全文获取类型
| 收费全文 | 1293篇 |
| 免费 | 366篇 |
| 国内免费 | 25篇 |
专业分类
| 林业 | 199篇 |
| 农学 | 112篇 |
| 基础科学 | 58篇 |
| 294篇 | |
| 综合类 | 581篇 |
| 农作物 | 64篇 |
| 水产渔业 | 86篇 |
| 畜牧兽医 | 142篇 |
| 园艺 | 66篇 |
| 植物保护 | 82篇 |
出版年
| 2025年 | 27篇 |
| 2024年 | 47篇 |
| 2023年 | 43篇 |
| 2022年 | 42篇 |
| 2021年 | 43篇 |
| 2020年 | 56篇 |
| 2019年 | 61篇 |
| 2018年 | 36篇 |
| 2017年 | 65篇 |
| 2016年 | 89篇 |
| 2015年 | 83篇 |
| 2014年 | 94篇 |
| 2013年 | 87篇 |
| 2012年 | 106篇 |
| 2011年 | 116篇 |
| 2010年 | 83篇 |
| 2009年 | 89篇 |
| 2008年 | 69篇 |
| 2007年 | 69篇 |
| 2006年 | 65篇 |
| 2005年 | 46篇 |
| 2004年 | 44篇 |
| 2003年 | 20篇 |
| 2002年 | 18篇 |
| 2001年 | 31篇 |
| 2000年 | 22篇 |
| 1999年 | 18篇 |
| 1998年 | 17篇 |
| 1997年 | 15篇 |
| 1996年 | 13篇 |
| 1995年 | 10篇 |
| 1994年 | 15篇 |
| 1993年 | 6篇 |
| 1992年 | 13篇 |
| 1991年 | 4篇 |
| 1990年 | 5篇 |
| 1989年 | 2篇 |
| 1988年 | 3篇 |
| 1987年 | 5篇 |
| 1986年 | 1篇 |
| 1985年 | 1篇 |
| 1984年 | 1篇 |
| 1981年 | 1篇 |
| 1980年 | 1篇 |
| 1977年 | 1篇 |
| 1956年 | 1篇 |
排序方式： 共有1684条查询结果，搜索用时 0 毫秒
1.
2.
Z. Villegas M. Peña-Claros B. Mostacedo A. Alarcón J.C.Licona C. Leaño W. Pariona U. Choque 《Forest Ecology and Management》2009
Silvicultural treatments are often needed in selectively logged tropical forest to enhance the growth rates of many commercial tree species and, consequently, for recovering a larger proportion of the initial volume harvested over the next cutting cycle. The available data in the literature suggest, however, that the effect of silvicultural treatments on tree growth is smaller in dry forests than in humid forest tree species. In this study, we analyze the effect of logging and application of additional silvicultural treatments (liana cutting and girdling of competing trees) on the growth rates of future crop trees (FCTs; i.e., trees of current and potentially commercial timber species with adequate form and apparent growth potential). The study was carried out in a tropical dry forest in Bolivia where a set of 21.25-ha plots were monitored for 4 years post-logging. Plots received one of four treatments that varied in intensity of both logging and silvicultural treatments as follows: normal (reduced-impact) logging; normal logging and low-intensity silviculture; increased logging intensity and high-intensity silviculture; and, unlogged controls. The silvicultural treatments applied to FCTs involved liberation from lianas and overtopping trees. Results showed that rates of FCT stem diameter growth increased with light availability, logging intensity, and intensity of silvicultural treatments, and decrease with liana infestation degree. Growth rate increment was larger in the light and intensive silvicultural treatment (22–27%). Long-lived pioneer species showed the strongest response to intensive silviculture (50% increase) followed by total shade-tolerant species (24%) and partial shade-tolerant species (10%). While reduced-impact logging is often not sufficient to guarantee the sustainability of timber yields, application of silvicultural treatments that substantially enhanced the growth rates of FCTs will help move the management of these forests closer to the goal of sustained yield. 相似文献
3.
This study was conducted to assess regional patterns of diversity of Ethiopian tetraploid wheat accessions and to identify areas of diversity that can be used as source of new germplasm for developing high yielding and stable varieties. A collection of 133 Ethiopian tetraploid wheat accessions and eight introduced cultivars was analysed using 29 wheat microsatellite markers. A total of 383 alleles were detected with an average value of 13.14 alleles per locus. Relatively more alleles were observed in the B genome than in the A genome. Gene diversity indices ranged from 0.08 to 0.95, with a mean value of 0.72. Accessions collected from the same region were pooled and the number of alleles and gene diversity were calculated over the 29 simple sequence repeats for each region. Higher numbers of alleles were detected in the Shewa region (8.72), followed by Tigray (5.86) and Hararghe (5.76). The highest average gene diversity value was found in Shewa (0.65), followed by Gondar (0.64). No significant correlation was observed between geographic distance and genetic distance. Out of 383 different alleles detected, 93 (24.4%) were observed to be region‐specific. Region‐specific alleles were found across all chromosomes except for Xgwm752, Xgwm155 and Xgwm148. Genetic similarity coefficients were estimated for all the possible 55 pairs of regional comparisons and they ranged from 0.16 to 0.52, with a mean value of 0.50. All provinces were differentiated in the UPGMA cluster diagram. 相似文献
4.
