全文获取类型
收费全文 | 7451篇 |
免费 | 575篇 |
国内免费 | 849篇 |
专业分类
林业 | 669篇 |
农学 | 964篇 |
基础科学 | 174篇 |
1421篇 | |
综合类 | 2429篇 |
农作物 | 721篇 |
水产渔业 | 644篇 |
畜牧兽医 | 1346篇 |
园艺 | 246篇 |
植物保护 | 261篇 |
出版年
2024年 | 52篇 |
2023年 | 181篇 |
2022年 | 269篇 |
2021年 | 318篇 |
2020年 | 350篇 |
2019年 | 363篇 |
2018年 | 285篇 |
2017年 | 405篇 |
2016年 | 416篇 |
2015年 | 346篇 |
2014年 | 423篇 |
2013年 | 492篇 |
2012年 | 561篇 |
2011年 | 560篇 |
2010年 | 458篇 |
2009年 | 479篇 |
2008年 | 413篇 |
2007年 | 451篇 |
2006年 | 309篇 |
2005年 | 283篇 |
2004年 | 209篇 |
2003年 | 180篇 |
2002年 | 139篇 |
2001年 | 132篇 |
2000年 | 120篇 |
1999年 | 95篇 |
1998年 | 65篇 |
1997年 | 75篇 |
1996年 | 71篇 |
1995年 | 68篇 |
1994年 | 53篇 |
1993年 | 61篇 |
1992年 | 36篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 26篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1973年 | 1篇 |
1962年 | 1篇 |
1956年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有8875条查询结果,搜索用时 15 毫秒
981.
Variation in quality and decomposability of red oak leaf litter along an urban-rural gradient 总被引:6,自引:0,他引:6
M. M. Carreiro K. Howe D. F. Parkhurst R. V. Pouyat 《Biology and Fertility of Soils》1999,30(3):258-268
This study tested whether urban land use can affect the chemistry and decomposability of Quercus rubra L. (red oak) leaf litter in forests within and near a large metropolitan area. Cities may affect the quality of leaf litter
directly through foliar uptake of atmospheric pollutants, and indirectly through alterations in local climate and changes
in soil fertility caused by pollutant loads and altered nutrient cycling regimes. Using a microbial bioassay, we tested whether
red oak leaf litter collected from urban and suburban forests in and near New York City differed in decomposability from litter
of the same species collected from rural forests 130 km from the city. We found that oak litter from the urban forests decayed
25% more slowly and supported 50% less cumulative microbial biomass in a laboratory bioassay than rural litter. Rural litter
contained less lignin and more labile material than urban litter, and the amounts of these chemical constituents were highly
correlated with the decay rate coefficients and integrated microbial growth achieved on the litter. The specific causes of
the variation in litter chemistry are not known. The results of this study suggest that decomposer activity and nutrient cycling
in forests near large cities may be affected both by altered litter quality and by altered biotic, chemical and physical environments.
The sensitivity of the microbial bioassay makes it useful for distinguishing differences in within-species litter quality
that result from natural or anthropogenic variation in the environment.
Received: 7 January 1999 相似文献
982.
Field studies on microbial populations and their activity in soil and litter layers of different sites revealed statistically
significant correlations with climatic factors, soil characteristics and the number of terrestrial isopods (Isopoda: Oniscidea).
The number of captured Porcellio scaber was negatively correlated with the density of soil microbiota, but positively correlated with both respiration and cellulolytic
activity of litter-colonizing microorganisms. By contrast, the number of Oniscus asellus was positively correlated with soil microbial density and respiration, and cellulolytic and respiratory activity of the litter.
These interspecific differences were unexpected between species that frequently occur sympatrically, and, thus, probably have
similar ecological requirements. Hence, the results suggested species-specific interactions of these species with soil and
litter-colonizing microbiota. However, further field studies are required to prove this.
Received: 16 December 1998 相似文献
983.
984.
撒坝猪血清淀粉酶与繁殖性能关系的研究 总被引:3,自引:1,他引:2
采用垂直板聚丙烯酰胺凝胶电泳法(PAGE)对115头撒坝猪血清淀粉酶(Am)多态性进行了检测,并用二因素有互作的最小二乘模型对不同淀粉酶基因型与繁殖性能的关系进行了分析。结果表明,所检测到的血清淀粉酶AA、AB和BB3种基因型的频率分别为0.1739、0.4261和0.4000,2个等位基因A和B的频率分别为0.3870和0.6130,未发现C基因。该位点的遗传多样性指数为0.4744。在3种基因型中,不同基因型公、母猪间的繁殖性能差异未达到显著水平(P>0.05),但公、母猪不同基因型交配组合间的繁殖性能则在仔猪断奶体重上存在着显著差异(P<0.05或P<0.01)。说明血清淀粉酶多态性与某些繁殖性状之间存在着一定的相关。 相似文献
985.
