全文获取类型
收费全文 | 6146篇 |
免费 | 390篇 |
国内免费 | 206篇 |
专业分类
林业 | 335篇 |
农学 | 443篇 |
基础科学 | 70篇 |
1104篇 | |
综合类 | 821篇 |
农作物 | 422篇 |
水产渔业 | 434篇 |
畜牧兽医 | 2396篇 |
园艺 | 142篇 |
植物保护 | 575篇 |
出版年
2023年 | 41篇 |
2022年 | 66篇 |
2021年 | 127篇 |
2020年 | 116篇 |
2019年 | 136篇 |
2018年 | 169篇 |
2017年 | 181篇 |
2016年 | 184篇 |
2015年 | 146篇 |
2014年 | 199篇 |
2013年 | 284篇 |
2012年 | 339篇 |
2011年 | 369篇 |
2010年 | 281篇 |
2009年 | 256篇 |
2008年 | 334篇 |
2007年 | 322篇 |
2006年 | 263篇 |
2005年 | 251篇 |
2004年 | 226篇 |
2003年 | 281篇 |
2002年 | 344篇 |
2001年 | 326篇 |
2000年 | 191篇 |
1999年 | 152篇 |
1998年 | 43篇 |
1997年 | 44篇 |
1996年 | 36篇 |
1995年 | 46篇 |
1994年 | 45篇 |
1993年 | 38篇 |
1992年 | 47篇 |
1991年 | 42篇 |
1990年 | 54篇 |
1989年 | 54篇 |
1988年 | 34篇 |
1987年 | 39篇 |
1986年 | 31篇 |
1985年 | 34篇 |
1984年 | 53篇 |
1983年 | 27篇 |
1982年 | 27篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 28篇 |
1977年 | 28篇 |
1975年 | 25篇 |
1974年 | 39篇 |
1972年 | 31篇 |
1971年 | 37篇 |
排序方式: 共有6742条查询结果,搜索用时 15 毫秒
151.
F. De Coninck 《Geoderma》1980,24(2):101-128
Current explanations of the formation of spodic horizons do not accomodate all features of the horizons in their natural state. In this paper, a more complete explanation of major mechanisms is proposed, using two principles of colloid chemistry: (1) organic substances may form hydrophylic colloids with surface charges, and (2) the hydrophylic character and negative surface charges determine the dispersibility of the colloids. The hydrophylic character is due to the presence of hydrophylic radicals as parts of the organic compounds in soils. The surface charges are the result of dissociation of -COOH and possibly phenol-OH radicals.The neutralization of the surface charge can in principle occur: (1) through electrostatic or physical adsorption and (2) through chemisorption. The first case is typical for monovalent alkali cations. The adsorbed cations are distributed in a double layer, which favours dispersion. Chemisorption occurs mostly with polyvalent cations. This process corresponds in reality to the formation of organo-metallic compounds. It results in a relatively complete disappearance of the double layer and in the formation of large immobile “polymerized” organo-metallic compounds. Because these compounds contain much hydrophylic water, they form a gel. Transition into the solid state is accompanied by the loss of most of the hydration water. The dehydration may be induced by a decrease in thickness of the double layer. At a certain stage of the dehydration process, Van der Waals bonds and protonic bridges can form and bring about a certain degree of hydrophoby.In soils, mobile organic substances are formed during breakdown of plant remains. If at the top of the mineral soil enough polyvalent cations, especially Al and Fe, are available, the mobile organic substances formed are immobilized immediately and no migration occurs. In case insufficient amounts of Al and/or Fe are available to completely immobilize the mobile compounds, these cations are complexed by the mobile compounds and transported downward. Immobilization may occur at some depth through supplementary fixation of cations, through dessication or on arrival at a level with different ionic concentration.In nature, spodic horizons range from loose, with many roots, to very cemented with few roots. These differences can be related to changes in microstructure. Loose spodic horizons have a predominance of polymorphic pellets and aggregates, whereas organans or monomorphic coatings prevail in cemented horizons. The former horizons have many features suggesting major biological influences during their formation, viz., high numbers of roots, thorough mixing of the organic units with clay and silt, the presence of pedotubules and relatively young mean residence times. The latter horizons have features consistent with organo-metallic compounds immobilized in a gel-state, viz., the coatings are strongly cracked, indicating the transition of a gel into a solid; they contain much Al or Al plus Fe but very little or no Si, and the mean residence time is considerably higher than in loose horizons.The two processes seem to operate simultaneously during the formation of spodic horizons and their relative intensities determine the composition of each spodic horizon at any moment in its evolution. As long as the biological activity predominates, the horizon remains loose; if the accumulation of mobile organo-metallic compounds starts to prevail, the horizon is gradually cemented and fossilized. 相似文献
152.
153.
盐碱胁迫下绒毛白蜡种子的萌发特性 总被引:5,自引:0,他引:5
将中性盐NaC l,Na2SO4和2种碱性盐NaHCO3,Na2CO3按不同比例混合,模拟出20种盐度、碱度各不相同的复杂盐碱逆境条件,进行绒毛白蜡种子发芽试验。测定其发芽率等6项指标,结果表明:在混合盐碱的胁迫作用下,绒毛白蜡种子的发芽受到明显的抑制,发芽率、发芽势、发芽指数、活力指数等指标呈下降趋势,并随盐碱度递增,下降趋势变大,高盐高碱的共同作用结果远远大于单纯的高盐或高碱,SOD的活性总体上有不同程度的增加,CAT在不同处理组间均呈现上升的趋势,在50,100mmol/L的组内呈上升趋势,但在200,300mmol/L组内则是先升后降。可见盐胁迫与碱胁迫是2种完全不同的胁迫,应将碱性盐胁迫称为碱胁迫,而通常所说的盐胁迫应仅指中性盐胁迫,而且两者具有明显的协同作用。 相似文献
154.
