全文获取类型
收费全文 | 1169篇 |
免费 | 47篇 |
国内免费 | 3篇 |
专业分类
林业 | 113篇 |
农学 | 18篇 |
基础科学 | 12篇 |
259篇 | |
综合类 | 131篇 |
农作物 | 110篇 |
水产渔业 | 34篇 |
畜牧兽医 | 409篇 |
园艺 | 28篇 |
植物保护 | 105篇 |
出版年
2022年 | 13篇 |
2021年 | 11篇 |
2020年 | 19篇 |
2019年 | 24篇 |
2018年 | 26篇 |
2017年 | 22篇 |
2016年 | 33篇 |
2015年 | 19篇 |
2014年 | 30篇 |
2013年 | 56篇 |
2012年 | 49篇 |
2011年 | 58篇 |
2010年 | 33篇 |
2009年 | 41篇 |
2008年 | 45篇 |
2007年 | 49篇 |
2006年 | 61篇 |
2005年 | 71篇 |
2004年 | 43篇 |
2003年 | 54篇 |
2002年 | 49篇 |
2001年 | 13篇 |
2000年 | 11篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1997年 | 19篇 |
1996年 | 19篇 |
1995年 | 15篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 18篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 11篇 |
1972年 | 7篇 |
1971年 | 8篇 |
1926年 | 11篇 |
1925年 | 10篇 |
1924年 | 7篇 |
排序方式: 共有1219条查询结果,搜索用时 0 毫秒
91.
Quantifying the most important mechanisms of acid buffering in carbonate-free soils From the release of cations from the soil matrix during pH-stat-titrations, characteristic values for assessing buffer rates were derived separately for single elements. Recorded buffer reactions became almost exclusively apparent in the release of Al and Fe, however the contribution of Fe to the total turnover only amounted to 1–3%. The release of each element and of DOC in the course of time was due to the overlapping of two first-order reactions, respectively. In the fast initial reaction all measured elements were released at nearly the same velocity, whereas in the 2nd, slower reaction the rate of Al-release corresponded to that of DOC but was at least seven times higher than that of Fe. Correlations between the maxima of release and soil chemical properties revealed a dominating contribution of organically bound Al (Alp) to proton buffering. It is assumed that also the amount of Fe released is originating in organic complexes. 相似文献
92.
We explored the extent of interspecific hybridisation between alien and native plant species in Germany with a special focus on the potential threat for native biodiversity. In total we listed 134 hybrids which are interpreted as products of hybridisation between 81 alien and 109 native plant species (including 13 archeophytes) that occur in Germany. Seventy-five of these hybrids have been recorded in Germany, while the remaining 59 hybrids have not been detected in Germany yet, although both parental species currently occur in Germany. Interspecific hybridisation between abundant alien and rare native species can threaten populations of the native species through outbreeding depression and/or through high rates of gene flow swamping native populations. We identified 37 threatened native plant species which hybridise with aliens. Seventeen of these threatened plant species may suffer from outbreeding depression when hybridising with a more abundant alien invader (minority disadvantage). Using hybrid abundance as an indicator of hybrid fitness we argue that introgression of alien genes may affect the gene pool of eight threatened native plant species. Consequently, hybridisation with aliens has to be considered as an additional risk potentially leading to a loss of biodiversity and should be included in the repertoire of causes for rare species extinction in German Red Lists of threatened plant species. 相似文献
93.
Two plant species, tomato (Lycopersicon esculentum Mill.) and bitter gourd (Momordica charantia), were used for in‐depth studies on the dynamics of silicon (Si) uptake and translocation to the shoots and compartmentation of Si in the roots. The experiments were conducted under controlled environmental conditions in nutrient solutions, which were partly amended with 1 mM Si in the form of silicic acid. At harvest, xylem exudates were collected, and Si concentrations and biomass of roots and shoots were determined. Mass flow of Si was calculated based on the Si concentration of the nutrient solution and transpiration determined in a parallel experiment. Plant roots were subjected to a fractionated Si analysis, allowing attributing Si to different root compartments. Silicon concentrations in the roots compared to the shoots were higher in tomato but lower in bitter gourd. A more ready translocation from the roots to the shoots in bitter gourd was in agreement with Si concentrations in the xylem exudates which were higher than in the external solution. In tomato, the xylem‐sap Si concentration was lower than in the nutrient solution. Calculated Si mass flow to the root exceeded Si uptake in tomato, which was consistent with the measured accumulation of Si in the root water‐free space (WFS). In contrast, Si concentration in the root WFS was lower than in the nutrient solution in bitter gourd, reflecting the calculated Si depletion at the root surface based on the comparison of Si mass flow and Si uptake. Within the roots, more than 80% of the total Si was located in the cell wall and less than 10% in the cytoplasmic fractions in tomato. In bitter gourd, between 60% and 70% of the total root Si was attributed to the cell‐wall fraction whereas the proportion of the cytoplasmic fraction reached more than 30%. Our results clearly confirm that tomato belongs to the Si excluders and bitter gourd to the Si‐accumulator plant species for which high Si concentrations in the cytoplasmic root fraction appear to be characteristic. 相似文献
94.
