全文获取类型
收费全文 | 490篇 |
免费 | 17篇 |
专业分类
林业 | 101篇 |
农学 | 4篇 |
基础科学 | 1篇 |
145篇 | |
综合类 | 49篇 |
农作物 | 15篇 |
水产渔业 | 10篇 |
畜牧兽医 | 130篇 |
园艺 | 19篇 |
植物保护 | 33篇 |
出版年
2023年 | 12篇 |
2022年 | 9篇 |
2021年 | 12篇 |
2020年 | 21篇 |
2019年 | 20篇 |
2018年 | 18篇 |
2017年 | 16篇 |
2016年 | 19篇 |
2015年 | 9篇 |
2014年 | 18篇 |
2013年 | 17篇 |
2012年 | 38篇 |
2011年 | 40篇 |
2010年 | 28篇 |
2009年 | 28篇 |
2008年 | 34篇 |
2007年 | 21篇 |
2006年 | 30篇 |
2005年 | 20篇 |
2004年 | 20篇 |
2003年 | 10篇 |
2002年 | 16篇 |
2001年 | 8篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1981年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1958年 | 1篇 |
1941年 | 2篇 |
1940年 | 1篇 |
1939年 | 7篇 |
1938年 | 3篇 |
1937年 | 5篇 |
排序方式: 共有507条查询结果,搜索用时 46 毫秒
1.
The cultivation of perennial instead of annual energy crops has received growing interest. Previous studies identified numerous beneficial effects of perennial energy crop (PEC) cultivation for the agricultural landscape such as promotion of agrobiodiversity, reduced requirements for agrochemicals and fertilizers as well as a large potential for carbon accumulation in soil. However, the mere presence of soil organic matter (SOM) accumulation gives no indication about the persistence of the SOM for example after a recultivation of the stands. Therefore, this study focused on SOM pools of different density fractions and soil microbial parameters. Six different PECs were tested against a typical benchmark system as feedstock for anaerobic digestion. The study has shown that all PEC species increased soil microbial activity and provided an insight how they sequester carbon in soil. Moreover, significant modifications in basic soil properties caused by plant growth were observed. For example, the cultivation of giant knotweed has lowered the soil pH by more than 0.5 pH units compared to the benchmark system. After 5 years of PEC cultivation, total soil organic carbon stocks were increased between 1,500 ± 400 and 4,500 ± 1,500 kg C ha-1 for the upper 10 centimetres of soil. The distribution among different soil fractions showed species-specific patterns. Tall wheatgrass and Virginia mallow showed particular high accumulation rates in the mineral-associated SOM fraction which indicates long residence times of the SOM after a possible recultivation of the fields. 相似文献
2.
3.
Christoph Hofmann 《Forstwissenschaftliches Centralblatt》1939,61(5):137-142
Ohne Zusammenfassung 相似文献
4.
5.
6.
Neis C Rohde J Valentin-Weigand P Baums CG 《Berliner und Münchener tier?rztliche Wochenschrift》2007,120(5-6):202-206
Streptococcus (S.) suis plays an important role in pig breeding causing invasive diseases such as meningitis and polyarthritis. Herd problems with Streptococcus suis are common in many pig farms and are frequently characterised by disease outbreaks. In this context, it is often important to identify chains of infection, e.g. between a farrow-to-wean and a grower farm. In the following case report a possible chain of infection among the different farms of a farrow-to-finish system was investigated. In two grower units herd problems with S. suis were present; the mortality was higher than 5 %. It appeared likely, that the streptococci causing these problems were transmitted from a single farrow-to-wean unit to the two different grower farms. In the respective farms swabs were taken from different healthy animals and, in the case of the grower farms, also from the infected animals. Genotypic profiling of strains by a multiplex-PCR and AFLP typing method revealed that two different S. suis pathotypes were responsible for the herd problems. Both pathotypes could not be detected in the farrow-to-wean farm.Thus, a chain of infection originating from the farrow-to-wean farm appeared unlikely. The multiplex PCR was in this case sufficient to elucidate the described problem. 相似文献
7.
8.
The effects of cytokinins, carbohydrate sources and cold pretreatment on the conversion of protocorm-like bodies (PLBs) to shoots were investigated for the enhancement of micropropagation of Dendrobium huoshanense C.Z. Tang et S.J. Cheng, an endangered medicinal plant in China. The effects of cytokinins and carbohydrate sources on the conversion of PLBs to shoots depended on their types and concentrations. The best results in terms of shoot development from PLBs occurred on 1/2 MS medium supplemented with 20 μM kinetin and 10 g l−1 maltose. Cold pretreatment at 10 °C for 1–2 weeks significantly enhanced the conversion of PLBs to shoots, and over 1300 shoots were obtained from one gram of PLBs after 3 months of culture. The developed shoots were rooted on growth regulator-free MS medium supplemented with 8 g/l banana paste to give complete plantlets, which were successfully acclimatized with a survival rate of approximately 65%. The results indicate that a suitable cold pretreatment (10 °C for 1 week) followed by the use of 20 μM kinetin and 10 g/l maltose in 1/2 MS medium would produce a large number of shoots from PLBs for plantlet regeneration of D. huoshanense. 相似文献
9.
Conversion of a tropical forest into agroforest alters the fine root-related carbon flux to the soil
Dietrich Hertel Marieke A. Harteveld Christoph Leuschner 《Soil biology & biochemistry》2009,41(3):481-490
Large areas of remaining tropical forests are affected by anthropogenic disturbances of various intensities. These disturbances alter the structure of the forest ecosystem and consequently its carbon budget. We analysed the role of fine root dynamics in the soil carbon budget of tropical moist forests in South-east Asia along a gradient of increasing disturbance intensity. Fine root production, fine root turnover, and the associated carbon fluxes from the fine root system to the soil were estimated with three different approaches in five stands ranging from an old growth forest with negligible anthropogenic disturbance to a cacao agroforestry system with planted shade trees. Annual fine root production and mortality in three natural forest sites with increasing canopy openness decreased continuously with increasing forest disturbance, with a reduction of more than 45% between the undisturbed forest and the forest with large timber extraction. Cacao agroforestry stands had higher fine root production and mortality rates than forest with large timber extraction but less than undisturbed forest. The amount of carbon annually transferred to the soil carbon pool through fine root mortality was highest in the undisturbed forest and generally decreased with increasing forest use intensity. However, root-related C flux was also relatively high in the plantation with planted shading trees. In contrast, the relative importance of C transfer from root death in the total above- and below-ground C input to the soil increased with increasing forest use intensity and was even similar to the C input via leaf litter fall in the more intensively managed agroforest. We conclude that moderate to heavy disturbance in South-east Asian tropical moist forests has a profound impact on fine root turnover and the related carbon transfer to the soil. 相似文献
10.
Kormann Urs G. Scherber Christoph Tscharntke Teja Batáry Péter Rösch Verena 《Landscape Ecology》2019,34(5):1045-1056
Landscape Ecology - Global change pressures (GCPs) imperil species and associated ecosystem functions, but studies investigating interactions of landscape-scale pressures remain scarce. Loss of... 相似文献