全文获取类型
收费全文 | 18549篇 |
免费 | 89篇 |
国内免费 | 247篇 |
专业分类
林业 | 3861篇 |
农学 | 1614篇 |
基础科学 | 291篇 |
3079篇 | |
综合类 | 1397篇 |
农作物 | 2201篇 |
水产渔业 | 1854篇 |
畜牧兽医 | 1389篇 |
园艺 | 1195篇 |
植物保护 | 2004篇 |
出版年
2024年 | 4篇 |
2023年 | 15篇 |
2022年 | 43篇 |
2021年 | 91篇 |
2020年 | 86篇 |
2019年 | 54篇 |
2018年 | 2786篇 |
2017年 | 2770篇 |
2016年 | 1241篇 |
2015年 | 170篇 |
2014年 | 114篇 |
2013年 | 105篇 |
2012年 | 941篇 |
2011年 | 2273篇 |
2010年 | 2243篇 |
2009年 | 1382篇 |
2008年 | 1446篇 |
2007年 | 1703篇 |
2006年 | 148篇 |
2005年 | 207篇 |
2004年 | 162篇 |
2003年 | 189篇 |
2002年 | 98篇 |
2001年 | 55篇 |
2000年 | 104篇 |
1999年 | 64篇 |
1998年 | 49篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 34篇 |
1994年 | 30篇 |
1993年 | 43篇 |
1992年 | 25篇 |
1991年 | 21篇 |
1990年 | 31篇 |
1989年 | 27篇 |
1988年 | 23篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1977年 | 4篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1968年 | 4篇 |
1967年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
后墙立体栽培草莓提高冬季日光温室内温度 总被引:1,自引:2,他引:1
在日光温室的后墙上,采用管道无土栽培方式进行蔬菜或草莓生产,可以提高温室空间利用率和作物种植量,但可能会出现因为管道和植物的挡光而减少后墙蓄热、降低冬季温室温度的问题。为此,通过冬季连续31 d的温度监测,在3种典型气象(晴天、阴天、雪天)条件下,对比分析了有后墙立体基质栽培的日光温室(solar greenhouse with equipment,ESG)和无后墙立体栽培的日光温室(solar greenhouse with no equipment,NSG)温度环境的变化。监测结果表明,ESG的月平均气温较NSG高0.84℃,其中最大日温差为2.22℃,最小日温差为0.14℃。晴天条件下,ESG的日平均冠层温度和1.5 m高度处的空气温度分别是12.72和13.04℃,NSG分别是10.68和11.04℃;ESG的冠层温度最低值是4.68℃,而NSG最低值是4.10℃。可见,ESG较NSG的气温要略高一些;阴天和雪天条件下,2种温室内的温度环境无显著差别。因此,利用日光温室后墙进行立体基质栽培草莓,不但没有降低反而提高了冬季温室内的温度,是一种可行、值得推广应用的温室高效栽培技术。 相似文献
112.
太阳能与发电余热复合沼气增温系统设计 总被引:2,自引:2,他引:2
为了实现大中型沼气工程在北方寒冷地区的应用推广。该文针对北方寒冷气候特点,以哈尔滨双城市沼气工程为例构建了1套太阳能-发电余热中温厌氧发酵增温系统,阐述了该增温系统的设计原理,并对沼气发酵热负荷、发电机组余热回收利用率、太阳能集热装置热效率等关键参数进行了理论计算。计算得出该沼气工程全年平均每日热量的损失为6659.2 MJ,太阳能-发电余热中温厌氧发酵增温系统全年平均每日集热量为7017.6 MJ。通过对增温效果与该工程的热量损失进行对比,表明12、1、2月份系统需沼气发电机组额外提供372.2、369.4、208.3 kWh电量增温,其余月份系统可以实现发酵工程每日热量损失的补充,保证该工程的稳定运行。在8月份对示范工程的一次发酵罐体进行了增温效果测试,表明该增温系统在28 d左右可将该发酵罐内物料温度提升到中温发酵水平。该文为北方寒冷地区大中型沼气增温保温设备的配套建设提供参考。 相似文献
113.
