全文获取类型
收费全文 | 6541篇 |
免费 | 362篇 |
国内免费 | 1篇 |
专业分类
林业 | 1145篇 |
农学 | 200篇 |
基础科学 | 16篇 |
1389篇 | |
综合类 | 679篇 |
农作物 | 192篇 |
水产渔业 | 270篇 |
畜牧兽医 | 2412篇 |
园艺 | 150篇 |
植物保护 | 451篇 |
出版年
2023年 | 41篇 |
2022年 | 51篇 |
2021年 | 112篇 |
2020年 | 110篇 |
2019年 | 69篇 |
2018年 | 208篇 |
2017年 | 197篇 |
2016年 | 180篇 |
2015年 | 161篇 |
2014年 | 175篇 |
2013年 | 397篇 |
2012年 | 403篇 |
2011年 | 423篇 |
2010年 | 194篇 |
2009年 | 181篇 |
2008年 | 371篇 |
2007年 | 380篇 |
2006年 | 370篇 |
2005年 | 326篇 |
2004年 | 313篇 |
2003年 | 317篇 |
2002年 | 252篇 |
2001年 | 189篇 |
2000年 | 181篇 |
1999年 | 106篇 |
1998年 | 43篇 |
1997年 | 37篇 |
1996年 | 50篇 |
1995年 | 80篇 |
1994年 | 46篇 |
1993年 | 25篇 |
1992年 | 26篇 |
1991年 | 24篇 |
1990年 | 26篇 |
1989年 | 18篇 |
1988年 | 25篇 |
1987年 | 14篇 |
1986年 | 30篇 |
1985年 | 29篇 |
1984年 | 22篇 |
1983年 | 18篇 |
1982年 | 14篇 |
1980年 | 26篇 |
1977年 | 23篇 |
1966年 | 14篇 |
1964年 | 13篇 |
1961年 | 13篇 |
1955年 | 15篇 |
1941年 | 14篇 |
1857年 | 13篇 |
排序方式: 共有6904条查询结果,搜索用时 8 毫秒
61.
Steen Magnussen Michael Köhl Konstantin Olschofsky 《European Journal of Forest Research》2014,133(6):1137-1155
According to the United Nations International Panel on Climate Change good practice guidance, an annual forest biomass carbon balance (AFCB) can be estimated by either the stock-difference (SD) or the gain–loss (GL) method. An AFCB should be accompanied by an analysis and estimation of uncertainty (EU). EUs are to be practicable and supported by sound statistical methods. Sampling and model errors both contribute to an EU. As sample size increases, the sampling error decreases but not the error due to errors in model parameters. Uncertainty in GL AFCB estimates is dominated by model-parameter errors. This study details the delta technique for obtaining an EU with the SD and the GL method applicable to the carbon in aboveground forest biomass. We employ a Brownian bridge process to annualize the uncertainty in SD AFCBs. A blend of actual and simulated data from three successive inventories are used to demonstrate the application of the delta technique to SD- and GL-derived AFCBs during the years covered by the three inventories (SD) and rescaled national wood volume harvest statistics (GL). Examples are limited to carbon in live trees with a stem diameter of 7 cm or greater. We confirm that a large contribution to the uncertainty in an AFCB comes from models used to estimate biomass. Application of the delta technique to summary statistics can significantly underestimate uncertainty as some sources of uncertainty cannot be quantified from the available information. We discuss limitations and problems with the Monte Carlo technique for quantifying uncertainty in an AFCB. 相似文献
62.
63.
64.
65.
Hadi Manghabati Wendelin Weis Axel Göttlein 《European Journal of Forest Research》2018,137(5):631-642
Phosphorus is an essential nutrient for forest growth. In this study, we assessed the impact of soil extractable phosphorus using two simple extraction methods on nutrition and productivity of Norway spruce in sixteen mature forest stands on different bedrocks and soils in Bavaria, Southern Germany. Representative trees were sampled for needles, twigs, branches, stem bark, and stem wood. Total phosphorus content in the tree parts and soil phosphorus stock extractable with citric acid and sodium bicarbonate up to a soil depth of 80 cm were determined. We found that easily soil extractable phosphorus is a suitable indicator for estimating phosphorus uptake and stand productivity in Norway spruce. In contrast, organic layer phosphorus showed no significant correlation with aboveground biomass phosphorus contents. In the biomass, the highest phosphorus contents were measured in young needles and twigs, but the highest correlation with soil phosphorus was detected for phosphorus contents in needles and bark. The stock of phosphorus extracted by citric acid down to 40 cm soil depth revealed the best correlation with phosphorus in needles and bark. Therefore, as a supplemental or alternative method to needle analysis, our study suggests the use of phosphorus contents in stem bark to evaluate tree phosphorus nutrition. These results highlight the suitability of the citric acid soil extraction method to characterize plant available phosphorus in Norway spruce ecosystems. 相似文献
66.
Karl Gösswald 《Journal of pest science》1943,19(3):30-34
Ohne Zusammenfassung
Mit 10 Abbildungen 相似文献
67.
68.
69.
70.
H. W. Fassbender L. Alpízar J. Heuveldop H. Fölster G. Enríquez 《Agroforestry Systems》1988,6(1-3):49-62
Models for cycles for organic matter and nutrients element (N, P, K, Ca and Mg) are presented for the agroforestry systems
of cacao (Theobroma cacao) withCordia alliodora orErythrina poeppigiana in Turrialba, Costa Rica.
For the models, system reserves (soil, humus, vegetation divided into leaves, branches, stems, fine roots, fruits) and transference
between compartments (production and decomposition of litter residues) inputs (fertilizer, rainfall) and outputs (harvests)
of the system are considered.
The implications of the models are discussed in detail.
Aspects of net primary production in the systems studied are considered.
N fixation is calculated on the basis of balances. Analysis of soil water showed high variations that coincided with rainfall
patterns and pruning of theE. poeppigiana.
For part I see Vol. 4, No. 3, 1986 For part II see this issue
Agroforestry Project, CATIE/GTZ (Tropical Agricultural Research and Training Center/Gesellschaft für Technische Zusammenarbeit),
Turrialba, Costa Rica 相似文献