全文获取类型
收费全文 | 5480篇 |
免费 | 327篇 |
国内免费 | 670篇 |
专业分类
林业 | 509篇 |
农学 | 483篇 |
基础科学 | 634篇 |
1496篇 | |
综合类 | 2147篇 |
农作物 | 198篇 |
水产渔业 | 259篇 |
畜牧兽医 | 352篇 |
园艺 | 84篇 |
植物保护 | 315篇 |
出版年
2024年 | 23篇 |
2023年 | 113篇 |
2022年 | 220篇 |
2021年 | 228篇 |
2020年 | 235篇 |
2019年 | 268篇 |
2018年 | 171篇 |
2017年 | 322篇 |
2016年 | 399篇 |
2015年 | 309篇 |
2014年 | 285篇 |
2013年 | 403篇 |
2012年 | 520篇 |
2011年 | 453篇 |
2010年 | 302篇 |
2009年 | 306篇 |
2008年 | 237篇 |
2007年 | 277篇 |
2006年 | 212篇 |
2005年 | 185篇 |
2004年 | 144篇 |
2003年 | 115篇 |
2002年 | 112篇 |
2001年 | 88篇 |
2000年 | 81篇 |
1999年 | 74篇 |
1998年 | 47篇 |
1997年 | 45篇 |
1996年 | 58篇 |
1995年 | 47篇 |
1994年 | 38篇 |
1993年 | 29篇 |
1992年 | 33篇 |
1991年 | 28篇 |
1990年 | 22篇 |
1989年 | 20篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有6477条查询结果,搜索用时 15 毫秒
121.
牛瑟氏泰勒虫P33表面蛋白基因的克隆与序列分析 总被引:3,自引:0,他引:3
为分析吉林省流行的牛瑟氏泰勒虫基因序列,根据GenBank上发表的牛瑟氏泰勒虫P33表面蛋白基因序列设计合成一对引物,用PCR方法扩增出牛瑟氏泰勒虫的基因片段,并成功地将该基因纯化后克隆到pGEM-TEasy载体上,将经EcoRⅠ酶切鉴定和PCR鉴定为阳性的重组质粒进行测序。结果表明克隆的基因片段长度为868bp,编码283个氨基酸,有2个潜在的糖基化位点。核苷酸同源性分析表明,该基因片段与韩国株(AF521557)、日本株(AB016280)、俄罗斯株(AB016279)的核苷酸序列同源性分别为99.4%、88.0%、88.1%。 相似文献
122.
Applied ecology could benefit from new tools that identify potential movement pathways of invasive species, particularly where
data are sparse. Cost surface analysis (CSA) estimates the permeability (friction) across a landscape and can be applied to
dispersal modelling. Increasingly used in a diversity of applications, several fundamental assumptions that might influence
the outputs of CSA (cost surfaces and least-cost pathways) have yet to be systematically examined. Thus, we explore two issues:
the presumed relationship between habitat preferences and dispersal behaviour as well as the degree of landscape fragmentation
through which an organism moves by modelling a total of 18 sensitivity and dispersal scenarios. We explored the effect of
fragmentation by altering the friction values (generally assigned using expert opinion) associated with patch and linear features.
We compared these sensitivity scenarios in two sites that differed in fragmentation. We also used eastern grey squirrels (Sciurus carolinensis) as an example invading species and compared diffusion models and two contrasting cost surface dispersal scenarios. The diffusion
model underestimated spread because squirrels did not move randomly through the landscape. Despite contrasting assumptions
regarding dispersal behaviour, the two cost surfaces were strikingly similar while the least-cost paths differed. Furthermore,
while the cost surfaces were insensitive to changes in friction values for linear features, they were sensitive to assumptions
made for patch features. Our results suggest that movement in fragmented landscapes may be more sensitive to assumptions regarding
friction values than contiguous landscapes. Thus, the reliability of CSA may depend not only on the range of friction values
used for patches but also the degree of contiguity in the landscape. 相似文献
123.
MODIS热红外数据在乌鲁木齐市热岛研究中的应用 总被引:4,自引:0,他引:4
利用MODIS热红外数据,在完成几何校正、辐射定标、大气校正、云检测等数据预处理后,利用分裂窗算法反演得到乌鲁木齐2000-2005年1,4,7,10月的地表温度空间数据.在分析典型热温场的基础上,进行了热岛的季节和昼夜变化特征研究.结果表明:时间上,乌鲁木齐在夏季的白天和全年四季的夜晚均存在明显的城市热岛现象;空间上,乌鲁木齐城市和附近的戈壁、荒漠温度较高,近处的中山区和远郊的高山区温度较低,区域内温差较大、热岛强度高.乌鲁木齐城市热岛虽与其他大城市热岛特征在时空分布规律上有较多相似之处,但也有其自身的特性:干旱区城市郊区下垫面较多为戈壁荒漠,因此城市热岛效应在日间并不明显,相反有时会出现"城市冷岛"现象. 相似文献
124.
