全文获取类型
收费全文 | 138930篇 |
免费 | 22813篇 |
国内免费 | 7330篇 |
出版年
2024年 | 231篇 |
2023年 | 1502篇 |
2022年 | 3382篇 |
2021年 | 6021篇 |
2020年 | 5556篇 |
2019年 | 7810篇 |
2018年 | 7899篇 |
2017年 | 7121篇 |
2016年 | 8541篇 |
2015年 | 10464篇 |
2014年 | 11501篇 |
2013年 | 12408篇 |
2012年 | 11933篇 |
2011年 | 10578篇 |
2010年 | 8537篇 |
2009年 | 6651篇 |
2008年 | 6495篇 |
2007年 | 5423篇 |
2006年 | 4826篇 |
2005年 | 3824篇 |
2004年 | 3418篇 |
2003年 | 3006篇 |
2002年 | 2665篇 |
2001年 | 2440篇 |
2000年 | 2184篇 |
1999年 | 2068篇 |
1998年 | 1083篇 |
1997年 | 1204篇 |
1996年 | 1051篇 |
1995年 | 968篇 |
1994年 | 977篇 |
1993年 | 698篇 |
1992年 | 1022篇 |
1991年 | 863篇 |
1990年 | 637篇 |
1989年 | 587篇 |
1988年 | 498篇 |
1987年 | 416篇 |
1986年 | 397篇 |
1985年 | 403篇 |
1984年 | 229篇 |
1983年 | 213篇 |
1982年 | 153篇 |
1981年 | 124篇 |
1980年 | 118篇 |
1979年 | 126篇 |
1978年 | 94篇 |
1977年 | 70篇 |
1974年 | 82篇 |
1972年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
利用蛋白质内源荧光和酶活性两种信号以及荧光偏振,HPLC和停流等方法研究了盐酸胍诱导的α淀粉酶去折叠与重折叠的平衡转变和动力学。实验结果表明α淀粉酶去折叠与重折叠是两个不同的过程;变性与复性过程中可能伴有聚集体生成;去折叠与重折叠均为双相过程,重折叠大约始于2秒之后。 相似文献
992.
<正> 我们用国内的新鲜酵母为材料,从中分离纯化到一种多肽——酵母多肽,其分子量为14kD。在低血清培养体系中能促使人成纤维细胞(HFB)和人脐静脉内皮细胞(HUVEC)的DNA合成。当培养液中酵母多肽的浓度为1μg/mL时能引起最大的刺激作用。但此多肽对胎牛心脏内皮细胞(FBHEC)的DNA合成则无作用。 相似文献
993.
Elevated tropospheric ozone (O3) affects the allocation of biomass aboveground and belowground and influences terrestrial ecosystem functions. However, how belowground functions respond to elevated O3 concentrations ([O3]) remains unclear at the global scale. Here, we conducted a detailed synthesis of belowground functioning responses to elevated [O3] by performing a meta-analysis of 2395 paired observations from 222 publications. We found that elevated [O3] significantly reduced the primary productivity of roots by 19.8%, 16.3%, and 26.9% for crops, trees and grasses, respectively. Elevated [O3] strongly decreased the root/shoot ratio by 11.3% for crops and by 4.9% for trees, which indicated that roots were highly sensitive to O3. Elevated [O3] impacted carbon and nitrogen cycling in croplands, as evidenced by decreased dissolved organic carbon, microbial biomass carbon, total soil nitrogen, ammonium nitrogen, microbial biomass nitrogen, and nitrification rates in association with increased nitrate nitrogen and denitrification rates. Elevated [O3] significantly decreased fungal phospholipid fatty acids in croplands, which suggested that O3 altered the microbial community and composition. The responses of belowground functions to elevated [O3] were modified by experimental methods, root environments, and additional global change factors. Therefore, these factors should be considered to avoid the underestimation or overestimation of the impacts of elevated [O3] on belowground functioning. The significant negative relationships between O3-treated intensity and the multifunctionality index for croplands, forests, and grasslands implied that elevated [O3] decreases belowground ecosystem multifunctionality. 相似文献
994.
