全文获取类型
收费全文 | 1221篇 |
免费 | 88篇 |
国内免费 | 144篇 |
专业分类
林业 | 101篇 |
农学 | 99篇 |
基础科学 | 65篇 |
303篇 | |
综合类 | 485篇 |
农作物 | 53篇 |
水产渔业 | 75篇 |
畜牧兽医 | 102篇 |
园艺 | 17篇 |
植物保护 | 153篇 |
出版年
2024年 | 15篇 |
2023年 | 43篇 |
2022年 | 65篇 |
2021年 | 52篇 |
2020年 | 60篇 |
2019年 | 73篇 |
2018年 | 51篇 |
2017年 | 74篇 |
2016年 | 78篇 |
2015年 | 55篇 |
2014年 | 66篇 |
2013年 | 91篇 |
2012年 | 94篇 |
2011年 | 115篇 |
2010年 | 91篇 |
2009年 | 74篇 |
2008年 | 58篇 |
2007年 | 67篇 |
2006年 | 41篇 |
2005年 | 26篇 |
2004年 | 20篇 |
2003年 | 30篇 |
2002年 | 18篇 |
2001年 | 10篇 |
2000年 | 16篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 16篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有1453条查询结果,搜索用时 15 毫秒
1.
Impact of grassland contract policy on soil organic carbon losses from alpine grassland on the Qinghai–Tibetan Plateau
下载免费PDF全文
![点击此处可从《Soil Use and Management》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J. Cao Y. Gong E. T. Yeh N. M. Holden J. F. Adamowski R. C. Deo M. Liu J. Zhou J. Zhang W. Zhang S. Zhang D. Sheng S. Yang X. Xu M. Li Q. Feng 《Soil Use and Management》2017,33(4):663-671
Carbon storage in the soils on the Qinghai–Tibetan Plateau plays a very important role in the global carbon budget. In the 1990s, a policy of contracting collective grasslands to smaller units was implemented, resulting in a change from the traditional collective grassland management to two new management patterns: a multi‐household management pattern (MMP: grassland shared by several households without enclosures) and a single‐household management pattern (SMP: grassland enclosed and used by only one household). In 2016, 50 MMP and 54 SMP winter pastures on the Qinghai–Tibetan Plateau were sampled to assess the differences in soil organic carbon (SOC) between the two management patterns. Results showed that average SOC was significantly greater under MMP than under SMP, with an estimated 0.41 Mg C/ha/yr lost due to SMP following the new grassland contract. Based on the government's grassland policy, four grassland utilization scenarios were developed for both summer and winter pastures. We found that if the grassland were managed under SMP, likely C losses ranged between 0.31 × 107 and 6.15 × 107 Mg C/yr across the Qinghai–Tibetan Plateau relative to MMP, which more closely resembles pre‐1990s grassland management. Previous estimates of C losses have only considered land use change (with cover change) and ignored the impacts driven by land management pattern changes (without cover change). The new data suggest that C losses from the Qinghai–Tibetan Plateau are greater than previously estimated, and therefore that the grassland contract policy should be reviewed and SMP households should be encouraged to reunite into the MMP. These findings have potential implications for land management strategies not only on the Qinghai–Tibetan Plateau but also other grazing regions globally where such practices may exist. 相似文献
2.
3.
4.
N. Fenner D. J. Dowrick M. A. Lock C. R. Rafarel & C. Freeman 《Soil Use and Management》2006,22(3):267-273
The temperature dependence of chemical reaction rates and microbial metabolism mean that temperature is a key factor regulating soil trace gas emissions and hydrochemistry. Here we evaluated a novel approach for studying the thermal response of soils, by examining the effects of temperature on gas emissions and hydrochemistry in (a) peat and (b) soil from a Sitka spruce plantation. A thermal gradient was applied along an aluminium bar, allowing soil to be incubated contemporaneously from 2 to 18 °C. The approach demonstrated clear differences in the biogeochemical responses of the two soil types to warming. The peat showed no significant emission of CH4 at temperatures below 6 °C, while above 6 °C, a marked increase in the rate of release was apparent up to 15 °C (Q10 = 2.5) with emissions being similar between 15 and 18 °C. Conversely, CH4 emissions from the forest soil did not respond to warming. Nitrate availability in the peat decreased by 90% between 2 and 18 °C (P < 0.01), whereas concentrations in the forest soil did not respond. Sulphate availability in the peat decreased significantly with warming (60%, P < 0.01), while the forest soil showed the opposite response (a 30% increase, P < 0.01). Conventionally, thermal responses are studied by incubating individual soil samples at different temperatures, involving lengthy preparation and facilities to incubate samples at different temperatures simultaneously. Data collected on a given thermal response is usually limited and thus interpolated or extrapolated. The thermal gradient method overcomes these problems, is simple and flexible, and can be adapted for a wide range of sample types (not confined to soil). Such apparatus may prove useful in the optimization of management practices to mitigate the effects of climate change, as thermal responses will differ depending on land use and soil type. 相似文献
5.
