全文获取类型
收费全文 | 3707篇 |
免费 | 228篇 |
国内免费 | 270篇 |
专业分类
林业 | 515篇 |
农学 | 196篇 |
基础科学 | 150篇 |
887篇 | |
综合类 | 1488篇 |
农作物 | 175篇 |
水产渔业 | 34篇 |
畜牧兽医 | 479篇 |
园艺 | 68篇 |
植物保护 | 213篇 |
出版年
2024年 | 20篇 |
2023年 | 72篇 |
2022年 | 117篇 |
2021年 | 137篇 |
2020年 | 152篇 |
2019年 | 160篇 |
2018年 | 117篇 |
2017年 | 153篇 |
2016年 | 211篇 |
2015年 | 181篇 |
2014年 | 219篇 |
2013年 | 220篇 |
2012年 | 330篇 |
2011年 | 314篇 |
2010年 | 218篇 |
2009年 | 244篇 |
2008年 | 167篇 |
2007年 | 201篇 |
2006年 | 168篇 |
2005年 | 122篇 |
2004年 | 97篇 |
2003年 | 80篇 |
2002年 | 63篇 |
2001年 | 55篇 |
2000年 | 53篇 |
1999年 | 39篇 |
1998年 | 28篇 |
1997年 | 41篇 |
1996年 | 31篇 |
1995年 | 39篇 |
1994年 | 31篇 |
1993年 | 23篇 |
1992年 | 24篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 2篇 |
排序方式: 共有4205条查询结果,搜索用时 93 毫秒
901.
退化喀斯特植被恢复对土壤微生物数量及群落功能多样性的影响 总被引:5,自引:0,他引:5
采集不同恢复阶段的土壤样品,采用微生物培养法研究了退化喀斯特植被恢复对土壤微生物数量、群落功能多样性的影响。研究结果表明随着退化喀斯特植被的恢复,土壤微生物数量增加,表现为乔木群落阶段灌木群落阶段草本群落阶段裸地阶段。土壤微生物群落代谢功能分析表明:植被恢复往往导致较高的平均颜色变化率、物种丰富度和功能多样性。乔木群落阶段的平均诱导底物利用率最高,明显地与其他3个恢复阶段不同。总之,植被恢复使得土壤微生物数量增加,碳源平均利用率增强。因此,创造了更好的土壤条件更有利于退化喀斯特植被的恢复。 相似文献
902.
不同林龄华北落叶松人工林枯落物储量及持水特性研究 总被引:4,自引:0,他引:4
对3种不同林龄阶段的华北落叶松人工林林下枯落物储量及其持水特性进行分析的结果表明:枯落物储量幼龄林、中龄林、近熟林分别为42.23、57.11、65.00t/hm2;各林龄林下枯落物最大持水量均是未分解层小于半分解层,未分解层最大持水量是其风干重的280%~374%,半分解层最大持水量是其风干重的408%~466%;3种林龄按林下枯落物未分解层持水量大小排序为近熟林>幼龄林>中龄林,按半分解层持水量大小排序为近熟林>中龄林>幼龄林。各林龄各层枯落物持水量与浸水时间之间的最佳拟合关系式为W=alnt+b;各林龄各层次枯落物吸水速率与浸水时间之间的关系式为S=ktn,在0—1h内吸水速率急剧下降,1h以后下降平缓,吸水作用逐渐减小。 相似文献
903.
子午岭林区辽东栎、油松、柴松群落特征及其枯枝落叶层水文效应研究 总被引:2,自引:0,他引:2
通过野外调查和室内分析相结合的方法,对子午岭林区辽东栎、油松、柴松3种群落的特征及其枯枝落叶层水文效应进行了研究,调查结果表明:(1)子午岭辽东栎林除局部林是与其它林木混交外,一般都是纯林;油松林和柴松林常形成以该种群为主的单优群落,但混有少量阔叶树种.(2)子午岭林区油松林物种多样性指数最大,而柴松林物种不丰富;乔木层、灌木层,草本层这3者的多样性指数呈现出由低到高的特点.(3)子午岭辽东栎林、油松林、柴松林的枯落物蓄积量分别为32.65,48.95和47.30 t/hm2.(4)辽东栎、油松、柴松枯枝落叶最大拦蓄量分别为50.93,70.15和61.68 t/hm2. 相似文献
904.
陕西省黄土高原地区侵蚀性降水变化特征 总被引:2,自引:0,他引:2
利用陕西省黄土高原地区42个站点的逐日降水观测资料,分析了1961-2007年该地区的年降水量、侵蚀性降水量、暴雨量的变化趋势和空间分布特征.研究表明,陕西省黄土高原地区年降水量、侵蚀性降水和暴雨的空间分布非常相似,具有北少南多的分布特征.1961-2007年陕西省黄土高原年降水和侵蚀性降水呈下降趋势,暴雨呈上升趋势.在空间分布上,延安市以北地区,降水量减少幅度小,延安市以南地区降水量减少明显;渭北地区暴雨量增加,而丘陵沟壑区暴雨量则呈减少趋势. 相似文献
905.