Walter H PiperMargaret Klich Keren B TischlerAmy Dolsen 《Biological conservation》2002,104(2):199-203
Many southern populations of the common loon (Gavia immer) face threats from lead and methylmercury contamination, lake acidification, shoreline development and human recreation. It is now clear that the task of conserving loon populations will depend upon mitigating these varied threats. In a controlled experiment, we examined the efficacy of using floating nest platforms to increase reproductive success of loons and thus help sustain local populations. Platforms were attractive nesting sites both on lakes that had consistently hatched chicks from natural sites and on lakes where chick production had been sporadic. When compared to natural nest sites, platforms increased hatching success by 69% and fledging success by 32%, apparently through reduction in mammalian egg predation. A well-managed effort to introduce nesting platforms might be a viable strategy to help sustain threatened populations. 相似文献
95.
Sewram V Mshicileli N Shephard GS Vismer HF Rheeder JP Lee YW Leslie JF Marasas WF 《Journal of agricultural and food chemistry》2005,53(12):4861-4866
Six strains of Fusarium verticillioides, two of F. oxysporum, one strain of F. proliferatum, and a strain of an unidentified species were cultured on maize patties and rice and evaluated for their ability to simultaneously produce fumonisin B (FB) and C (FC) series analogues. Fumonisins were quantified by LC-MS-MS using positive ion electrospray ionization. FC1 provided characteristic fragment ions at m/z 690, 672, 654, 532, 514, and 338 corresponding to sequential loss of H2O and tricarboxylic acid moieties from the alkyl backbone, while FC3 and FC4 provided equivalent product ions 16 and 32 amu lower than the corresponding FC1 fragments, respectively. All isolates cultured on maize produced FC4. All isolates except for that of F. proliferatum also produced FC1, and three of the six strains of F. verticillioides produced FC3. All isolates except those of F. oxysporum produced detectable amounts of FB1, FB2, and FB3. Isolates that produced fumonisin B analogues produced at least 10 fold more of the B series analogues than they did of the C series analogues. The results confirm that at least some strains of F. oxysporum produce FC, but not FB, fumonisin analogues and also suggest that the genetics and physiological regulation of fumonisin production may be more complicated than previously envisaged since some strains of F. verticillioides and F. proliferatum as well as the strain of the unidentified species can simultaneously produce both FB and FC analogues. 相似文献
96.
Hu J Zheng YL Hyde W Hendrich S Murphy PA 《Journal of agricultural and food chemistry》2004,52(9):2689-2696
The metabolism of soyasaponin I (3-O-[alpha-L-rhamnopyranosyl-beta-D-galactopyranosyl-beta-D-glucuronopyranosyl]olean-12-ene-3beta,22beta,24-triol) by human fecal microorganisms was investigated. Fresh feces were collected from 15 healthy women and incubated anaerobically with 10 mmol soyasaponin I/g feces at 37 degrees C for 48 h. The disappearance of soyasaponin I in this in vitro fermentation system displayed apparent first-order rate loss kinetics. Two distinct soyasaponin I degradation phenotypes were observed among the subjects: rapid soyasaponin degraders with a rate constant k = 0.24 +/- 0.04 h(-)(1) and slow degraders with a k = 0.07 +/- 0.02 h(-)(1). There were no significant differences in the body mass index, fecal moisture, gut transit time, and soy consumption frequency between the two soyasaponin degradation phenotypes. Two primary gut microbial metabolites of soyasaponin I were identified as soyasaponin III (3-O-[beta-D-galactopyranosyl-beta-D-glucuronopyranosyl]olean-12-ene-3beta,22beta,24-triol) and soyasapogenol B (olean-12-ene-3beta,22beta,24-triol) by NMR and electrospray ionized mass spectroscopy. Soyasaponin III appeared within the first 24 h and disappeared by 48 h. Soyasapogenol B seemed to be the final metabolic product during the 48 h anaerobic incubation. These results indicate that dietary soyasaponins can be metabolized by human gut microorganisms. The sugar moieties of soyasaponins seem to be hydrolyzed sequentially to yield smaller and more hydrophobic metabolites. 相似文献
97.
