全文获取类型
收费全文 | 7722篇 |
免费 | 271篇 |
国内免费 | 332篇 |
专业分类
林业 | 3442篇 |
农学 | 390篇 |
基础科学 | 146篇 |
1003篇 | |
综合类 | 2530篇 |
农作物 | 130篇 |
水产渔业 | 83篇 |
畜牧兽医 | 208篇 |
园艺 | 272篇 |
植物保护 | 121篇 |
出版年
2024年 | 35篇 |
2023年 | 103篇 |
2022年 | 110篇 |
2021年 | 125篇 |
2020年 | 150篇 |
2019年 | 219篇 |
2018年 | 82篇 |
2017年 | 159篇 |
2016年 | 248篇 |
2015年 | 233篇 |
2014年 | 348篇 |
2013年 | 308篇 |
2012年 | 522篇 |
2011年 | 634篇 |
2010年 | 553篇 |
2009年 | 617篇 |
2008年 | 551篇 |
2007年 | 538篇 |
2006年 | 383篇 |
2005年 | 356篇 |
2004年 | 298篇 |
2003年 | 274篇 |
2002年 | 225篇 |
2001年 | 177篇 |
2000年 | 172篇 |
1999年 | 151篇 |
1998年 | 98篇 |
1997年 | 92篇 |
1996年 | 94篇 |
1995年 | 86篇 |
1994年 | 99篇 |
1993年 | 64篇 |
1992年 | 59篇 |
1991年 | 39篇 |
1990年 | 49篇 |
1989年 | 34篇 |
1988年 | 27篇 |
1987年 | 10篇 |
1986年 | 2篇 |
1977年 | 1篇 |
排序方式: 共有8325条查询结果,搜索用时 15 毫秒
21.
22.
Existing spatial patterns of a forest are in part a product of its disturbance history. Using laser altimetry and field measures
of canopy top height to represent pre- and post-hurricane canopy topography, respectively, we measured changes in spatial
patterns of stand structure of a United States southern mixed coniferous-deciduous for est. Autocorrelative and fractal properties
were measured in this opportunistic study to quantify changes in canopy architecture along twelve, 190-250 m transects that
were subjected to moderate to high levels of wind disturbance. Prior to the hurricane, canopy heights were autocorrelated
at scales <40 m with an average fractal dimension of 1.71. After the disturbance, autocorrelation disappeared; the average
fractal dimension rose to 1.94. This shift towards spatial randomness illustrates part of the cyclical nature of ecosystem
development. It shows how a catastrophic collapse of biomass accumulation corresponds to a decrease in ecosystem organization
across a landscape.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
23.
SSR技术及其在果树上的应用 总被引:27,自引:4,他引:27
SSR(Simple sequence repeat)技术以其丰富的多态性、共显性遗传、重复性好和操作简便等优点日益受到重视,已成为植物遗传和育种研究中不可缺少的分子标记。对SSR技术的原理和特点作了简要的介绍,较详细地分析了如何获得SSR引物,特别是综述了果树上SSR引物的研究现状,同时将其与其它几种主要的分子标记进行了比较分析,认为SSR标记检测的位点多态性水平明显高于RFLP,而且重复性优于RAPD;着重介绍了SSR技术在果树种质资源和构建果树遗传图谱及基因定位等研究中的应用现状;指出SSR技术将在果树科研上起到重要的作用。 相似文献
24.
Spatial dependence and the relationship of soil organic carbon and soil moisture in the Luquillo Experimental Forest,Puerto Rico 总被引:11,自引:0,他引:11
Wang Hongqing Hall Charles A.S. Cornell Joseph D. Hall Myrna H.P. 《Landscape Ecology》2002,17(8):671-684
We used geo-spatial statistical techniques to examine the spatial variation and relationship of soil organic carbon (SOC) and soil moisture (SM) in the Luquillo Experimental Forest (LEF), Puerto Rico, in order to test the hypothesis that mountainous terrain introduces spatial autocorrelation and crosscorrelation in ecosystem and soil properties. Soil samples (n = 100) were collected from the LEF in the summer of 1998 and analyzed for SOC, SM, and bulk density (BD). A global positioning system was used to georeference the location of each sampling site. At each site, elevation, slope and aspect were recorded. We calculated the isotropic and anisotropic semivariograms of soil and topographic properties, as well as the cross-variograms between SOC and SM, and between SOC and elevation. Then we used four models (random, linear, spherical and wave/hole) to test the semi-variances of SOC, SM, BD, elevation, slope and aspect for spatial dependence. Our results indicate that all the studied properties except slope angle exhibit spatial dependence within the scale of sampling (200 – 1000 m sampling interval). The spatially structured variance (the variance due to the location of sampling sites) accounted for a large proportion of the sample variance for elevation (99%), BD (90%), SOC (68%), aspect (56%) and SM (44%). The ranges of spatial dependence (the distances within which parameters are spatially dependent) for aspect, SOC, elevation, SM, and BD were 9810 m, 3070 m, 1120 m, 930 m and 430 m, respectively. Cross correlograms indicate that SOC varies closely with elevation and SM depending on the distances between samples. The correlation can shift from positive to negative as the separation distance increases. Larger ranges of spatial dependence of SOC, aspect and elevation indicate that the distribution of SOC in the LEF is determined by a combination of biotic (e.g., litterfall) and abiotic factors (e.g., microclimate and topographic features) related to elevation and aspect. This demonstrates the importance of both elevation and topographic gradients in controlling climate, vegetation distribution and soil properties as well as the associated biogeochemical processes in the LEF.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
25.
