全文获取类型
收费全文 | 2884篇 |
免费 | 194篇 |
国内免费 | 4篇 |
专业分类
林业 | 198篇 |
农学 | 70篇 |
基础科学 | 22篇 |
486篇 | |
综合类 | 701篇 |
农作物 | 62篇 |
水产渔业 | 241篇 |
畜牧兽医 | 1018篇 |
园艺 | 90篇 |
植物保护 | 194篇 |
出版年
2023年 | 20篇 |
2022年 | 23篇 |
2021年 | 66篇 |
2020年 | 77篇 |
2019年 | 91篇 |
2018年 | 81篇 |
2017年 | 77篇 |
2016年 | 89篇 |
2015年 | 71篇 |
2014年 | 79篇 |
2013年 | 108篇 |
2012年 | 192篇 |
2011年 | 225篇 |
2010年 | 147篇 |
2009年 | 97篇 |
2008年 | 224篇 |
2007年 | 214篇 |
2006年 | 198篇 |
2005年 | 191篇 |
2004年 | 194篇 |
2003年 | 184篇 |
2002年 | 177篇 |
2001年 | 31篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 5篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1976年 | 5篇 |
1974年 | 8篇 |
1973年 | 3篇 |
1970年 | 3篇 |
1968年 | 2篇 |
1961年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有3082条查询结果,搜索用时 15 毫秒
71.
Animal surveys were conducted in six habitat types within the timber concession of PT International Timber Corporation Indonesia, East Kalimantan. Species diversity was found to be similar in undisturbed forest and in 3- to 5-year-old selectively logged forest, although densities in the latter were considerably less. Both diversity and density declined drastically in forestry plantations, but certain of the species remaining caused significant damage to young trees. Although few animals were observed in areas where there was active road construction and timber felling, their densities in adjacent undisturbed forest areas did not increase, suggesting that the level of local migration actually is low. It is suggested that recolonisation of old logged forest can occur successfully provide that the level of hunting is low and that adjacent areas of undisturbed forest remain to provide a population pool from which colonists can spread. 相似文献
72.
Recent studies of the effects of deer fences on tetraonids have concluded that fences are an important cause of mortality in woodland grouse. This 2-year study involving 16 sections of fences in the Scottish Highlands evaluates the effectiveness of making fences highly visible by using orange netting to reduce bird collisions with fences. A total of 437 collisions involving 13 bird species were recorded. Red grouse (Lagopus lagopus scoticus) formed 42% of all collisions, with black grouse (Tetrao tetrix) 29% and capercaillie (T. urogallus) 20%. Allowing for corpse removal by scavengers, an estimated 70% of red grouse and 29% of black grouse collisions were fatal. Black grouse and capercaillie both collided with 11 of the 16 fences at mean rates of 1.3 and 0.9 collisions km−1 year−1, red grouse collisions occurred at 13 fences, with a mean rate of 1.6 collisions km−1 year−1. Fewer grouse collisions occurred in the summer. Three quarters of black grouse collisions were by males. Collision rates were positively correlated with indices of black grouse and capercaillie abundance. Fence marking reduced capercaillie collisions by 64%, black grouse by 91% and red grouse by 49%. Although marked fences reduced capercaillie collision rates, they still remained an important cause of mortality. To conserve capercaillie, fences need to be removed altogether pending increased deer culls that would allow woodland regeneration without fences, or “grouse friendly” fences designed. 相似文献
73.
Spatial modeling of wind speed around windbreaks 总被引:3,自引:0,他引:3
This paper presents a model to integrate windbreak shelter effects into a Geographic Information System (GIS). The GIS procedure incorporates the 1999 version windbreak sub-model of the Wind Erosion Prediction System (WEPS). Windbreak shelter is modeled in terms of friction velocity reduction, which is a function of wind speed and direction, distance from the barrier, windbreak height, porosity, width, and orientation. A first application of the model was conducted at a study area with an extensive windbreak network in England (Thetford, East Anglia). Windbreak characteristics (windbreak type, height, width, porosity, and location) were recorded. Porosity was estimated from digitized B/W silhouettes. To evaluate the network effectiveness, a windbreak network shelter index (SI) was proposed in terms of average reduction of friction velocity over the area due to network shelter. The network was found to give good protection, but the windbreak distribution was not optimal in relation to the wind vector distribution. 相似文献
74.
Effect of storage on wall-bound phenolics in green asparagus 总被引:2,自引:0,他引:2
Rodriguez-Arcos RC Smith AC Waldron KW 《Journal of agricultural and food chemistry》2002,50(11):3197-3203
The cell walls of green asparagus spears have been analyzed for their phenolic and carbohydrate composition as modified by postharvest storage. Esterified phenolic components were released by sequential alkaline hydrolysis and identified and quantified by diode array HPLC. Significant quantities of ferulic acid (FA) and its derivatives were found to increase at least 3-fold during storage, particularly in walls from the lower parts of the stem, where accompanying changes in sugar composition were also observed. In fresh asparagus, >60% of the total FA was in the form of diferulic acid, and this increased to approximately 70% after 3 days of storage. The main FA dehydrodimers were 8-8-, 8-O-4-, and 8-5-diferulates. These have been detected in other monocotyledonous and dicotyledonous plants, but as a smaller proportion of the total FA. The possible roles phenolic esters might have in relation to the mechanical, textural, and wound-response properties of asparagus spears are discussed. 相似文献
75.
