全文获取类型
收费全文 | 1934篇 |
免费 | 119篇 |
国内免费 | 122篇 |
专业分类
林业 | 259篇 |
农学 | 129篇 |
基础科学 | 87篇 |
300篇 | |
综合类 | 733篇 |
农作物 | 68篇 |
水产渔业 | 276篇 |
畜牧兽医 | 175篇 |
园艺 | 61篇 |
植物保护 | 87篇 |
出版年
2024年 | 6篇 |
2023年 | 28篇 |
2022年 | 46篇 |
2021年 | 47篇 |
2020年 | 62篇 |
2019年 | 74篇 |
2018年 | 41篇 |
2017年 | 96篇 |
2016年 | 97篇 |
2015年 | 54篇 |
2014年 | 112篇 |
2013年 | 122篇 |
2012年 | 132篇 |
2011年 | 135篇 |
2010年 | 120篇 |
2009年 | 119篇 |
2008年 | 112篇 |
2007年 | 105篇 |
2006年 | 98篇 |
2005年 | 91篇 |
2004年 | 60篇 |
2003年 | 50篇 |
2002年 | 49篇 |
2001年 | 42篇 |
2000年 | 35篇 |
1999年 | 24篇 |
1998年 | 28篇 |
1997年 | 25篇 |
1996年 | 33篇 |
1995年 | 26篇 |
1994年 | 14篇 |
1993年 | 22篇 |
1992年 | 12篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1983年 | 2篇 |
排序方式: 共有2175条查询结果,搜索用时 15 毫秒
61.
62.
Japanese red pine (Pinus densiflora) seedlings taken from a nursery were grown in liquid culture with all roots entirely submerged in water. The aims of the present study were to observe the reactions of preformed ectomycorrhizae to liquid culture and to examine new infections by ectomycorrhizal fungi on newly formed root tips in liquid culture. Inoculation levels were controlled by trimming the ectomycorrhizal roots to one of three selected root lengths. The results showed that the mantles of pre-formed ectomycorrhizae were lost during the 8 weeks of the liquid culture and that these preformed root tips became blackish and wrinkled, but Hartig nets remained in these blackish root tips. On the newly formed lateral root tips, no ectomycorrhizal mantles were formed, although Hartig nets were found. In addition, the level of inoculum did not affect the frequency of Hartig nets in newly formed root tips, suggesting that the density of inoculum was more important than the amount of inoculum for new infection. In conclusion, ectomycorrhizal fungi endured and infected new roots while submerged; however, only Hartig nets were formed and not mantles. 相似文献
63.
银木(Cinnamomum septentrionale Hand.Mazz.)系樟科樟属的常绿乔木,主要分布在年平均气温14℃,降水量800 mm以上的四川、陕西南部、甘肃南部、湖南等地区,是自然分布最北面的樟科植物.该树种系典型的北亚热带树种,对水、热条件的要求低于其它樟科植物,既能耐-10℃的低温,又能耐干旱[1].银木树皮灰色、光滑,叶近革质,互生,椭圆状、倒圆状披针形,具有较好的观赏性和较高的开发价值,在湖南、四川等地已人工栽培多年,形成特色鲜明的城市行道树. 相似文献
64.
65.
66.
Francis O. C. Nwonwu 《Agroforestry Systems》1987,5(4):455-462
The costs and benefits of the use of three labour alternatives: direct permanent, direct casual and taungya labour in Gmelina arborea pulpwood plantation establishment in Nigeria were investigated. Results show that cost per hectare is lowest with taungya labour and highest with permanent labour with savings in cost of as much as 30% and 47% by taungya labour over casual and permanent labour respectively. The net present value per hectare of investment is highest when taungya labour is used and lowest and negative with permanent labour. Also, the internal rate of return of investment is highest with taungya labour and lowest with permanent labour. The net present values and internal rates of return for Gmelina plantations calculated for the three pulp and paper mills in Nigeria show the same trend in favour of taungya labour. 相似文献
67.
68.
A study on dry matter production and nutrient cycling in agroforestry systems of mandarin grown in association with N2-fixing Albizia and mixed tree species (non-N2-fixing) was carried out in the Sikkim Himalaya. A site with Albizia was referred asAlbizia-mandarin stand and the other site with mixed tree species as mandarin stand. The stand total biomass, net primary productivity and mandarin fruit production was higher under the influence of Albizia. Agronomic yield of crops remained nearly the same in both the stands. Nitrogen and phosphorus concentrations of different components of Albizia were higher than those of mixed tree species, whereas their back translocation from leaf to branch before abscission was lower inAlbizia. The mandarin-based agroforestry is a highly nutrient-exhaustive system evaluated in terms of nutrient exit through the removal of agronomic yield. This system, under the influence of Albizia, was more productive with faster rates of nutrient cycling. Nutrient use efficiency increased under the influence of Albizia, in contrary to the hypothesis that efficiency should decrease with increasing rate of uptake. The poor nutrient conservation of Albizia, and malleability of nutrient cycling under its influence make it an excellent association which promotes higher availability and faster cycling of nutrients.Albizia should be utilised more extensively in the management of mandarin-based agroforestry systems. 相似文献
69.
本文讨论了山东省沂南县沂河林场不同密度Ⅰ-72杨人工林的投入产出及经济成熟龄。投入分为一次性投入和每年投入。一次性的总投入量为2 33 1.00元/ha,其中整地占84%;平均年投入为253.21元/ha·a,其中追肥占74%。文中还计算了长期占用资金的复利。对各密度的年投入量、年生长量、年产值、年均净产值及其年动态进行了研究,同时做了林木的年均生长量和年均净产值数学模拟。杨树人工林在造林后第三年才能获得利润。数量成熟龄为6~10年,年均最高生长量可达25.252 1 m~3/ha·a。经济成熟龄为6~7年,年均最高净产值可达6 110.52元/ha·a。杨树人工林的数量成熟龄与经济成熟龄主要受密度及投入成本的影响。 相似文献
70.
田间和盆栽核桃与柿子苗木光合特性的初步研究 总被引:3,自引:0,他引:3
研究了核桃(Juglans regia L.)、柿子(Diospyros kaki L.f.)两种苗木在田间和盆栽栽培条件下的光合特性。通过室内人工控制光强和CO2浓度,测得盆栽的核桃、柿子的光补偿点分别是62 μmol CO2/m2·s、40μmol CO2/ m2·s;饱和点分别是1 582 μmol CO2/m2·s、1 064 μmol CO2/m2·s;CO2补偿点分别是135 ml/L、230 ml/L。6月的晴天,核桃、柿子盆栽栽培中的净光和速率日变化呈"双峰"型;田问栽培中仅出现1次高峰。两种栽培均出现"午休"现象。 相似文献