In an attempt to evaluate whether breeding and selection for high yielding capacity changed the P requirement of modem wheat cultivars. the response of two wheat cultivars to different levels of P supply was investigated. A traditional cultivar (‘Peragis’) and a modern spring-wheat cultivar (‘Cosir’) were cultivated in a C-loess low in available P and high in CaCO3 in 120 cm high PVC tubes. In addition and for comparison, nutrient solution experiments were also conducted. Shoot growth, root growth. P uptake. P translocation and P distribution within the shoot at different developmental stages were compared. The grain yield of the modern cultivar ‘Cosir’ was higher at limiting and non-limiting P supply and. therefore, this cultivar can be considered as more P-efficient than the traditional cultivar. Grain yield reduction at low P supply was mainly due to an inhibition of tillering and thus lower number of ears per plant, whereas the number of grains per ear was hardly affected. Reduced tillering at low P supply could not be related to P concentrations in the shoot meristematic tissues which were generally much higher than in other plant tissues and kept at an elevated level even at limiting P supply. Root branching (1st order laterals) was reduced at limiting P supply in ‘Cosir’ but not in ‘Peragis’ which, generally, had lower numbers of laterals at the beginning of tillering. From the results it can be concluded that the main factors contributing to the higher P efficiency of the modern cultivar ‘Cosir’ are (i) efficient use of assimilates for root-growth characteristics which enhance P acquisition: enhanced root branching and thus smaller mean root diameter and longer root hairs, (ii) an efficient P uptake system, (iii) efficient remobilization of P from vegetative plant organs to the grains, and most importantly (iv) lower P requirement for grain yield formation because of lower ear number per plant but higher grain number per ear. 相似文献
98.
Restriction fragment length polymorphisms of the rRNA genes were investigated among 20 genotypes ofP. vulgaris, P. coccineus, P. polyanthus, P. microcarpus, P. glabellus, P. acutifolius, P. maculatus, P. oligospermus andP. lunatus. Detection of the polymorphisms was performed nonradioactively with a digoxigenin-labeled rDNA probe. RFLP-based phylogenetic trees for the rDNA of the species studied were computed with several distance matrix and parsimony methods. The estimated molecular relationships within the genusPhaseolus coincide, on principle, with the classical taxonomical investigations. Hence, RFLP analyses of the rRDA have been proven useful for systematic studies in the genusPhaseolus. 相似文献
99.
Walter W. Piegorsch Kelly A. Richwine 《Journal of Agricultural, Biological & Environmental Statistics》2001,6(3):305-325
In studies examining patterns of mutational damage, the primary variables of interest are expressed typically as discrete counts within defined categories of damage. The multinomial distribution is a common model for such data settings. Of interest is statistical comparison of the pattern, or spectrum, of mutation among the various mutant categories. A specific question in such instances is the identification of differences among the spectral categories, identified, e.g., via simultaneous confidence intervals for pairwise differences in or ratios of the multinomial response probabilities. Herein, a selection of possible methods for constructing such intervals is described and compared via computer simulations to determine which are most appropriate for practical use in the evaluation of mutant spectra. 相似文献
100.
Walter R. Fischer 《植物养料与土壤学杂志》1990,153(2):93-95
The interpretation of K/Ca exchange isotherms by a simple GAPON equation K/Ca exchange isotherms of various soils can be described by a GAPON equation, where the coefficients being fitted to the measured values by iteration. The resulting values for the “labile potassium” are very similar to those of exchangeable K, whereas only 5–25% of the cec is involved in the short-term K/Ca exchange. 相似文献