袁海军 《上海交通大学学报(农业科学版)》2013,(4):57-60
为提高潜水蒸发计算精度,利用潜水蒸发强度、埋深为零时的土面蒸发强度和水面蒸发强度的实测资料对阿维里扬诺夫公式、叶水庭公式和雷志栋公式中E0值的选取进行了探讨,同时对比分析了E0分别为水面蒸发强度和埋深为零时土面蒸发强度两种情况下的潜水蒸发强度。结果表明,针对新疆地区粘壤土类型,对于上述三种公式,若用埋深为零的土面蒸发强度代替水面蒸发强度,计算结果会产生误差,精度偏低,应选用E0为水面蒸发强度进行计算,获得了η与地下水埋深的指数函数关系。该研究为潜水蒸发经验模型中E0值的选取提供参考,同时为求解雷志栋公式中参数η提供一种新方法。 相似文献
125.
《Land Degradation \u0026amp; Development》2017,28(5):1704-1716
Phosphorus (P) loss from arable sloping land due to water erosion causes off‐site environment pollution in the Three Gorges area in China. This study aims to estimate the effects of agricultural practices on the reduction of P losses during water erosion processes under natural precipitation conditions: contour culturing with organic matter addition (CT + OM), contour culturing with wheat straw mulching (CT + SM), contour culturing with straw mulching and organic matter addition (CT + OM + SM) and conventional downslope culturing framework as a control (CK). Results demonstrated that CT + OM + SM could viably diminish runoff, sediment yields and P losses from arable sloping land. Compared with CK, CT + OM, CT + SM and CT + OM + SM lessened the surface runoff depths by 19%, 34% and 50%, respectively. A high curve number value induced a notable runoff potential from the plot. The surface‐runoff‐related total P (TP) loads for CK with yearly measures of 1·56 kg ha−1 were significantly higher than those for CT + OM, CT + SM and CT + OM + SM at 1·07, 0·77 and 0·51 kg ha−1 (p < 0·05), respectively. With all things considered, the particulate P losses represented a significant division of TP losses, adding up to almost 80% in 2014 and 79% in 2015. The outcomes demonstrated that surface protection and organic matter addition are two of the greatest agricultural protection practices that lessen P losses in the surface runoff from sloping farmland by water erosion. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
126.
《食用菌学报》2007,(2)
The composition of fermentation medium for producing Cordyceps brasiliensis mycelium was optimized using response surface methodology. A Plackett-Burman design was applied to screen potential key factors selected on the basis of preliminary experiments, and the steepest ascent method was adopted to approach the approximate optimal medium composition. Finally, central composite design and response surface methodology were applied to define more precisely the optimal composition of the fermentation medium. Our data revealed glucose and beef extract to be the most influential parameters, and the composition of the optimal fermentation medium (100 mL) was determined to consist of 4.46 g glucose, 2 g soluble starch, 4 g silkworm powder, 2.93 g beef extract, 0.10 g KH2PO4 and 0.15 g MgSO4·7H2O. 相似文献
127.
128.
《Communications in Soil Science and Plant Analysis》2012,43(17-18):2784-2799
Cooperative adsorption involving anions and cations, termed ion‐pair adsorption (IPA), is reported to increase the retention of some ions in certain soils. Sulfate and calcium can exhibit such interaction, and this affects their movement through the soil. Ion‐pair adsorption is shown here in miscible displacement experiments with a variable‐charge soil. The relevance of IPA under more realistic conditions is further investigated in a pot experiment. Rapeseed (Brassica napus) was grown at two different irrigation regimes and with two sulfur fertilizer sources. Calcium sulfate (CaSO4·2H2O) was used to induce IPA in contrast to potassium sulfate (K2SO4). The results suggest that IPA reduces sulfate and calcium leaching only in the short term. Continued irrigation dissipates the differences between the two fertilizer sources. Final soil ion concentrations and the plant uptake could not be related to IPA, evidencing the short‐term relevance of IPA. The influence of IPA on the bioavailability of calcium and sulfate to plants still demands further study. 相似文献
129.
基于EnKF-3DVar模型的海淀区地表温度模拟 总被引:1,自引:0,他引:1
以城市化程度较深的北京市海淀区为研究区,基于2005年、2010年和2015年的遥感影像数据,利用基于影像(IB)的算法反演城市地表温度空间分布。将数据同化算法EnKF-3DVar与CA/Markov模型集成,将海淀区的多年平均臭氧浓度空间分布数据同化进行城市地表温度的模拟预测。结果表明海淀区的城市地表温度10年间呈现先下降后上升的趋势,但总体呈现下降趋势,其中2015年的平均温度为31.139 3℃。引入EnKF-3DVar的预测模型能够显著提升模型的模拟精度,所预测的2015年的数据结果 Kappa系数达到0.821 6。在有城市公园绿地的模式下,地表温度高值区呈现缓解趋势,在无城市绿地公园的模式下,城市地表温度高值区呈现出明显的扩张趋势,最高温度达到了56.142 3℃,城市生态绿地对于城市地表温度的空间分布影响巨大,合理布局城市绿地公园意义重大。 相似文献
130.