Mountain regions are known to be especially vulnerable to climatic changes; however, information on the climate sensitivity of alpine ecosystems is still scarce to date. In this study, we investigate the impacts of climate and vegetation composition on soil organic‐matter (SOM) stocks and characteristics along an elevation gradient (900 to 1900 m asl) in the Austrian Limestone Alps. The soils classified as Leptic Histosols, i.e., organic soils directly overlying the calcareous bedrock. Soil organic‐carbon stocks (SOC; mean ± standard deviation) to bedrock increased in the low‐elevation forest sites from 19 ± 3 kg m–2 (900 m asl) to 31 ± 3 kg m–2 (1300 m asl), reached a maximum (38 ± 5 kg m–2) in the shrubland at 1500 m asl, but decreased again in the high‐elevation grassland sites (26 ± 3 kg m–2 at 1700 m asl and 13 ± 3 kg m–2 at 1900 m asl). Thermogravimetic measurements and Fourier‐transform infrared spectroscopy (FTIR) suggest that the upper soil layers were dominated by more labile organic compounds, whereas more persistent materials increased with depth. Along the studied climosequence, the aliphatic FTIR band (2920 cm–1) was lower in the low‐elevation forest sites compared to the high‐elevation grassland sites. Most other FTIR bands did not change with altitude, but were related to specific site conditions, such as vegetation composition and associated differences in soil pH. Our results demonstrate that differences in SOM stocks and characteristics are not consistently related to variations in climatic conditions along the studied elevation gradient, but are strongly affected by the vegetation composition, their C input and litter quality. This, in turn, is expected to shift in response to climate change. 相似文献
906.
M. Rubino J.A.J. Dungait R.P. Evershed P. De Angelis A. Lagomarsino A. Merola M.F. Cotrufo 《Soil biology & biochemistry》2010,42(7):1009-1016
Partitioning of the quantities of C lost by leaf litter through decomposition into (i) CO2 efflux to the atmosphere and (ii) C input to soil organic matter (SOM) is essential in order to develop a deeper understanding of the litter-soil biogeochemical continuum. However, this is a challenging task due to the occurrence of many different processes contributing to litter biomass loss. With the aim of quantifying different fluxes of C lost by leaf litter decomposition, a field experiment was performed at a short rotation coppice poplar plantation in central Italy. Populus nigra leaf litter, enriched in 13C (δ13C ∼ +160‰) was placed within collars to decompose in direct contact with the soil (δ13C ∼ −26‰) for 11 months. CO2 efflux from within the collars and its isotopic composition were determined at monthly intervals. After 11 months, remaining litter and soil profiles (0-20 cm) were sampled and analysed for their total C and 13C content. Gas chromatography (GC), GC-mass spectrometry (MS) and GC-combustion-isotope ratio (GC/C/IRMS) were used to analyse phospholipid fatty acids (PLFA) extracted from soil samples to identify the groups of soil micro-organisms that had incorporated litter-derived C and to determine the quantity of C incorporated by the soil microbial biomass (SMB). By the end of the experiment, the litter had lost about 80% of its original weight. The fraction of litter C lost as an input into the soil (67 ± 12% of the total C loss) was found to be twice as much as the fraction released as CO2 to the atmosphere (30 ± 3%), thus demonstrating the importance of quantifying litter-derived C input to soils, in litter decomposition studies. The mean δ13C values of PLFAs in soil (δ13C = −12.5‰) showed sustained incorporation of litter-derived C after one year (7.8 ± 1.6% of total PLFA-C). Thus, through the application of stable 13C isotope analyses, we have quantified two major C fluxes contributing to litter decomposition, at macroscopic and microscopic levels. 相似文献
907.
908.
909.
In order to explore the methods of recovering native grasses into exotic vegetation, the response of riparian vegetation to the removal of the above‐ground shoots and/or litter of Solidago gigantea in a flood plain in Hokkaido, northern Japan, was investigated. The four treatments were: the removal of the above‐ground shoots of S. gigantea (A); the removal of the litter of S. gigantea (L); the removal of both the above‐ground shoots and litter of S. gigantea (AL); and a control (C). The vegetation cover and S. gigantea cover decreased in the A and AL treatments and increased in the L and C treatments. The understory plant cover increased in the A and AL treatments, but did not change in the L and C treatments. The increases in the understory cover in the A and AL treatments were associated with increases in Phalaris arundinacea. The seedling emergence of P. arundinacea was promoted by AL. In a greenhouse, the S. gigantea litter tended to decrease the seed germination of P. arundinacea. The AL treatment also increased the abundance of the other exotic plant, Solidago altissima. The continuous removal of the above‐ground shoots and litter of S. gigantea long term is effective for promoting the recovery and emergence of native riparian grassland vegetation. However, this method also promotes the recovery of other exotics. 相似文献
910.
烤烟“3414”肥料效应 总被引:1,自引:0,他引:1
[目的]为不同烟区土壤测土配方施肥提供理论指导。[方法]采用"3414"完全试验方案设计,研究了不同肥料效应对烤烟生长发育以及产量、质量等方面的影响。[结果]不同处理烟株田间长势表现具有一定的差异性。烤后烟叶总氮、总植物碱和K2O含量与肥料效应关系密切。不同肥料效应对烟叶经济性状有较大的影响,不施肥处理烟叶产量最低,中上等烟比例较高。氮肥用量最多处理经济性状没有表现出最好的优势。[结论]建立肥料效应三元二次数学模拟方程:Y=138.36+23.18X1-0.83X12-4.32X2+0.34X22-4.40X3+0.09X32-1.02X1X2-0.35X1X3+0.64X2X3(式中,X1、X2,X3分别代表N、P2O5、K2O用量)。该研究可以预测最大施肥量和最佳施肥量,用于指导当地烟叶生产。 相似文献