全文获取类型
收费全文 | 2385篇 |
免费 | 115篇 |
国内免费 | 144篇 |
专业分类
林业 | 300篇 |
农学 | 106篇 |
基础科学 | 129篇 |
745篇 | |
综合类 | 720篇 |
农作物 | 89篇 |
水产渔业 | 35篇 |
畜牧兽医 | 161篇 |
园艺 | 148篇 |
植物保护 | 211篇 |
出版年
2024年 | 14篇 |
2023年 | 70篇 |
2022年 | 85篇 |
2021年 | 69篇 |
2020年 | 93篇 |
2019年 | 93篇 |
2018年 | 68篇 |
2017年 | 107篇 |
2016年 | 108篇 |
2015年 | 94篇 |
2014年 | 111篇 |
2013年 | 147篇 |
2012年 | 186篇 |
2011年 | 179篇 |
2010年 | 148篇 |
2009年 | 180篇 |
2008年 | 138篇 |
2007年 | 140篇 |
2006年 | 124篇 |
2005年 | 74篇 |
2004年 | 61篇 |
2003年 | 62篇 |
2002年 | 45篇 |
2001年 | 39篇 |
2000年 | 34篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 13篇 |
1994年 | 17篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有2644条查询结果,搜索用时 31 毫秒
61.
Abstract— The microhabitat selection of stone loach ( Barbatula barbatula ) in relation to instream cover and predation risk was investigated in artificial streams. In one experiment, stone loach were presented different combinations of shelter structures, two at a time, that provided visual isolation, flow refuge, both, or none. In all cases, visual isolation shelters were selected by the fish, with little use of clear shelters and no apparent selection of flow refuges. In a second experiment, diel activity patterns and habitat selection of stone loach were measured when visual isolation structures were placed in the riffle only, pool only, both riffle and pool or in no habitat. Stone loach were marked with passive integrated transponder tags and a sensor plate was placed between the riffle and pool habitats to measure diel activity patterns. Habitat use was measured by releasing drop gates between the two habitats at the end of each trial. More stone loach used the pool than the riffle in all treatments, but use of the riffle increased when cover was present only there. Furthermore, stone loach were most active between 2100 and 0300. To determine the effect of predation threat on habitat use by stone loach, one adult brown trout ( Salmo trutta ) was added to each pool. The presence of trout caused stone loach to move into the riffle, especially the smaller loach. Overall, the study showed that stone loach used cover in a visual isolation context, preferring deeper water when there was no predation risk, but made more use of shallower water in the presence of brown trout. 相似文献
62.
阳坡山地花椒园的立地条件和丰产试验 总被引:3,自引:0,他引:3
阳坡山地花椒园光热充足,但土壤水分、养分条件较差,因而,限制了单株产量。采用园地覆膜、尿素叶面喷肥技术措施能明显提高单株产量,是石灰岩山地花椒园的重要增产措施。 相似文献
63.
在广州秋播玉米采用稻草覆盖与地膜覆盖和无覆盖相比,稻草覆盖的玉米产量和穗行数性状明显优越,穗长,行粒数,穗粗,千料重等性状也有较好的趋势。稻草和黑色地膜覆盖,均可减少杂草萌发,稻草覆盖处理中萌发的杂草,仍然会不断生长,但个体数量已大大减少。 相似文献
64.
65.