在菲律宾的雨养旱地、雨养低地和灌溉田种植制度研究基点上,分别对各种农民和拟推广的实验种植方式,进行了生物量及其组成、作物分器官的 C、N、P、K 含量与分布比率、作物残茬燃烧后 C、N、P、K 损失测定。从添加与取走两个方面,分三种添加与取走情况,计算了土壤N、P、K 的部分养分平衡与有机质平衡。结果显示:在当地施肥水平下,即使归还全部残茬,很难维持 N、P、K 的养分平衡,但能较好满足土壤有机质平衡。为维持土壤有机质的年平衡,雨养低地和雨养旱地需分别归还1400和1900公斤生物物质/公顷.年。并对当地维持养分平衡的途径作了分析与探讨。 相似文献
5.
6.
T. A. Quine A. Navas D. E. Walling J. Machin 《Land Degradation u0026amp; Development》1994,5(1):41-55
The semiarid regions of Spain, including the central part of the Ebro River basin, are under threat due to desertification. Severe erosion, as a result of poor land management, has led to degradation of the soil resource, and there is a clear need for quantitative erosion rate data to evaluate the problem. This study aimed to examine the potential for using caesium-137 to identify the patterns and rates of soil erosion and redistribution within this semiarid environment. Samples for the determination of caesium-137 were collected from uncultivated slopes and cultivated valley floor sites near the head and outlet of a small representative basin in the Las Bardenas area. The measured patterns of caesium-137 mobilization, redistribution and export provide a semiquantitative indication of the variation in erosion within the study site. Calibration of the caesium-137 measurements, taking account of the differing behaviour of radiocaesium on cultivated and uncultivated land, allows estimation of the actual rates of erosion and deposition involved. The results show (1) the erosion rates on the cultivated land (1.6–2.5 kg m−2 yr−1) are typically more than five times those seen on the uncultivated land (0.2–0.4 kg m−2 yr−1), and (2) erosion on the uncultivated land is significantly less severe at the head of the basin than at the outlet. Study of the vegetation cover suggests that lower growing shrubs and grasses may be more effective in reducing erosion in this environment than trees. 相似文献
7.
Fabien Thomas Jean-Pierre Rossi Thibaud Decaëns Michel Grimaldi Patrick Lavelle Paulo Fernando da Silva Martins Evelyne Garnier-Zarli 《European Journal of Soil Biology》2008,44(5-6):545
Soil ecosystem engineers produce biogenic structures (casts, mounds, galleries) that strongly affect soil processes. A comparative analysis of the physical characteristics of the casts produced by the earthworm Andiodrilus pachoensis was carried out in four pastures sowed with Brachiaria brizantha and four primary forests on the deforestation front of the Amazonian forest. In both systems, we recorded surface cast density and spatial patterns as well as surface cast physical properties. Cast and vegetation distributions were mapped within plots of sizes ranging from 25 to 50 m2, and cast production was monitored during 2 months. Cast spatial distribution varied greatly among plots and across spatial scales ranging from regular at small spatial scales (0–20 cm) to clumped for larger distance ranges (>80 cm). Cast density was not significantly correlated with grass tuft density and their production was independent from the presence of grass tufts in pastures or litter quantity in forests. Although bulk soil properties (pH, C content, structural stability, etc) differed between pastures and forests, cast physical properties did not differ significantly among the studied ecosystem. 相似文献
8.
M. Pittarello M. Probo M. Lonati D. W. Bailey G. Lombardi 《Grass and Forage Science》2016,71(4):529-539
Effects of traditional salt placement (TS) and strategically placed mineral mix supplements (MMS) on cattle distribution were compared in the Italian Alps. Salt was placed within flat and herbaceous sites following the traditional method of farmers, whereas MMS blocks were placed on steep and shrub‐encroached sites within three large pastures (44 ha on average). Each TS and MMS site was paired with a control site, and eleven cows were tracked with GPS collars. Within 10 and 50 m of TS and MMS, treatment sites were used more than control sites (P < 0·05). No differences were detected (P > 0·05) in the number of visits by cows within 10 and 50 m of TS and MMS placements. No differences were detected in time spent within 10 m of TS and MMS placements, but cows spent more time (P < 0·05) within 50 m of TS. With either TS or MMS, cattle preferred areas with gentler terrain, higher forage pastoral value and closer to water (P < 0·001). When MMS was available cattle also preferred areas near MMS (P < 0·001). Shrub and herbaceous cover decreased near MMS placements (P < 0·05) from cattle grazing and trampling. Placement of MMS is a promising tool to enhance cattle distribution and potentially reduce shrub encroachment in rugged mountain pastures. 相似文献
9.
针对目前粮食产后处理模式存在的问题,利用网络分析法进行了优化分析,并提出了改进措施,经比较分析,改进后的处理模式在处理效率、成本、数量损失、粮食品质以及益本比等方面均有较双改善。 相似文献
10.