全文获取类型
收费全文 | 104038篇 |
免费 | 5312篇 |
国内免费 | 769篇 |
专业分类
林业 | 7185篇 |
农学 | 3960篇 |
基础科学 | 857篇 |
14223篇 | |
综合类 | 18272篇 |
农作物 | 5367篇 |
水产渔业 | 5908篇 |
畜牧兽医 | 44942篇 |
园艺 | 2216篇 |
植物保护 | 7189篇 |
出版年
2021年 | 801篇 |
2020年 | 904篇 |
2019年 | 1107篇 |
2018年 | 4385篇 |
2017年 | 4572篇 |
2016年 | 2731篇 |
2015年 | 1479篇 |
2014年 | 1637篇 |
2013年 | 3075篇 |
2012年 | 3935篇 |
2011年 | 6083篇 |
2010年 | 4873篇 |
2009年 | 3978篇 |
2008年 | 4887篇 |
2007年 | 5244篇 |
2006年 | 2864篇 |
2005年 | 2845篇 |
2004年 | 2550篇 |
2003年 | 2692篇 |
2002年 | 2397篇 |
2001年 | 2519篇 |
2000年 | 2598篇 |
1999年 | 1970篇 |
1998年 | 812篇 |
1997年 | 720篇 |
1995年 | 753篇 |
1993年 | 691篇 |
1992年 | 1574篇 |
1991年 | 1733篇 |
1990年 | 1657篇 |
1989年 | 1616篇 |
1988年 | 1504篇 |
1987年 | 1533篇 |
1986年 | 1583篇 |
1985年 | 1402篇 |
1984年 | 1201篇 |
1983年 | 1020篇 |
1982年 | 685篇 |
1979年 | 1059篇 |
1978年 | 835篇 |
1977年 | 694篇 |
1976年 | 716篇 |
1975年 | 748篇 |
1974年 | 1009篇 |
1973年 | 984篇 |
1972年 | 956篇 |
1971年 | 884篇 |
1970年 | 854篇 |
1969年 | 808篇 |
1967年 | 710篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Tania Khandakar Chris Guppy Sheikh M. F. Rabbi Heiko Daniel 《Soil Use and Management》2021,37(1):120-125
Soil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (<53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (>15 g kg?1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil. 相似文献
2.
3.
Freshwater fishes are threatened globally, and often too little is known about threatened species to effectively guide their conservation. Habitat complexity is linked to fish species diversity and persistence, and degraded streams often lack habitat complexity. Beaver Castor spp., in turn, have been used to restore streams and increase habitat complexity. The northern leatherside chub Lepidomeda copei is a rare, small‐bodied, drift‐feeding minnow that has anecdotally been observed to use complex habitats associated with beaver dams in the western United States. To investigate this anecdote, we conducted fish and habitat surveys, the latter focusing on quantifying habitat complexity, in a sub‐basin of the Upper Snake River Basin in the USA. Complementary generalised linear model and path analyses revealed that northern leatherside chub occurred more often at sites with complex streamflows, and streamflows were more complex when beaver dams were present and pools were deeper. Northern leatherside chubs were also more likely to occur when temperatures were warmer, aquatic macrophytes were abundant and stream channels were narrow and deep. The linkage between chubs, complex streamflows and beaver dams needs to be evaluated more broadly to completely understand its role in the rangewide status of the species. However, it does suggests that increased use of beaver reintroductions and dam analogues for stream restoration could be a boon for the northern leatherside chub, but such efforts should be monitored to determine their effectiveness to help adapt beaver‐based restoration approaches to best benefit the species. 相似文献
4.
5.
Still Only One Earth. Progress in the 40 years since the First UN Conference on the Environment. By R.E. Hester & R.M. Harrison (eds). Published by Royal Society of Chemistry,Cambridge, 2015. vii + 285 pp. £67.50. Hardback (ISBN 978‐1‐78262‐076‐1).
下载免费PDF全文
R. L. Hough 《Soil Use and Management》2017,33(1):176-176
6.
Do cover crops change the lability of phosphorus in a clayey subtropical soil under different phosphate fertilizers? 下载免费PDF全文
A. P. B. Teles M. Rodrigues W. F. Bejarano Herrera A. Soltangheisi L. R. Sartor P. J. A. Withers P. S. Pavinato 《Soil Use and Management》2017,33(1):34-44
Plants have developed different mechanisms to absorb and solubilize phosphorus (P) in the soil, especially in environments with low P availability. This study evaluated the effects of different winter cover crops on soil P availability in a clayey subtropical (Hapludox) soil receiving soluble P fertilizer and a rock phosphate applied to the summer crop, under no‐tillage. The experiment was carried out over 3 yrs (2009–2011) with five different cover crop species: common vetch, fodder radish, ryegrass, black oat, white clover and fallow as control. The soil was sampled after the third year of cover crop cultivation and analysed for inorganic and organic P forms according to the well‐established Hedley fractionation procedure. Phosphate fertilizers promoted accumulation of both labile and nonlabile P pools in soil in the near surface layer, especially under rock phosphate. Fertilizer applications were not able to change P fractions in deeper layers, emphasizing that the Brazilian clayey soils are a sink of P from fertilizer and its mobility is almost nil. Although the cover crops recycled a great amount of P in tissue, in a short‐term evaluation (3 yrs) they only changed the content of moderately labile P in soil, indicating that long‐term studies are needed for more conclusive results. 相似文献
7.
Mark D. Rawling Nicola Pontefract Ana Rodiles Ilektra Anagnostara Eric Leclercq Marion Schiavone Mathieu Castex Daniel L. Merrifield 《Journal of the World Aquaculture Society》2019,50(6):1108-1122
Fish were fed a single‐strain yeast fraction (SsYF; 2 g/kg) or a multistrain yeast fraction (MsYF; 0.8 g/kg) for 10 weeks. The results demonstrated significant (p ≤ 0.03) elevations in weight gain, specific growth rate, protein efficiency ratio, and feed conversion ratio in fish fed the yeast fraction‐supplemented diets. In the distal intestine, a significant elevation in microvilli density was observed after 5 and 10 weeks of dietary supplementation with MsYF and SsYF, respectively, compared to control fed fish (p < 0.001). A significant elevation (p = 0.02) in the perimeter ratio was observed in fish fed diets supplemented with the yeast fractions. After 10 weeks of feeding on the experimental diets, Rt‐qPCR demonstrated a significant downregulation (p < 0.05) in the stress response genes, heat‐shock protein 70 (hsp70) and proliferating cell nuclear antigen (pcna), in fish fed diets supplemented with the yeast fractions. Significant (p < 0.05) elevations in interleukin 1‐beta (il1β) and interleukin‐10 (il10) gene expression were observed in fish fed diets supplemented with the MsYF compared to the other dietary groups. These findings suggest that feeding an MsYF specifically at a lower incorporation rate < 1 g/kg, compared to a commercial SsYF at 2 g/kg, is effective in improving the intestinal health status and growth performance of European seabass. 相似文献
8.
10.