全文获取类型
收费全文 | 96907篇 |
免费 | 559篇 |
国内免费 | 814篇 |
出版年
2023年 | 13篇 |
2022年 | 21篇 |
2021年 | 39篇 |
2020年 | 31篇 |
2019年 | 44篇 |
2018年 | 11885篇 |
2017年 | 10709篇 |
2016年 | 7525篇 |
2015年 | 771篇 |
2014年 | 437篇 |
2013年 | 501篇 |
2012年 | 4473篇 |
2011年 | 13052篇 |
2010年 | 12173篇 |
2009年 | 8363篇 |
2008年 | 9972篇 |
2007年 | 11586篇 |
2006年 | 499篇 |
2005年 | 736篇 |
2004年 | 1176篇 |
2003年 | 1242篇 |
2002年 | 981篇 |
2001年 | 299篇 |
2000年 | 197篇 |
1999年 | 87篇 |
1998年 | 70篇 |
1997年 | 72篇 |
1996年 | 63篇 |
1995年 | 46篇 |
1994年 | 43篇 |
1993年 | 80篇 |
1992年 | 59篇 |
1991年 | 63篇 |
1990年 | 25篇 |
1989年 | 40篇 |
1988年 | 30篇 |
1987年 | 40篇 |
1986年 | 20篇 |
1985年 | 14篇 |
1984年 | 29篇 |
1983年 | 44篇 |
1982年 | 17篇 |
1981年 | 23篇 |
1980年 | 16篇 |
1975年 | 12篇 |
1972年 | 250篇 |
1971年 | 276篇 |
1965年 | 14篇 |
1962年 | 25篇 |
1944年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Manuella Strukelj Suzanne Brais Marc J. Mazerolle David Paré Pierre Drapeau 《Ecosystems》2018,21(1):68-84
Litter decomposition is a major driver of carbon (C) and nitrogen (N) cycles in forest ecosystems and has major implications for C sequestration and nutrient availability. However, empirical information regarding long-term decomposition rates of foliage and wood remains rare. In this study, we assessed long-term C and N dynamics (12–13 years) during decomposition of foliage and wood for three boreal tree species, under a range of harvesting intensities and slash treatments. We used model selection based on the second-order Akaike’s Information Criterion to determine which decomposition model had the most support. The double-exponential model provided a good fit to C mass loss for foliage of trembling aspen, white spruce, and balsam fir, as well as aspen wood. These litters underwent a rapid initial phase of leaching and mineralisation, followed by a slow decomposition. In contrast, for spruce and fir wood, the single-exponential model had the most support. The long-term average decay rate of wood was faster than that of foliage for aspen, but not of conifers. However, we found no evidence that fir and spruce wood decomposed at slower rates than the recalcitrant fraction of their foliage. The critical C:N ratios, at which net N mineralisation began, were higher for wood than for foliage. Long-term decay rates following clear-cutting were either similar or faster than those observed in control stands, depending on litter material, tree species, and slash treatment. The critical C:N ratios were reached later and decreased for all conifer litters following stem-only clear-cutting, indicating increased N retention in harvested sites with high slash loads. Partial harvesting had weak effects on C and N dynamics of decaying litters. A comprehensive understanding of the long-term patterns and controls of C and N dynamics following forest disturbance would improve our ability to forecast the implications of forest harvesting for C sequestration and nutrient availability. 相似文献
122.
Takahiro Ishii Tatsufumi Okino Yosuke Mino Hiroaki Tamiya Fuyuhiko Matsuda 《Plant Growth Regulation》2007,52(2):131-139
Starfish waste has been shown to be an effective compost material not only in the promotion of plant growth but also in terms
of having insecticidal activity. In the present study, plant growth regulation by chemicals from starfish was examined. The
aqueous fraction from a hot water extract of the starfish Asterias amurensis Lütken showed plant-growth activity, while the aqueous fraction from a methanol extract inhibited growth of Brassica campestris. The lipophilic fraction from the methanol extract also exhibited a plant growth-promoting effect. The active components
from each extract were identified. Asterubine from the hot water extract promoted plant growth. A ceramide from the lipophilic
fraction showed root growth promoting effect, and three glucocerebrosides had promotive effects on the entire plant. Asterosaponins
were identified as the main growth inhibitors in the aqueous fraction of the methanol extract. These active compounds from
starfish waste could be analyzed as potential plant growth regulators in agricultural applications in the future. 相似文献
123.
This editorial addresses the debate concerning the origin of adult nucleus pulposus cells in the light of profiling studies
by Minogue and colleagues. In their report of several marker genes that distinguish nucleus pulposus cells from other related
cell types, the authors provide novel insights into the notochordal nature of the former. Together with recently published
work, their work lends support to the view that all cells present within the nucleus pulposus are derived from the notochord.
Hence, the choice of an animal model for disc research should be based on considerations other than the cell loss and replacement
by non-notochordal cells. 相似文献
124.
125.
Kevin B. Potts 《Journal of Mammalian Evolution》2010,17(3):221-222
126.
127.
128.
Background
Arsenic contamination is widespread throughout the world and this toxic metalloid is known to cause cancers of organs such as liver, kidney, skin, and lung in human. In spite of a recent surge in arsenic related studies, we are still far from a comprehensive understanding of arsenic uptake, detoxification, and sequestration in plants. Crambe abyssinica, commonly known as 'abyssinian mustard', is a non-food, high biomass oil seed crop that is naturally tolerant to heavy metals. Moreover, it accumulates significantly higher levels of arsenic as compared to other species of the Brassicaceae family. Thus, C. abyssinica has great potential to be utilized as an ideal inedible crop for phytoremediation of heavy metals and metalloids. However, the mechanism of arsenic metabolism in higher plants, including C. abyssinica, remains elusive. 相似文献129.
Background
Meiotic prophase is a critical stage in sexual reproduction. Aberrant chromosome recombination during this stage is a leading cause of human miscarriages and birth defects. However, due to the experimental intractability of mammalian gonads, only a very limited number of meiotic genes have been characterized. Here we aim to identify novel meiotic genes important in human reproduction through computational mining of cross-species and cross-sex time-series expression data from budding yeast, mouse postnatal testis, mouse embryonic ovary, and human fetal ovary. 相似文献130.
Kasper Stovgaard Christian Andreetta Jesper Ferkinghoff-Borg Thomas Hamelryck 《BMC bioinformatics》2010,11(1):429