基于植被覆盖率的农村居民点整理潜力估算及实证 总被引:1,自引:4,他引:1
为合理估算农村居民点整理潜力,该文提出基于植被覆盖率的农村居民点整理潜力测算新思路。借助植被覆盖率对农村居民点斑块进行分类,通过分析抽样农村居民点斑块的植被覆盖与内部用地结构得到整理潜力修正系数,从而建立基于植被覆盖率修正的农村居民点整理潜力估算模型,并以北京市门头沟区为例展开实证研究。研究结果表明,门头沟区无整理潜力的农村居民点占大多数,具有整理潜力的农村居民点主要集中在经济相对落后的乡镇,并且这部分农村居民点多远离主干道;山区具有中、高整理潜力的农村居民点略多于平原、浅山区,而低整理潜力的农村居民点明显多于平原、浅山区;门头沟区规划整理的农村居民点内部包含有大量农用地,其中建设用地比例只占63.19%~71.54%,但是,第二次土地调查将整个农村居民点都作为建设用地调绘;因而,一些不考虑植被覆盖修正的农村居民点整理潜力测算方法夸大了整理潜力,放大了城乡建设用地增减挂钩的效果。该文为进一步合理估算农村居民点整理理论潜力提供参考。 相似文献
114.
Yaying Li Juan Wang Fuxiao Pan Stephen James Chapman Huaiying Yao 《Journal of Soils and Sediments》2016,16(5):1472-1480
Purpose
Soil microorganisms are important in the cycling of plant nutrients. Soil microbial biomass, community structure, and activity are mainly affected by carbon substrate and nutrient availability. The objective was to test if both the overall soil microbial community structure and the community-utilizing plant-derived carbon entering the soil as rhizodeposition were affected by soil carbon (C) and nitrogen (N) availability.Materials and methods
A 13C-CO2 steady-state labeling experiment was conducted in a ryegrass system. Four soil treatments were established: control, amendment with carboxymethyl cellulose (CMC), amendment with ammonium nitrate (NF), combined CMC and NF. Soil phospholipid fatty acid (PLFA) and 13C labeling PLFA were extracted and detected by isotope ratio mass spectrometer.Results and discussion
The combined CMC and NF treatment with appropriate C/N ratio (20) significantly enhanced soil microbial biomass C and N, but resulted in lower soil inorganic N concentrations. There was no significant difference in soil PLFA profile pattern between different treatments. In contrast, most of the 13C was distributed into PLFAs 18:2ω6,9c, 18:1ω7c, and 18:1ω9c, indicative of fungi and gram-negative bacteria. The inorganic-only treatment was distinct in 13C PLFA pattern from the other treatments in the first period of labeling. Factor loadings of individual PLFAs confirmed that gram-positive bacteria had relatively greater plant-derived C contents in the inorganic-only treatment, but fungi were more enriched in the other treatments.Conclusions
Amendments with CMC can improve N transformation processes, and the ryegrass rhizodeposition carbon flux into the soil microbial community is strongly modified by soil N availability.115.
Yong Wang Shuqing Gao Cuilan Li Jinjing Zhang Lichun Wang 《Journal of Soils and Sediments》2016,16(7):1849-1857
Purpose
Under a global warming scenario, understanding the response of soil organic carbon fractions and aggregate stability to temperature increases is important not only for better understanding and maintaining relevant ecosystem services like soil fertility and crop productivity, but also for understanding key environmental processes intimately related with the maintenance of other regulatory ecosystem services like global climate change mitigation through carbon sequestration. An increase in temperature would accelerate the mineralization of soil organic carbon. However, the properties of organic carbon remained in soil after mineralization is not well known.Materials and methods
Mollisol was collected at 0–20-cm depth from maize (Zea mays L.) field in Northeast China. A 180-day incubation experiment was conducted at three different temperatures (10, 30, and 50 °C) under constant soil moisture (60 % water holding capacity). Soil samples were assayed for total organic carbon (TOC), water-soluble organic carbon (WSOC), easily oxidizable organic carbon (EOC), humic fractions carbon, aggregate-associated carbon, and water stability of aggregates. Elemental analysis and solid-state 13C nuclear magnetic resonance spectroscopy were used to characterize humic acid and humin fractions.Results and discussion
The contents of soil TOC, EOC, humic fractions carbon, and aggregate-associated carbon decreased with the increase in temperature. The proportion of 2–0.25-mm macroaggregate and the mean weight diameter (MWD) of aggregates also decreased. The C, H, N, S, alkyl C, and O-alkyl C contents of humic acid and humin decreased, whereas the O, aromatic C, and carbonyl C contents increased. The H/C, aliphatic C/aromatic C, and O-alkyl C/aromatic C ratios in humic acid and humin fractions decreased.Conclusions
The increase in temperature has a negative impact on soil organic carbon content, soil aggregation, and aggregate stability. Moreover, humic acid and humin molecules become less aliphatic and more decomposed with the increase in temperature.116.
