全文获取类型
收费全文 | 539篇 |
免费 | 61篇 |
国内免费 | 49篇 |
专业分类
林业 | 395篇 |
农学 | 10篇 |
基础科学 | 1篇 |
51篇 | |
综合类 | 151篇 |
农作物 | 9篇 |
水产渔业 | 1篇 |
畜牧兽医 | 9篇 |
园艺 | 10篇 |
植物保护 | 12篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2020年 | 4篇 |
2019年 | 12篇 |
2018年 | 6篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 15篇 |
2014年 | 18篇 |
2013年 | 36篇 |
2012年 | 35篇 |
2011年 | 23篇 |
2010年 | 35篇 |
2009年 | 38篇 |
2008年 | 32篇 |
2007年 | 46篇 |
2006年 | 32篇 |
2005年 | 41篇 |
2004年 | 23篇 |
2003年 | 14篇 |
2002年 | 15篇 |
2001年 | 16篇 |
2000年 | 16篇 |
1999年 | 18篇 |
1998年 | 15篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 6篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 1篇 |
1986年 | 3篇 |
排序方式: 共有649条查询结果,搜索用时 31 毫秒
51.
Forage and wood yield of Acacia cyanophylla, also known as Acacia saligna, was studied in a 300–400 mm precipitation zone in Tunisia. Yields were measured during and after drought. This short (2–8
m) evergreen leguminous tree is used as a forage drought reserve in frost free regions where mean annual precipitation exceeds
250 mm. The standing crop of leafy forage builds up year after year for at least four years or until the tree is cut. It rapidly
regrows after cutting from coppice shoots. The leaves provide high protein forage for sheep and goats during the long dry
summer season typical of the Mediterranean climate as well as emergency forage during drought. The tree is used to stabilize
moving sand dunes, and as a windbreak to protect cropland. It also provides fuelwood and increasaes soil nitrogen by fixation.
The need for supplemental irrigation during establishment is a major constraint.
Research in the 350 mm precipitation zone of Tunisia found 3.2 year old trees to yield over 1400 kg of forage standing crop
per hectare after a severe drought. Trees harvested at 2.5 years of age in May, during the worst drought in over 30 years,
yielded a standing forage crop of 724 kg per hectare. Forage regrowth 8 months after cutting and 4 months after rains returned
was 700 kg/ha. The forage standing crop for trees harvested only once during the 3.2 year period was double the amount of
forage regrowth from trees harvested the previous year, but mean annual forage yield similar. This demonstrates that it is
possible for forage to be conserved as a living forage reserve for later use during drought. Total wood yield was only 1621
kg/ha for trees cut twice compared to 3683 kg/ha for trees cut only once.
Annual cutting will substantially reduce the amount of forage available during drought and reduce the production of fuelwood.
It may also reduce the vigor, productivity and life of the tree. Acacia cyanophylla alley cropped on cereal farmland can protect the soil from erosion, protect the associated crop from wind damage, fix nitrogen,
provide fuelwood and provide a reserve of high quality forage for use during drought.
This work was supported by the Government of Tunisia and the U.S.A.I.D. Tunisia Range Development and Management Project (664-0312.8). 相似文献
52.
53.
以东南沿海防护林6种相思树种为研究对象,通过跟踪测定其不同季节叶绿素荧光参数,进一步评价各树种对沿海沙地气候环境的适应能力。结果显示:卷荚相思夏季和冬季PSⅡ最大光能转化率和潜在活性均低于春季和秋季,表明夏季高温干旱和冬季低温均会制约其生长;大叶、厚荚及马占相思冬季PSⅡ最大光能转化率和潜在活性较小,生长状况较差;肯氏相思和纹荚相思的Fv/Fm及Fv/Fo值冬季相对较高,其中季节更替对肯氏相思Fv/Fm及Fv/Fo,值变化影响最小,整体生长状况较好。结合林分生长情况可知,大叶、厚荚及马占相思对东南沿海沙地环境适应能力较差;肯氏相思和纹荚相思能够适应沿海防护林的复杂环境,具有较高的推广种植潜力。 相似文献
54.
55.
为了解灰木莲、巨尾桉和厚荚相思3种人工林地的土壤肥力变化,为制定相应的营林措施提供参考,对3种8年生人工林0 ~ 20cm土层的微生物数量及土壤酶活性进行比较.结果表明:(1)不同树种的土壤微生物总量为灰木莲>厚荚相思>巨尾桉.同一树种在一年四季中的变化是:灰木莲林地夏季>冬季>秋季>春季;巨尾桉林地秋季>春季>夏季>冬季;厚荚相思林地秋季>春季>冬季>夏季.(2)3种林地土壤的三大类群微生物数量特征表现为细菌最多,真菌数量最少,但在不同季节里有变化.灰木莲林地和厚荚相思林地的细菌数量明显大于巨尾桉林地;而巨尾桉林地的真菌和放线菌数量在春夏两季均大于另外两种林地,但秋冬两季则是3种林地中最少的.(3)3种林地的土壤蛋白酶活性在春季基本持平,但在夏季和冬季的差异却达到极显著水平;而3种林地的土壤过氧化氢酶活性均在夏季和冬季出现两次高峰,冬季的酶活性最大,差异达显著水平.分析表明不同树种对林地土壤微生物数量、组成以及土壤酶活性有重要影响. 相似文献
56.