SSR-based genetic linkage analysis of resistance to crown rust (Puccinia coronata f. sp. lolii) in perennial ryegrass (Lolium perenne) 总被引:2,自引:0,他引:2
Crown rust (caused by Puccinia coronata f. sp. lolii) is a serious foliar disease of the pasture and turfgrass perennial ryegrass (Lolium perenne). Previous genetic studies have detected both qualitative and quantitative resistance mechanisms, and interpretation of the genetic system is complicated by variation within the sexually reproducing pathogen. Resistant and susceptible parental genotypes of ryegrass were identified using a composite urediniospore population collected from three geographically distinct locations. A two-way pseudo-testcross mapping population was obtained as the F1 progeny of the pair-cross between ryegrass parental genotypes Vedette6 and Victorian9. Both parents showed intermediate resistance against a pathogen population collected in a single geographical zone (Hamilton, Victoria), but in the F1 population, significant variation for a range of resistance-associated characters was detected. Statistical analysis of phenotypic data suggested a major gene effect, hence bulked segregant analysis with map-assigned simple sequence repeat (SSR) markers was used to scan the genome. A marker showing strong association with resistance was assigned to linkage group (LG) 2 of perennial ryegrass. Analysis of 11 LG2 SSR markers defined an interval between loci xlpssrh03f03 and xlpssrk02e02 as containing the gene or genes (LpPc1) conferring crown rust resistance. Resistance gene determinants were inherited from both parents, with up to 80% of the total phenotypic variation explained by markers segregating from Vedette6 and up to 26% of the variation explained by markers segregating from Victorian9. The two contributions together resulted in an additive increase in effect, with fully resistant individuals requiring determinants from both parents. A conserved syntenic relationship was observed with linkage group B of Avena strigosa, which is the location of a cluster of resistance genes to the oat form of crown rust. The implications of this study for marker-assisted selection of disease resistance in perennial ryegrass are discussed. 相似文献
26.
森林生态保护与效益补偿法律机制研究 总被引:14,自引:0,他引:14
回顾了森林生态保护与效益补偿机制研究现状与存在问题 ,分析了传统的生态保护政策法律手段的作用和历史局限性 ,指出在市场经济条件下 ,法律机制是生态保护和效益补偿机制的主要保障。着重研究了森林生态补偿法律关系中的主体、客体、补偿强度、补偿方法 ;从立法、执法、监督以及产权改革等方面 ,系统提出了建立我国森林生态补偿法律机制的途径和措施 相似文献
27.
Maurice Keith R. Welch Joan M. Brown Christopher P. Latham Roger Earl 《Landscape Ecology》2004,19(6):603-620
The Pocono mesic till barrens (PMTB) are a unique assemblage of fire-maintained shrub communities that support numerous rare species. Historically these barrens covered a large area in the vicinity of Long Pond, Pennsylvania, USA. However, due largely to regional fire suppression instituted in the early 1960s, over 70% of the area covered by barrens succeeded to fire-intolerant forest that does not support the rare species. We investigated the influence of forest proximity on barrens succession across three geomorphic types during periods of high fire frequency and fire suppression, testing the hypothesis that forest processes such as seed rain, shading, and detrital enrichment of soils enhances barrens succession through a contagion effect. Evidence of a forest contagion effect should be shown by increased rates of barrens succession with increasing proximity to the nearest forest edge. In order to detect a forest contagion effect, barrens persistence and barrens succession were modeled in proximity zones of 0-50 m, 50-100 m, 100-200 m, and greater than 200 m from the nearest forest edge. We used existing GIS data layers for fire, geomorphology, and vegetation distribution in 1938, 1963, and 1992. The layers were modified and overlain using ArcView software to determine persistence and succession rates for each unique combination of layers in each proximity zone from 1938 to 1963 (pre-fire suppression) and 1963 to 1992 (post-fire suppression). ANCOVA results indicate that proximity to the nearest forest edge significantly affected barrens persistence rates in both time periods, but succession rates were significantly affected in 1938 to 1963 only. Twenty-eight percent of the 1938 barrens succeeded to forest by 1963; 56% of the 1963 barrens became forest by 1992. Results support previous findings that barrens persistence is enhanced by increased fire frequency, and that barrens persist longer where they overlie flat glacial till than on other geomorphology types. 相似文献
28.
