全文获取类型
收费全文 | 6508篇 |
免费 | 398篇 |
国内免费 | 583篇 |
专业分类
林业 | 583篇 |
农学 | 560篇 |
基础科学 | 383篇 |
1238篇 | |
综合类 | 2391篇 |
农作物 | 321篇 |
水产渔业 | 576篇 |
畜牧兽医 | 1062篇 |
园艺 | 147篇 |
植物保护 | 228篇 |
出版年
2024年 | 28篇 |
2023年 | 108篇 |
2022年 | 154篇 |
2021年 | 187篇 |
2020年 | 220篇 |
2019年 | 236篇 |
2018年 | 170篇 |
2017年 | 310篇 |
2016年 | 329篇 |
2015年 | 289篇 |
2014年 | 322篇 |
2013年 | 447篇 |
2012年 | 523篇 |
2011年 | 467篇 |
2010年 | 381篇 |
2009年 | 368篇 |
2008年 | 339篇 |
2007年 | 406篇 |
2006年 | 339篇 |
2005年 | 251篇 |
2004年 | 220篇 |
2003年 | 165篇 |
2002年 | 154篇 |
2001年 | 139篇 |
2000年 | 121篇 |
1999年 | 96篇 |
1998年 | 88篇 |
1997年 | 68篇 |
1996年 | 85篇 |
1995年 | 87篇 |
1994年 | 62篇 |
1993年 | 67篇 |
1992年 | 58篇 |
1991年 | 48篇 |
1990年 | 50篇 |
1989年 | 53篇 |
1988年 | 18篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1976年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有7489条查询结果,搜索用时 453 毫秒
151.
Abstract – The native and non-native freshwater fish species richness as well as the proportion of native fish species to overall fish species were examined to test a hypothesis of a spatial linkage between the distributions of dams and non-native fishes based on survey data collected in rivers and streams in Hokkaido, Japan. Non-native fish species have steadily increased in Hokkaido since the mid 1970s, as indicated by the decreasing proportion of native fish species per fish survey. Non-native species richness was significantly higher in areas above dams (i.e., reservoirs and their inlet streams) compared to areas without dams. As a result, the proportion of native fish species was lower above dams. This proportion was lower in developed and agricultural areas compared with forest areas, although both native and non-native species richness were higher in areas with development and agriculture. Non-native cyprinids, catfish and snakehead occurred primarily in a particular river basin with the largest drainage area, the longest history of reclamation and highest human population in the island. Non-native salmonids on the other hand, occurred in forest areas at higher elevations and their occurrence was associated with the existence of dams. There was a spatial linkage between the distribution of dams and non-native species, although it was applicable only to salmonids. The management of dams and reservoirs needs a strategy to prevent further expansion of exotic salmonids such as rainbow and brown trout. These species can be a significant threat to native Japanese salmonids as a result of various biological mechanisms. 相似文献
152.
通过DSC -TG综合热分析、X衍射分析、线膨胀测试仪、扫描电镜等测试方法研究了ZnO/Na 2 O(mol)( Z 值)对Li 2 O -K 2 O -SiO 2 -Al 2 O 3 -B 2 O 3 系陶瓷结合剂及立方氮化硼砂轮性能的影响,结果发现:ZnO能够抑制α -石英晶相析出,诱导析出低膨胀晶体Li 2 OAl 2 O 3 7.5SiO 2 ,并随着 Z 值的增加,α -石英减少,Li 2 OAl 2 O 3 7.5SiO 2 晶体逐渐增多, Li 2 OAl 2 O 3 7.5SiO 2 晶型结构趋于完整,晶粒尺寸减小,结构更加致密;当砂轮配方一定时,砂轮的体积密度、抗弯曲强度、洛氏硬度均随着 Z 值的增加而增大,气孔率减小,磨削(干磨)铸铁(QT400)时磨耗比先增加后减小;当 Z =0.75时,结合剂的膨胀系数为6.902 9×10 -6 K - 1 ,耐火度为805 ℃,砂轮的烧结温度为845 ℃,抗弯曲强度为73.97 MPa,气孔率为15.65%,磨耗比最大为156.42%. 相似文献
153.
