其它3县降水量快速减少,降水量倾向率平均为24.82 mm/10 8.尖扎县、同仁县NDVI与降水量和气温具有较好的相关性,与降水量呈正相关,与气温呈负相关;泽库县NDVI主要受蒸发量、冰雹期和无霜期共同作用;河南县NDVI主要与日照时数正相关.建议植被恢复时,尖扎县和同仁县应注重保湿工作;在小尺度上开展类似研究针对性更强,对生态建设的指导意义显著. 相似文献