沼肥替代化肥对设施蔬菜产量和土壤养分及重金属累积的影响 总被引:5,自引:0,他引:5
针对盲目施用沼肥可能带来的土壤环境问题,该研究采用5年8茬的设施番茄-甜椒田间轮作试验,研究不同沼肥和化肥配比对蔬菜产量、设施土壤氮磷养分及重金属累积的影响.结果表明:在等氮钾养分投入下,沼肥和化肥以不同比例配施的各处理没有影响蔬菜产量;氮素盈余和0~180cm土层土壤全氮含量随沼肥施用比例增加没有显著差异;磷素盈余随沼肥施用比例增加显著增加,完全沼肥处理0~30cm土层土壤Olsen-P及CaCl2-P含量分别达到151和8.0mg/kg,高于其他2个处理,且明显超出环境阈值.与完全沼肥处理相比,减量沼肥施用(3/5沼肥处理)明显降低了土壤氮素淋洗风险和磷素累积.与不施肥处理相比,施用沼肥后30~60、60~90、150~180cm土层土壤全Hg含量均低于不施肥处理;除30~60cm土层使用沼肥处理土壤中全As含量显著高于不施肥处理,其他土层各处理间均没有显著差异,而完全沼肥处理全Cr、全Cd、全Pb含量有所下降,且全Pb含量下降最为显著,但均没有出现重金属污染风险.沼肥替代化肥施用可有效实现废弃物中的养分资源循环利用,但增加菜田土壤磷素累积和淋失风险,本研究为设施菜田合理施用沼肥提供技术支持,为实现沼肥资源的循环利用和化肥替代模式提供理论参考. 相似文献
117.
为深入了解添加含氧燃料和废气再循环技术(exhaust gas recirculation,EGR)共同作用对颗粒结构及分形特征的影响,运用颗粒分级采样装置采集了EGR率分别为0、10%、20%时,柴油机燃用正丁醇质量百分比为10%的正丁醇/柴油混合燃料(N10)的燃烧颗粒(N10EGR0、N10EGR10%、N10EGR20%),通过电镜试验和图像处理技术,开展了不同EGR率对正丁醇/柴油燃烧颗粒的影响研究,分析了颗粒群的微观结构、平均粒径、分形维数,基本碳粒子的层面间距、微晶尺寸等物理结构参数的变化规律,结果表明,N10EGR0、N10EGR10%、N10EGR20%燃烧颗粒群整体呈现团簇状结构,颗粒粒径范围主要集中在30~70 nm之间,呈正态单峰分布;随着EGR率的增加,粒径范围向大粒径方向移动,平均粒径逐渐增大,N10EGR20%与N10EGR0相比,平均粒径增大约为19%;计盒维数逐渐减小,表明颗粒间的团簇程度逐渐减弱。不同EGR条件下的基本碳粒子结构相似,呈指纹状球形碳层结构,随EGR率的增加,基本碳粒子的内核碳层排列无序性和外壳石墨晶体结构无规则性增强,平均层面间距和弯曲度逐渐增大,微晶尺寸逐渐减小。该研究为含氧燃料与EGR共同作用对颗粒的形成机理以及降低柴油机颗粒排放的措施提供理论参考。 相似文献
118.
Mario Augusto Pagnotta Linda Mondini Pasquale Codianni Clara Fares 《Genetic Resources and Crop Evolution》2009,56(3):299-310
Emmer wheat (Triticum dicoccon Schrank, 2n = 4x = 28) consists in a hulled wheat; its cultivation has been drastically reduced during the last century as a consequence of
its low yield. Recently, its agronomic and nutritive values, as well as the increase of popularity of organic agriculture,
have led to a renewed interest making its cultivation economically viable in the marginal lands with an increase of the cultivated
areas. In Italy, it mainly survives in few marginal lands of central and southern Italy, where local varieties, adapted to
the natural environment from where they originate, are used; moreover, some selected lines have also been developed. In the
present work, agro-morphological and qualitative traits, together with molecular analyses of 20 emmer accessions consisting
of Italian landraces, breeding lines, and cultivars, were performed. The field experiments were conducted for two consecutive
years (2001/2002–2002/2003) in two locations: Viterbo in central Italy, and Foggia in south Italy. The analyzed emmer wheat
accessions showed a good amount of genetic variability for both evaluated agro-morphological and molecular traits. This study
illustrates an increase in earliness, GY, TW, TKW, and YI going from landraces, breeding lines to cultivars, while the variability
does not change proportionally. 相似文献
119.