Dryland sustainable agriculture in the arid zone of India depends upon the choice of suitable cultivars for pure and mixed crop stands. Field experiments were conducted for two years to examine the response of two contrasting cultivars each of pearl millet (Pennisetum glaucum) and clusterbean (Cyamopsis tetragonoloba) in pure stands and in mixed pearl millet‐clusterbean stands. The differential response of cultivars of both crops to pure and mixed stands resulted in a significant genotype × cropping system interaction. Reduction in seed yield of both clusterbean cultivars was greater in mixed stands with tall and long duration pearl millet MH 179 than with medium statured and early maturing HHB 67. The degree of reduction was greater in Naveen, the branched clusterbean cultivar, than in RGC 197, the single stemmed cultivar. Mixing of pearl millet HHB 67 with medium duration clusterbean cultivar Naveen produced maximum pearl millet equivalent total yield. Higher land equivalent ratios (LERs) were also observed when clusterbean cultivars were mixed with early maturing and short statured pearl millet HHB 67. 相似文献
154.
《Plant Production Science》2013,16(3):235-242
AbstractThe objective of this study was to clarify the effects of soil temperature in the stage from late tillering to panicle initiation (SI) and during the grain-filling stage (SII) on grain setting, dry matter production, photosynthesis, non-structural carbohydrate (NSC), xylem exudation and abscisic acid (ABA) levels in rice (Oryza sativa L. cv. Koshihikari). Rice plants were exposed to four different soil temperatures during SI or SII: 17.5, 25, 31.5 and 36.5°C (ST18, ST25, ST32 and ST37, respectively). The yield, yield components, grain filling and quality in SI were negatively influenced by high soil temperature of 37°C. On the other hand, there was no significant difference in those characters among temperature treatments in SII. The root/shoot ratio was smallest in the ST37 plants in both SI and SII, mainly due to their lighter root weight. At 7 days after initiation of treatment (DAT) in both SI and SII, the photosynthetic and xylem exudation rate tended to increase slightly as soil temperature increased up to 32°C. At 21 DAT, however, the photosynthetic rate was lowest in ST37, with concurrent decrease of diffusion conductance and SPAD value. In addition, decrease of NSC concentration in stem and xylem exudation rate, and increase of ABA level in leaves and xylem exudate were observed in ST37 plants at 21 DAT. These results suggested that high soil temperature before heading especially influenced yield, grain quality and plant growth. Possible mechanisms of the effect of soil temperature are discussed. 相似文献
155.
小型木材除湿干燥机由除湿机、干燥机机体、风机和加热器组成,具有结构简单、操作容易、性价比高等特点。一次性干燥木材的容量为10~12m3,干燥周期为12~14d,平均能耗为87.50kW.h.m-3,成本为120.00元.m-3。符合国家标准锯材干燥质量中的有关规定,且木材保持天然色泽。 相似文献
156.
新疆国际客源市场时空发展变化分析研究 总被引:1,自引:0,他引:1
利用集中度和亲景度、竞争态指标,对新疆国际客源市场13个主体客源国1997~2004年统计数据进行分析,揭示国际客源时空演替规律,指出新疆主要的3个国际客源市场是日本、俄罗斯和美国,并提出了拓展国际客源市场的建议。 相似文献
157.
提出了一种有实际背景的最小费用箱子覆盖问题──每个物品有长度和费用2个参数.针对局外最小费用箱子覆盖问题,给出了一个求解该问题的最坏情况渐近性能比为1/2算法C-FF1.同时给出了一个求解该问题的局内算法C-FF2,其绝对性能比为1/2,并证明了不存在绝对性能比大于1的算法. 相似文献
158.
159.
160.
V.R. Smith 《Soil biology & biochemistry》2007,39(2):709-713
Indigenous soil macroinvertebrates (moth larvae, weevil larvae, earthworms) are cardinal agents of nutrient release from litter on sub-Antarctic Marion Island (47°S, 38°′E). Their populations are threatened through predation by introduced house mice, which do not prey on an introduced slug Deroceras panormitanum. A microcosm study was carried out to explore whether slugs affect rates of carbon and inorganic nutrient mineralisation from plant litter differently to an indigenous caterpillar (larva of a flightless moth Pringelophaga marioni). Caterpillars stimulated N, Ca, Mg and K mineralisation from plant litter two to five times more than slugs did, whereas the two invertebrate types stimulated C and P mineralisation to the same degree. Consequently, ratios of C:N and N:P released from the litter were different for slugs and caterpillars. Such differences might affect peat nutrient quality and ultimately the peat accumulation-decomposition balance, an important driver of ecological succession. This suggests that slugs cannot simply replace caterpillars without consequences for ecosystem structure and functioning on the island. 相似文献