We evaluated the spatial structures of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) fluxes in an Acacia mangium plantation stand in Sumatra, Indonesia, in drier (August) and wetter (March) seasons. A 60 × 100-m plot was established in an A. mangium plantation that included different topographical elements of the upper plateau, lower plateau, upper slope and foot slope. The plot was divided into 10 × 10-m grids and gas fluxes and soil properties were measured at 77 grid points at 10-m intervals within the plot. Spatial structures of the gas fluxes and soil properties were identified using geostatistical analyses. Averaged N2O and CO2 fluxes in the wetter season (1.85 mg N m−2 d−1 and 4.29 g C m−2 d−1, respectively) were significantly higher than those in the drier season (0.55 mg N m−2 d−1 and 2.73 g C m−2 d−1, respectively) and averaged CH4 uptake rates in the drier season (−0.62 mg C m−2 d−1) were higher than those in the wetter season (−0.24 mg C m−2 d−1). These values of N2O fluxes in A. mangium soils were higher than those reported for natural forest soils in Sumatra, while CO2 and CH4 fluxes were in the range of fluxes reported for natural forest soils. Seasonal differences in these gas fluxes appears to be controlled by soil water content and substrate availability due to differing precipitation and mineralization of litter between seasons. N2O fluxes had strong spatial dependence with a range of about 18 m in both the drier and wetter seasons. Topography was associated with the N2O fluxes in the wetter season with higher and lower fluxes on the foot slope and on the upper plateau, respectively, via controlling the anaerobic-aerobic conditions in the soils. In the drier season, however, we could not find obvious topographic influences on the spatial patterns of N2O fluxes and they may have depended on litter amount distribution. CO2 fluxes had no spatial dependence in both seasons, but the topographic influence was significant in the drier season with lowest fluxes on the foot slope, while there was no significant difference between topographic positions in the wetter season. The distributions of litter amount and soil organic matter were possibly associated with CO2 fluxes through their effects on microbial activities and fine root distribution in this A. mangium plantation. 相似文献
57.
目前我国被发现或被利用于优良紫胶虫种4号胶虫(Kerria lacca)的优良寄主植物种类较少,制约了我国优质紫胶的发展。对苏门答腊金合欢(Acacia montana var. Sumatorana.)、光叶合欢(Albizia meyeri)、复羽叶栾树(Koelreuteria bipinnata)的放养胶虫试验结果表明,这3种树种可以作为4号胶虫的良好寄主予以利用或推广。 相似文献
58.
本文对厦门地区灰叶荆群落的生长和更新能力进行了研究。该植物3、4年生群落植株基径年增量为0.71~0.91cm;当年播种植株高和基径年增量分别为299.4和3.38cm;4、5年生群落母桩萌条的高和基径年增量分别为304.4~264.0cm和2.25~1.55cm。光照,温度和雨量是影响该植物生长和萌更的主要生态因子。光照的差异使群落内个体分化为被压木和优势木两类。该植物自然更新能力差,但其种子发芽力强,耐贮;影响种子发芽的主要因素是气温和采种期。 相似文献
59.
建设速丰林,推进漳州林业跨越式发展 总被引:1,自引:1,他引:0
提出以发展桉树、相思树为主的速生丰产工业用材林基地,是实现漳州林业跨越式发展的突破口;分析了漳州发展速丰林具备的自然优势、技术条件;指出发展速丰林是实现其整体效益的最佳途径,并提出建设速丰林的对策建议。 相似文献
60.
Anti-uveal melanoma activity-guided fractionation of the MeOH extract of Acacia nilotica pods resulted in the isolation of the new compound gallocatechin 5-O-gallate (5) in addition to methyl gallate (1), gallic acid (2), catechin (3), catechin 5-O-gallate (4), 1-O-galloyl-β-D-glucose (6), 1,6-di-O-galloyl-β-D-glucose (7) and digallic acid (8). The structures of the isolated compounds were elucidated on the basis of HRESIMS, NMR spectroscopy and CD data. In addition to uveal melanoma, the antiproliferative activities of the isolated compounds and the related compound epigallocatechin 3-O-gallate (EGCG) were evaluated against cutaneous melanoma, ovarian cancer, glioblastoma and normal retinal pigmented cells. 相似文献