Nahla Bassil Kim E. Hummer Joseph D. Postman Gennaro Fazio Angela Baldo Isabel Armas Roger Williams 《Genetic Resources and Crop Evolution》2009,56(3):339-352
Heritage apple (Malus domestica Borkh. hybrids) and pear (Pyrus communis L. hybrid) trees grow in villages throughout Terceira Island, Azores, Portugal. Some of these pears have different names but similar morphology. The objective of this study was to determine synonymy, homology, and phylogeny of apples and pears from Terceira and to examine potential relationships of the island pears with standard apples and pears of Portuguese or American descent. Nine apple microsatellite markers were used to determine genetic relationships. Distance- and parsimony-based cluster analysis grouped these genotypes into separate apple and pear clades. The Terceira apples were divided into two clades: the maçā and the reineta-reinette. Among the 17 heritage apple genotypes, seven unique accessions were identified and four groups of synonyms, or possibly clones, were detected including: ‘Reineta Agosto’ and ‘Reineta Verde’ from Altares; ‘Reineta Castanha’ and ‘Reineta Verde Miuda’; ‘Maçā Pêra,’ ‘Maçā Calhau’, ‘Pêro Branco’ from Salga and from Terra-Chā and ‘Maçā Marmelo’; and the five genotypes ‘Maçā Sao Joao’, ‘Malápio Rosa’, ‘Maçā Gaspar’, ‘Maçā Branca’ and ‘Maçā Pato’. In addition, two homonyms were detected. ‘Pêro Vermelho’ from Terra Chā was a separate genotype from a tree from Doze Ribeiras of the same name, but Pêro Branco from Terra Chā appears to be a clone that can be distinguished by an additional allele at CH1F07a from a tree with that name from Salga. One pair of apple clones, ‘Reineta Agosto’ and ‘Reineta Verde’ from Altares appear to be derived from an unreduced gamete of ‘Golden Delicious.’ Another apple genotype ‘Maçā Acida’ could be a sibling of the ‘Maçā Pêra’ clonal group. Other tested standard apples from the US genebank were unrelated to Terceira genotypes. Of the seven heritage pears, five unique genotypes and one pair of synonyms were detected. ‘Pêra Papo Pintassilgo’ from Raminho and ‘Pêra Vermelha’ from the nursery of Serviço de Desenvolvimento Agario da Terceira (SDAT) were synonyms. ‘Passans du Portugal’ was related to ‘Pêra Cabaca’ but other standard pears from the US genebank were unrelated to Terceira genotypes. Future studies will include additional apple and pear cultivars from other Islands of the Azores and continental Portugal, and wild Asian species to further explore genetic relationships. 相似文献
120.
An understanding of the effects of salinity and sodicity on soil carbon (C) stocks and fluxes is critical in environmental
management, as the areal extents of salinity and sodicity are predicted to increase. The effects of salinity and sodicity
on the soil microbial biomass (SMB) and soil respiration were assessed over 12weeks under controlled conditions by subjecting
disturbed soil samples from a vegetated soil profile to leaching with one of six salt solutions; a combination of low-salinity
(0.5dSm−1), mid-salinity (10dSm−1), or high-salinity (30dSm−1), with either low-sodicity (sodium adsorption ratio, SAR, 1), or high-sodicity (SAR 30) to give six treatments: control (low-salinity
low-sodicity); low-salinity high-sodicity; mid-salinity low-sodicity; mid-salinity high-sodicity; high-salinity low-sodicity;
and high-salinity high-sodicity. Soil respiration rate was highest (56–80mg CO2-C kg−1 soil) in the low-salinity treatments and lowest (1–5mg CO2-C kg−1 soil) in the mid-salinity treatments, while the SMB was highest in the high-salinity treatments (459–565mg kg−1 soil) and lowest in the low-salinity treatments (158–172mg kg−1 soil). This was attributed to increased substrate availability with high salt concentrations through either increased dispersion
of soil aggregates or dissolution or hydrolysis of soil organic matter, which may offset some of the stresses placed on the
microbial population from high salt concentrations. The apparent disparity in trends in respiration and the SMB may be due
to an induced shift in the microbial population, from one dominated by more active microorganisms to one dominated by less
active microorganisms. 相似文献