Yohei Kato 《Pesticide biochemistry and physiology》2004,79(2):64-73
A hydrophilic form of acetylcholinesterase (AChE) was purified from N-methyl carbamate susceptible (SA) and highly N-methyl carbamate-resistant (N3D) strains of the green rice leafhopper (GRLH), Nephotettix cincticeps Uhler. Both of purified AChE from SA and N3D strains displayed the highest activities toward acetylthiocholine (ATCh) at pH 8.5. In the SA strain, the optimum concentrations for ATCh, propionylthiocholine (PTCh), and butyrylthiocholine (BTCh) were about 1 × 10−3, 2.5 × 10−3, and 1 × 10−3 M, respectively. However, in the N3D strain, substrate inhibition was not identified for ATCh, PTCh, and BTCh to 1 × 10−2 M. The Km value in the SA strain was 51.1, 39.1, and 41.6 μM and that in the N3D strain was 91.8, 88.1, and 85.2 μM for ATCh, PTCh, and BTCh, respectively. The Km value in the N3D strain indicated about 1.80-, 2.25-, and 2.05-fold lower affinity than that of the SA strain for ATCh, PTCh, and BTCh, respectively. The Vmax value in the SA strain was 70.2, 30.5, and 4.6 U/mg protein and that in the N3D strain was 123.0, 27.0, and 14.5 U/mg protein for ATCh, PTCh, and BTCh, respectively. The Vmax value in the N3D strain was 1.75- and 3.15-fold higher for ATCh and BTCh than that in the N3D strain. However, it was 1.13-fold lower for PTCh. The increased activity of AChE in the N3D strain is due to the qualitatively modified enzyme with a higher catalytic efficiency. The bimolecular rate constant (ki) for propoxur was 27.1 × 104 and 0.51 × 104 M−1 min−1 in the SA and N3D strain and that for monocrotophos was 0.031 × 104 and 2.0 × 104 M−1 min−1 in the SA and N3D strain. AChE from the N3D strain was 53-fold less sensitive than SA strain to inhibition by propoxur. In contrast, AChE from the N3D strain was 65-fold more sensitive to inhibition by monocrotophos than AChE from the SA strain. This indicated negatively correlated cross-insensitivity of AChE to propoxur and monocrotophos. 相似文献
29.
30.
本研究以感(岷山红三叶)、抗(澳大利亚红三叶品种♀Sensation×Renegade♂杂交新品系“甘农PR1”)白粉病红三叶材料为父母本杂交并种植成苗经人工接菌后筛选出抗、感白粉病的F1群体为作图群体,利用AFLP标记构建红三叶高密度遗传图谱,并利用区间作图法对抗白粉病QTL进行了定位分析,可以为红三叶抗白粉病基因克隆和转基因等分子辅助育种奠定基础。结果表明,149个AFLP标记构建得到的遗传图谱包含7个连锁群(LG1,LG2,LG3,LG4,LG5,LG6和LG7),遗传图谱的总距离为640.5 cM。其中,LG1连锁群的遗传距离(140.6 cM)和标记间平均距离(9.4 cM)均最大;LG4连锁群的遗传距离(55.2 cM)和标记间平均距离(1.8 cM)最小。应用区间作图法对红三叶抗白粉病基因进行QTL分析定位,共检测到5个抗白粉病相关QTL位点(qrp-1,qrp-2,qrp-3,qrp-4和qrp-5),其中qrp-1、qrp-2、qrp-3和qrp-4位于LG4连锁群上,qrp-5位于LG5连锁群上。5个QTL位点对抗白粉病的贡献率为29%~90%,qrp-1对红三叶白粉病抗性的贡献率最大(90%),为主效QTL。 相似文献