为绿色防控蛴螬,于2006-2018年秋作物收获后、小麦播种前,调查河南省漯河市5个区县不同茬口秋作物田内金龟子幼虫种类、虫口密度以及主要秋作物种植面积,同时在漯河市农业科学院试验基地内设置1台黑光灯,监测金龟子成虫的上灯种类和数量。结果表明,2006-2018年,铜绿丽金龟Anomala corpulenta幼虫平均密度由2006年的2.06头/m2下降到2018年的1.01头/m2,所占比例从2006年的94.12%下降到2018年的38.37%;暗黑鳃金龟Holotrichia parallela幼虫平均密度由2006年的0.06头/m2上升到2018年的1.59头/m2,所占比例从2006年的2.61%增加到2018年的60.46%,表明金龟子幼虫优势种由铜绿丽金龟变为暗黑鳃金龟。灯下金龟子成虫种类和数量变化与金龟子幼虫变化趋势相同。暗黑鳃金龟幼虫密度与玉米种植面积、红薯种植面积呈显著负相关,相关系数分别为-0.657和-0.690,暗黑鳃金龟幼虫密度与大豆种植面积、花生种植面积呈显著正相关,相关系数分别为0.753和0.657,表明大豆、花生种植面积增加,玉米、红薯种植面积减少,是暗黑鳃金龟成为优势种的主要原因。 相似文献
66.
Logging pattern and landscape changes over the last century at the boreal and deciduous forest transition in Eastern Canada 总被引:4,自引:0,他引:4
Forestry practices associated with the industrial era (since ~1900) have altered the natural disturbance regimes and greatly
impacted the world’s forests. We quantified twentieth century logging patterns and regional scale consequences in three sub-boreal
forest landscapes of Eastern Canada (117,000, 49,400 and 92,300 ha), comparing forestry maps depicting age and forest cover
types for early industrial (1930) and present-day (2000) conditions. Results were similar for the three landscapes, indicating
large-scale forest change during the twentieth century. In 1930, previous logging activities had been concentrated in the
lowlands and along the main hydrographical network, as compared to a more even distribution over the landscapes in 2000, reflecting
a decreasing influence of the environmental constraints on forest harvesting. In 1930, old-aged forests (>100 years) accounted
for more than 75% of the unlogged areas of the three landscapes, as compared to less than 15% for the present-day conditions.
Logging practices have thus inverted the stand age distribution of the landscapes that are currently dominated by young and
regenerating stands. The 1930 forest cover types showed a clear relationship with elevation, with conifers located in the
lowlands and mixed and deciduous stands restricted to the upper slopes. Between 1930 and 2000, 58–64% of the conifer areas
transformed to mixed and deciduous forests, such that no clear altitudinal relationships remained in 2000. We conclude that
twentieth century logging practices have strongly altered the preindustrial vegetation patterns in our study area, to the
point that ecosystem-based management strategies should be developed to restore conifer dominance, altitudinal gradients,
as well as the irregular structure inspired from old forest stands. 相似文献
67.
68.
《Land Degradation \u0026amp; Development》2017,28(4):1363-1375
Overgrazing contributes to rangeland degradation altering plant community composition, erosion and biodiversity. Little unanimity in the literature exists on the effects of livestock grazing on soil carbon and biodiversity, in part, due to uncontrolled grazing pressure from native and feral animals. Paired paddock contrasts at three, long‐term (>8 years) study locations in the southern Australian rangelands were used to examine the effects of managing grazing intensity through the use of exclusion fencing and rotational grazing on soil organic carbon (SOC), soil nitrogen (TN), ground cover and biodiversity (flora and invertebrates). Grazing management had no effect on SOC or TN on grey soils (Vertisols), but for red soils (Lixisols), significantly higher levels of SOC were found for both the 0 to 5 and 5 to 10‐cm soil depths (0·3% and 0·27% respectively) and associated with increased TN. We found strong and consistent relationships among SOC and higher perennial (p < 0·001), higher litter (p < 0·05) cover and close proximity to trees (p < 0·05). Managing grazing intensity resulted in significantly higher perennial ground cover (p < 0·001) on Vertisols (8·9 to 11%) and Lixisols (12·5 to 15%) and higher plant diversity (both native and exotic) but negatively impacted invertebrate diversity, indicating trade‐offs between production and resources. We provide evidence that the effects of grazing management on SOC are mediated by ground cover and increased organic matter supply and/or reduced soil carbon redistribution (erosion), which indicates that the management of grazing intensity may provide a tool to avoid soil carbon loss in rangelands. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
70.