Liping Jiang Jie Yu Shanyun Wang Xiaomin Wang Lorenz Schwark Guibing Zhu 《Global Change Biology》2023,29(7):1984-1997
The contribution of agriculture to the sustainable development goals requires climate-smart and profitable farm innovations. Increasing the ammonia fertilizer applications to meet the global food demands results in high agricultural costs, environmental quality deterioration, and global warming, without a significant increase in crop yield. Here, we reported that a third microbial ammonia oxidation process, complete ammonia oxidation (comammox), is contributing to a significant ammonia fertilizer loss (41.9 ± 4.8%) at the rate of 3.53 ± 0.55 mg N kg−1 day−1 in agricultural soils around the world. The contribution of comammox to ammonia fertilizer loss, occurring mainly in surface agricultural soil profiles (0–0.2 m), was equivalent to that of bacterial ammonia oxidation (48.6 ± 4.5%); both processes were significantly more important than archaeal ammonia oxidation (9.5 ± 3.6%). In contrast, comammox produced less N2O (0.98 ± 0.44 μg N kg−1 day−1, 11.7 ± 3.1%), comparable to that produced by archaeal ammonia oxidation (16.4 ± 4.4%) but significantly lower than that of bacterial ammonia oxidation (72.0 ± 5.1%). The efficiency of ammonia conversion to N2O by comammox (0.02 ± 0.01%) was evidently lower than that of bacterial (0.24 ± 0.06%) and archaeal (0.16 ± 0.04%) ammonia oxidation. The comammox rate increased with increasing soil pH values, which is the only physicochemical characteristic that significantly influenced both comammox bacterial abundance and rates. Ammonia fertilizer loss, dominated by comammox and bacterial ammonia oxidation, was more intense in soils with pH >6.5 than in soils with pH <6.5. Our results revealed that comammox plays a vital role in ammonia fertilizer loss and sustainable development in agroecosystems that have been previously overlooked for a long term. 相似文献
995.
大鼠出生后脑内钙调神经磷酸酶的研究 总被引:1,自引:0,他引:1
本文用BA-ELISA.immunoblotting及酶活力测定等方法,研究了大鼠脑中钙调神经磷酸酶在大鼠出生后的变化情况。结果表明,钙调神经磷酸酶的含量在大鼠出生后第二周和第三周显著增加,其活力也在出生后第二周达到顶峰。钙调神经磷酸酶这种有规律的变化与脑中突触形成在时间上是一致的,暗示钙调神经磷酸酶可能参与突触功能的调节。 相似文献
996.
Shengen Liu César Plaza Raúl Ochoa-Hueso Chanda Trivedi Juntao Wang Pankaj Trivedi Guiyao Zhou Juan Piñeiro Catarina S. C. Martins Brajesh K. Singh Manuel Delgado-Baquerizo 《Global Change Biology》2023,29(22):6276-6285
The decomposition of litter and the supply of nutrients into and from the soil are two fundamental processes through which the above- and belowground world interact. Microbial biodiversity, and especially that of decomposers, plays a key role in these processes by helping litter decomposition. Yet the relative contribution of litter diversity and soil biodiversity in supporting multiple ecosystem services remains virtually unknown. Here we conducted a mesocosm experiment where leaf litter and soil biodiversity were manipulated to investigate their influence on plant productivity, litter decomposition, soil respiration, and enzymatic activity in the littersphere. We showed that both leaf litter diversity and soil microbial diversity (richness and community composition) independently contributed to explain multiple ecosystem functions. Fungal saprobes community composition was especially important for supporting ecosystem multifunctionality (EMF), plant production, litter decomposition, and activity of soil phosphatase when compared with bacteria or other fungal functional groups and litter species richness. Moreover, leaf litter diversity and soil microbial diversity exerted previously undescribed and significantly interactive effects on EMF and multiple individual ecosystem functions, such as litter decomposition and plant production. Together, our work provides experimental evidence supporting the independent and interactive roles of litter and belowground soil biodiversity to maintain ecosystem functions and multiple services. 相似文献
997.
