收费全文 | 21740篇 |
免费 | 1882篇 |
国内免费 | 1406篇 |
2024年 | 28篇 |
2023年 | 201篇 |
2022年 | 320篇 |
2021年 | 852篇 |
2020年 | 626篇 |
2019年 | 789篇 |
2018年 | 841篇 |
2017年 | 629篇 |
2016年 | 939篇 |
2015年 | 1416篇 |
2014年 | 1642篇 |
2013年 | 1725篇 |
2012年 | 2039篇 |
2011年 | 1962篇 |
2010年 | 1156篇 |
2009年 | 1101篇 |
2008年 | 1269篇 |
2007年 | 1158篇 |
2006年 | 1054篇 |
2005年 | 894篇 |
2004年 | 878篇 |
2003年 | 692篇 |
2002年 | 562篇 |
2001年 | 367篇 |
2000年 | 293篇 |
1999年 | 264篇 |
1998年 | 198篇 |
1997年 | 161篇 |
1996年 | 150篇 |
1995年 | 121篇 |
1994年 | 109篇 |
1993年 | 64篇 |
1992年 | 91篇 |
1991年 | 71篇 |
1990年 | 82篇 |
1989年 | 64篇 |
1988年 | 44篇 |
1987年 | 45篇 |
1986年 | 27篇 |
1985年 | 28篇 |
1984年 | 33篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1967年 | 2篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1962年 | 1篇 |
A viable option for increasing nitrogen (N) use efficiency and mitigation of negative impacts of N on the environment is to capitalize on multi-element interactions through implementation of nutrient management programs that provide balanced nutrition. Numerous studies have demonstrated the immediate efficacy of this approach in the developing regions like China and India as well as developed countries in North America. Based on 241 site-years of experiments in these countries, the first-year N recovery efficiency (RE) for the conventional or check treatments averaged 21% while the balanced treatments averaged 54% RE, for an average increase of 33% in RE due to balanced nutrition. Effective policies to promote adoption are most likely those that enable site-specific approaches to nutrient management decisions rather than sweeping, nation-wide incentives supporting one nutrient over another. Local farmers, advisers and officials need to be empowered with tools and information to help them define necessary changes in practices to create more balanced nutrient management.
相似文献Soil samples were collected from 7 sites in the up-, mid-and down-reach along and nearby the wastewater irrigation channel, western Shenyang of China. The concentrations of selected pollutants (mineral oil, PAHs - polycycle aromatic hydrocarbons and Cd) were determined by UV spectrometer, HPLC and AAS (atomic adsorption spectrometer) spectrometer, respectively. Toxicity effects of soils were evaluated by seedling emergence test with root length of wheat as the end-point and by earthworms test with the mortality rate and inhibition rates of body weight as endpoints. Results showed accumulation of pollutants for most soils with concentration of 200.2 mg.kg−1∼1600 mg.kg−1 for mineral oil, 0.33 mg.kg−1∼1.81 mg.kg−1 for Cd and 900.16 mg.kg−1 ∼ 2737.91 mg.kg−1 for PAHs. The inhibition rates of root elongation were from −20% up to 40 %, and mortality rates of earthworms ranged from 0%∼40% from the exposure period of two weeks to eight weeks by sampling interval of two weeks, the inhibition rates of earthworm growth were from −19.36% to 34.53%, showing effects of stimulation at 2 weeks to an increasing effects of inhibition at 4, 6 and 8 weeks, respectively. Mortality rates correlated with the loss of body weight of earthworms.
This study indicated the potential risk of pollutants of environmental low content in soil by the determination of selected chemicals combined with toxicity indexes.
相似文献