改进了Griewank(1986)提出了关于求解非线性方程组的一种线性搜索方式。在理论上保证了线性搜索的实现,使得算法是适定的,而且,在改进的线性搜索条件下,Broyden算法仍具有全局收敛性和局部超线性收敛性。 相似文献
6.
Urban trees can favorably affect factors underlying global warming by storing carbon and by reducing energy needs for cooling and heating buildings. To estimate carbon stored in roots and above-ground portions of trees, data was collected consisting of whole tree sampling of Amelanchier, Malus, Pyrus, and Syringa cultivars. Roots were excavated using an Air-Spade™. Regression analysis resulted in two equations for predicting total carbon storage based on height and diameter of trees up to 20 cm dbh: Y = 0.05836 (dbh2) for root carbon storage, and Y = 0.0305 (dbh2 × h)0.9499 for above-ground carbon storage, explaining 97% and 96% of the variation, respectively. Average carbon stored in roots of various cultivars ranged from 0.3 to 1.0 kg for smaller trees, those 3.8 to 6.4 cm dbh, to more than 10.4 kg for trees 14.0 cm to 19.7 cm dbh. Average total carbon stored by cultivars ranged from 1.7 to 3.6 kg for trees less than 6.4 cm dbh to 54.5 kg for trees larger than 14.0 cm. The data from these equations apply mainly to trees in nurseries and recently transplanted trees. Comparisons showed that above-ground estimates from previous studies using a sampling technique overestimated values obtained from actual above-ground weights. 相似文献
7.
全球气候暖干化对秦岭南北河流径流泥沙的影响研究 总被引:8,自引:12,他引:8
以秦岭南侧汉江和北侧渭河多年的径流泥沙观测资料,分析了全球气候暖干化对秦岭南北径流泥沙的变化。分析指出在80年代后,由于全球气候变化的影响,秦岭南北河流年均径流量均减少,与1935-1980年相比,汉江河流年均径流量减少1.9%,渭河河流年均径流量减少27.4%;同时汉江河流泥沙含量明显减少,但渭河河流泥沙含量呈增加趋势,是汉江河流泥沙含量133倍,表明了秦岭南北两侧在全球气候暖干化表现出明显的区域响应性。 相似文献
8.
目前我国饲料产业地位不断提升,国际贸易出口总量保持稳定,与进口量上升形成鲜明对比,贸易逆差较大,其根本背景是国内、国际的产业格局发展不同。我国饲料产业的国际产能合作处于初级阶段,在全球价值链中处于不利位置。在价值链视角下,饲料企业不能把出口视为单纯的饲料产品向终端销售,而是应注重于价值链的总体布局,在现代化“大农业”的背景下,对畜牧业、水产养殖业等饲料行业下游产业与饲料产业进行布局。饲料企业应形成海外布局,提升其在全球价值链中的地位,选择海外的“走出去”策略,优化国际贸易的出口机制,同时按照企业价值最大化的要求控制风险,用好政策性金融支持,并重视对国际投资和贸易人才的培养。
[关键词] 全球价值链|产业升级|“走出去” 相似文献
9.
We discuss the problem related to the long time behavior of solutions of the partly dissipative reaction-diffusion equations in unbounded domain,and prove the existence of the compact attractors.The coefficient of the reaction-term depends on space variables,which extend Wang B and Marion M's results. 相似文献
10.
Zhao Yunbin Duan Yurong 《保鲜与加工》1993,(5):125-130
A new globally convergent algorithm was presented for a continuous differen-tiable nonlinear programming by defining a measure function deviating from the Kuhn-Tucker point. With this algorithm which can be used to get the optimum solution of the problem,the optimal lagrangian multiplier corresponding the optimal solution of the problem was also obtained. A new iterative algorithm for quadratic programming is obtained when applying the general algorithm to quadratic programming. Finally,a numerical example was given. 相似文献