155.
156.
Immunostimulatory effects of dietary poly‐β‐hydroxybutyrate in European sea bass postlarvae 下载免费PDF全文
Andrea Franke Catriona Clemmesen Peter De Schryver Linsey Garcia‐Gonzalez Joanna J Miest Olivia Roth 《Aquaculture Research》2017,48(12):5707-5717
The stable production of high‐quality fry in marine aquaculture is still hampered by unpredictable mortality caused by infectious diseases during larval rearing. Consequently, the development of new biocontrol agents is crucial for a viable aquaculture industry. The bacterial energy storage compound poly‐β‐hydroxybutyrate (PHB) has been shown to exhibit beneficial properties on aquatic organisms such as enhanced survival, growth, disease resistance and a controlling effect on the gastrointestinal microbiota. However, the effect of PHB on the developing immune system of fish larvae has so far not been investigated. In this study, the effect of feeding PHB‐enriched Artemia nauplii on survival, growth and immune response in European sea bass (Dicentrarchus labrax) postlarvae was examined. Amorphous PHB was administered to 28‐day‐old sea bass postlarvae over a period of 10 days. The survival and growth performance were monitored, and the expression of 29 genes involved in immunity, growth, metabolism and stress‐response was measured. While the expression of the insulin‐like growth factor 1 (igf1), an indicator of relative growth, was upregulated in response to feeding PHB, the larval survival and growth performance remained unaffected. After 10 days of PHB treatment, the expression of the antimicrobial peptides dicentracin (dic) and hepcidin (hep) as well as mhc class IIa and mhc class IIb was elevated in the PHB fed postlarvae. This indicates that PHB is capable of stimulating the immune system of fish early life stages, which may be the cause of the increased resistance to diseases and robustness observed in previous studies. 相似文献
157.
S.H.S. Dananjaya H.P.S.U. Chandrarathna L.A.U. Nayanaransi M. Edussuriya A.S. Dissanayake Ilson Whang Mahanama De Zoysa 《Aquaculture Research》2020,51(8):3109-3118
Pectin is a biodegradable polysaccharide, and it has been recently applied as a gene delivery, drug delivery, wound healing and tissue engineering agent. In this study, pectin was extracted from pomelo (Citrus maxima) peel and characterized. The extraction recovery of pectin form pomelo peel was 14.5%, and it had 72.56% degree of esterification, 1,245.56 equivalent weight, 7.82% methoxyl and 68.27% anhydrouronic acid contents. Use of pomelo pectin as a hatching enhancing agent for fish embryos and its effect on hatching enzyme 1 (ZHE1) was investigated. The pectin‐exposed zebrafish embryos (100 µg/ml) showed significantly (p < .05) higher hatching rate (96.6%) compared with untreated (control) embryos (66.6%) at 60 hpf. The mRNA expression of ZHE1 was also significantly (p < .05) elevated up to 55.6‐fold in pectin‐exposed embryos at 24 hpf. In situ hybridization results revealed remarkably strong expression of ZHE1 in pectin‐exposed embryos compared with the control group. In addition, considerably larger size of the hatching gland was observed in pectin‐exposed larvae than that of the unexposed larvae group. These results clearly indicate that pectin isolated from pomelo peel has an ability to enhance the hatching process of zebrafish embryos via upregulation of ZHE1. 相似文献
158.
Three approaches for multivariate analysis of fish growth in aquaculture experiments with Nile tilapia (Oreochromis niloticus niloticus L.) based on the von Bertalanffy growth curve are presented and compared. The approaches are: an extended Gulland‐and‐Holt (GH) plot, a forced extended GH plot and a multilinear regression analysis for the growth parameter K. All three models provide valuable insight into the major environmental factors influencing the daily growth rate and explain 28–46% of the variance of the observed daily growth rate of the used data set. For all three methods, the modelled parameter is significantly related to the net yield of Nile tilapia and can, therefore, be used for the predictive modelling of management scenarios. The extended GH plot loads the influence of environmental parameters upon L∞, while the forced extended GH plot and Direct fitting of K load the influence on the growth parameter K. The latter is more in the tradition of aquaculture research. But the forced extended GH plot and Direct fitting of K can only be applied if L∞ of the cultured species is known, as the selected L∞ influences the variance in the regression variables. 相似文献
159.
160.
Large‐scale inoculation with arbuscular mycorrhizal fungi (AMF) is generally impractical in most regions and we have little understanding of the factors that determine inoculation success. Nevertheless, the ability to take full advantage of indigenous AMF for sustainable production needs to be developed within cropping systems. We used part of a long‐term field experiment to understand the influence of tillage and the preceding crop on AMF colonization over the growing season. Arbuscular mycorrhiza colonization rate was more affected by treatment (tillage or the combination of crop and preceding crop) than by the total number of AMF spores in the soil. Conventional tillage (CT) had a statistically significant negative effect (P ≤ 0.05) on spore numbers isolated from the soil, but only in the first year of study. However, the AMF colonization rate was significantly reduced by CT, and the roots of wheat, Triticum aestivum, L, cv. Coa after sunflower, Helianthus annuus L., were less well colonized than were those of triticale, X Triticosecale Wittmack, cv. Alter after wheat, but the affect of tillage was more pronounced than was the effect of crop combination. Under no‐till there was a significant increase in AMF colonization rate throughout the sampling period in both wheat and triticale, indicating that the extraradical mycelium previously produced acted as a source of inoculum. In general, triticale showed greater AMF colonization than wheat, despite the preceding crop being less mycotrophic. Under these experimental conditions, typical of Mediterranean agricultural systems, AMF colonization responded more strongly to tillage practices than to the combination of crop and preceding crop. 相似文献