在同一条件下研究比较杜洛克(D) 与A、B、C3 个新品系猪杂交后代的肉用性能。结果显示, DA、DB、DC 的生长速度分别为757-32 、718-81 、686-67 g/d 。料重比依次为3-58 、3-54、3-70 。屠宰测定, DA、DB 的胴体瘦肉率比DC分别高7-51 % ( P< 0-05) 和13-38 % ( P< 0-01) ,脂肪率DC比DA、DB 分别高14-84 % ( P < 0-05) 和31-15% ( P < 0-01) , DA 和DB 无显著性差异。其它胴体性状水平各组间无显著性差异。DA、DC 的肉质性能良好, DB 表现轻度的PSE 肉 相似文献
986.
陶建平 《西南大学学报(自然科学版)》1999,21(6):500-504
通过人工播种的方法,研究了四川大头茶在不同覆土厚度以及不同枯叶覆盖下种子萌发及幼苗发生的差异。结果表明,在0.5cm,2cm和4cm在种覆土处理中,0.5cm覆土厚度情况下,幼苗发生及存活状况最好,不覆土或覆土过厚都不利于幼苗的发生及存活。枯叶主要通过改变幼苗发生的微环境而对处子萌发、幼苗发生及存活状况产生影响。撒播土表的种子较撒播枯叶表现的种子萌发情况好,随着枯叶覆盖层厚度的增加,幼苗发生数显著 相似文献
987.
988.
为探究青藏高原地区间影响青稞产量的主要因素和对青藏高原青稞种植区进行生态区划,以15个青藏高原主推青稞品种为试验材料,选取9个具有代表性的地区连续两年进行种植,测定株高、平均生育期、千粒重等8个青稞农艺性状,同时收集了9个地区的日照时数、降水量、温差等8个气象因素数据,利用相关性分析、单因素方差分析、隶属函数分析对被测指标进行统计分析。结果表明,平均生育期、千粒重、月均最低温度和温差与青稞产量显著相关,且在9个地区中均存在差异。通过聚类分析将9个地区划分为4个青稞种植生态区,分别为高寒旱作青稞生态区、柴达木盆地灌溉青稞生态区、河谷非饱灌青稞生态区和农林混合山地青稞生态区,其中高寒旱作青稞生态区综合表现最好。利用隶属函数分析对9个地区进行综合评价,其中贵南综合评价得分最高,且不同生态区均存在优势青稞品种。 相似文献
989.
Robson Elpídio Pereira Ribeiro Alexandre Carneiro Leão de Mello Márcio Vieira da Cunha Suellen Brandão de Miranda Costa Janerson José Coelho Rayanne Thalita de Almeida de Souza Mércia Virginia Ferreira dos Santos 《Grass and Forage Science》2023,78(1):194-203
This study hypothesised that different elephant grass genotypes respond differently in terms of their morphological development to irrigation. The objective of this study was to evaluate how water availability could affect the morphological development of different genotypes of elephant grass. The treatments consisted of four genotypes of elephant grass [Cenchrus purpureus (Schumach.) Morrone], managed under irrigation or not, during a two-year field trial. The experiment was arranged in a randomised block design in a split-plot, with four replicates per treatment (n = 4). Between the genotypes, two of them were classified as tall-sized (IRI 381 and Elephant B), and two as dwarfs (Taiwan A-146 2.37 and Mott). A three-way interaction between season, irrigation, and genotype affected the variables plant height, stem diameter, number of dead tillers, light interception (LI), and leaf area index (LAI) (p < .05). The morphological development of all genotypes was negatively impacted by the dry season. All genotypes grew taller during the rainy season (p < .05) and had a greater light interception (62%–80%) compared to the dry period (28%–59%). The genotype Mott, showed comparable LI and LAI to the tall-sized genotypes, whilst Taiwan A-146 2.37 was characterized by the lowest values, despite being the genotype with the greatest tiller density (60 tillers m−2) (p < .05). The use of irrigation mitigated the negative effects of the dry season on the plants, however, the type and size (tall vs. dwarf) of the elephant grass genotypes defined the extent of the responses to the use of the irrigation during the dry period. 相似文献
990.
不同温度下哲罗鲑幼鱼生长性状的遗传参数估计 总被引:5,自引:1,他引:4
采用每4个雄性亲本与1个雌性亲本交配的巢式设计方法,建立哲罗鲑(Hucho taimen)9组母系半同胞、34个父系全同胞家系.19个家系置于(13±1.5)℃(高温),15个家系置于(9±1.5)℃(低温)条件下饲养,计算幼鱼各月龄的体质量和体长性状的遗传力和遗传相关.结果表明,哲罗鲑幼鱼在低温条件下体质量遗传力为0.413~0.675,体长遗传力为0.297~0.777;高温条件下体质量遗传力为0.396~0.558,体长遗传力为0.194~0.624,均属于中高遗传力.在2个温度条件下母本间遗传方差组分均大于父本间遗传方差组分,存在较大的母本效应或显性效应,而根据父本间遗传方差组分估计的遗传力较为无偏.低温条件下体长、体质量遗传力估计值高于高温条件下的估计值,表明基因和环境互作效应较为明显,在低温条件下对体长和体质量进行选择能达到更好的效果.体长和体质量在不同生长时期、不同温度条件下都具有显著的遗传正相关和表型正相关(P<0.05),表型相关系数为0.815~0.939,遗传相关系数为0.794~0.939,表明通过体质量或体长进行选育均能达到改良生长性状的目的. 相似文献