Huili Feng Jiahuan Guo Changhui Peng Daniel Kneeshaw Gabrielle Roberge Chang Pan Xuehong Ma Dan Zhou Weifeng Wang 《Global Change Biology》2023,29(14):3970-3989
A significant increase in reactive nitrogen (N) added to terrestrial ecosystems through agricultural fertilization or atmospheric deposition is considered to be one of the most widespread drivers of global change. Modifying biomass allocation is one primary strategy for maximizing plant growth rate, survival, and adaptability to various biotic and abiotic stresses. However, there is much uncertainty as to whether and how plant biomass allocation strategies change in response to increased N inputs in terrestrial ecosystems. Here, we synthesized 3516 paired observations of plant biomass and their components related to N additions across terrestrial ecosystems worldwide. Our meta-analysis reveals that N addition (ranging from 1.08 to 113.81 g m−2 year−1) increased terrestrial plant biomass by 55.6% on average. N addition has increased plant stem mass fraction, shoot mass fraction, and leaf mass fraction by 13.8%, 12.9%, and 13.4%, respectively, but with an associated decrease in plant reproductive mass (including flower and fruit biomass) fraction by 3.4%. We further documented a reduction in plant root-shoot ratio and root mass fraction by 27% (21.8%–32.1%) and 14.7% (11.6%–17.8%), respectively, in response to N addition. Meta-regression results showed that N addition effects on plant biomass were positively correlated with mean annual temperature, soil available phosphorus, soil total potassium, specific leaf area, and leaf area per plant. Nevertheless, they were negatively correlated with soil total N, leaf carbon/N ratio, leaf carbon and N content per leaf area, as well as the amount and duration of N addition. In summary, our meta-analysis suggests that N addition may alter terrestrial plant biomass allocation strategies, leading to more biomass being allocated to aboveground organs than belowground organs and growth versus reproductive trade-offs. At the global scale, leaf functional traits may dictate how plant species change their biomass allocation pattern in response to N addition. 相似文献
998.
Dianye Zhang Lu Wang Shuqi Qin Dan Kou Siyu Wang Zhihu Zheng Josep Peñuelas Yuanhe Yang 《Global Change Biology》2023,29(14):3910-3923
The status of plant and microbial nutrient limitation have profound impacts on ecosystem carbon cycle in permafrost areas, which store large amounts of carbon and experience pronounced climatic warming. Despite the long-term standing paradigm assumes that cold ecosystems primarily have nitrogen deficiency, large-scale empirical tests of microbial nutrient limitation are lacking. Here we assessed the potential microbial nutrient limitation across the Tibetan alpine permafrost region, using the combination of enzymatic and elemental stoichiometry, genes abundance and fertilization method. In contrast with the traditional view, the four independent approaches congruently detected widespread microbial nitrogen and phosphorus co-limitation in both the surface soil and deep permafrost deposits, with stronger limitation in the topsoil. Further analysis revealed that soil resources stoichiometry and microbial community composition were the two best predictors of the magnitude of microbial nutrient limitation. High ratio of available soil carbon to nutrient and low fungal/bacterial ratio corresponded to strong microbial nutrient limitation. These findings suggest that warming-induced enhancement in soil nutrient availability could stimulate microbial activity, and probably amplify soil carbon losses from permafrost areas. 相似文献
999.
1000.
Kang Wang Hui Lin Conghui Peng Lin Sun Yahui Gao Baohong Chen 《Global Change Biology》2023,29(17):5099-5113
Noctiluca scintillans is one of the most common harmful algal species and widely known due to its bioluminescence. In this study, the spatial distribution, seasonal variations, and long-term trends of N. scintillans blooms in China and the related drivers were analyzed and discussed. From 1933 to 2020, a total of 265 events of N. scintillans blooms were recorded in Chinese coastal waters, with a total duration of 1052 days. The first N. scintillans bloom occurred in Zhejiang in 1933, and only three events were recorded before 1980. From 1981 to 2020, N. scintillans caused harmful algal blooms (HABs) almost every year, both the average duration and the proportion of multiphase HABs showed an increasing trend. 1986–1992, 2002–2004, and 2009–2016 were the three peak periods with a frequency of no less than five events of N. scintillans blooms per year. In terms of spatial distribution, N. scintillans blooms spread from the Southeast China Sea to the Bohai Sea after 2000, Guangdong, Fujian, and Hebei were the three provinces with the highest numbers of recorded events of N. scintillans blooms. Moreover, 86.8% of the events of N. scintillans blooms occurred in spring (March, April, and May) and summer (June, July, and August). Among environmental factors, the dissolved inorganic phosphate, dissolved silicate and chemical oxygen demand were significantly correlated with the cell density of N. scintillans during N. scintillans blooms, and most of N. scintillans blooms were recorded in the temperature range of 18.0–25.0°C. Precipitation, hydrodynamics, water temperature, and food availability might be the main factors affecting the spatial–temporal distribution of N. scintillans blooms along the Chinese coast. 相似文献