Andrew J. Friedland Arthur H. Johnson Thomas G. Siccama 《Water, air, and soil pollution》1986,29(3):233-243
A regional study was conducted to assess current levels of trace metals in the forest floor, and to establish a baseline for future studies of metal accumulation. Quantitative forest floor samples collected from 78 sites in 9 states in the northeastern United States in 1978 and 1980 were analyzed for Zn, Cu, Ni, and Cd. Mean amounts present in the forest floor were 9.8 ± 1.9 (standard error of the mean), 1.7 ± 0.2, 0.86 ± 0.08, and 0.13 ± 0.01 kg ha?1, for Zn, Cu, Ni, and Cd, respectively. Mean concentrations were 133 ± 25, 19.5 ± 1.6, 11 ± 0.8, and 1.7 ± 0.1 mg kg?1, respectively. Differences in metal and organic matter concentrations and amounts among forest types were attributed to geographic location of specific forest types rather than to direct biotic influence. No element was strongly correlated with elevation for the entire sample area. Regional patterns of elemental amounts showed that trace metal levels are slightly higher in the southern part than in the northern part of the study region. Regional variation of Zn, Cu, Ni, and Cd in the forest floor was not indicative of atmospheric deposition except near point sources of pollution. 相似文献
76.
Nutakul W Sobers HS Qiu P Dong P Decker EA McClements DJ Xiao H 《Journal of agricultural and food chemistry》2011,59(20):10964-10970
The effects of resveratrol and pterostilbene (two structurally related stilbene compounds) on three human colon cancer cells were systematically compared. Cell viability tests indicated that IC(50) values of pterostilbene were 2-5-fold lower than those of resveratrol in all three cancer cells. Pterostilbene was also more potent in inhibiting colony formation of all three cancer cells. Annexin V/propidium iodide costaining assay and Western blotting analysis showed pterostilbene had a stronger apoptosis-inducing effect, which was evidenced by the higher percentage of annexin V positive cells and higher levels of cleaved caspase-3 and poly(ADP-ribose) polymerase proteins in cancer cells treated with pterostilbene compared with resveratrol. High-performance liquid chromatography analysis demonstrated that intracellular levels of pterostilbene were 2-4-fold higher than those of resveratrol after treatments with individual compounds at the same concentration. Overall, the results demonstrated that pterostilbene had more potent inhibitory effects on colon cancer cells than resveratrol, which may be associated with the superior bioavailability of pterostilbene to resveratrol. 相似文献
77.
Photoirradiation of soy oil with UV/visible light has been shown to produce significant amounts of trans,trans conjugated linoleic acid (CLA) isomers through conversion of various synthesized intermediate cis,trans isomers. The objective of this study was to determine the kinetics of CLA isomers synthesis to better understand the production of various isomers. Soy oil was irradiated with UV/visible light for 144 h in the presence of an iodine catalyst and CLA isomers analyzed by gas chromatography (GC). Arrhenius plots were developed for the conversion of soy oil linoleic acid (A) to form cis-, trans/trans-, cis-CLA (B), conversion of cis-, trans/trans-, cis-CLA to form trans,trans-CLA (C) with respect to B, and formation of trans,trans-CLA isomers with respect to C. The kinetics of consumption of linoleic acid (LA) to form cis-, trans/trans-, cis-CLA was found to be of second-order with a rate constant of 9.01 x 10-7 L/mol s. The rate of formation of cis-, trans/trans-, cis-CLA isomers depends on the rate of formation from LA and its rate of consumption to form trans,trans-CLA isomers. The conversion of cis-, trans/trans-, cis-CLA isomers to trans,trans-CLA isomers was found to be of first-order with a rate constant of 2.75 x 10-6 s-1. However, the formation of thermodynamically stable trans,trans-CLA isomers (C) with respect to C was found to be a zero-order reaction with a rate constant of 10.66 x 10-7 mol/L s. The consumption of LA was found to be the rate-determining step in the CLA isomers formation reaction mechanism. The findings provide a better understanding of the mechanism of CLA isomers synthesis by photoirradiation and the factors controlling the ratio of various isomers. 相似文献
78.
The lipid content and fatty acid composition in the edible meat of twenty-nine species of wild and cultured freshwater and marine fish and shrimps were investigated. Both the lipid content and fatty acid composition of the species were specified due to their unique food habits and trophic levels. Most of the marine fish demonstrated higher lipid content than the freshwater fish, whereas shrimps had the lowest lipid content. All the marine fish and shrimps had much higher total n-3 PUFA than n-6 PUFA, while most of the freshwater fish and shrimps demonstrated much lower total n-3 PUFA than n-6 PUFA. This may be the biggest difference in fatty acid composition between marine and freshwater species. The cultured freshwater fish demonstrated higher percentages of total PUFA, total n-3 PUFA, and EPA + DHA than the wild freshwater fish. Two freshwater fish, including bighead carp and silver carp, are comparable to the marine fish as sources of n-3 PUFA. 相似文献
79.
We report a conceptual framework for a simple screening tool to evaluate the vulnerability of different fields in the landscape to phosphorus (P) loss within the European agricultural context. Our work builds on the principles established for the P Index system developed in the United States. We examine the differing needs and criteria for a range of EU member states and EFTA countries and highlight essential and optional criteria for inclusion in the decision support framework depending on their ecoregional characteristics. A framework such as this, meets in principle, the needs of the COST832 Action ‘Quantifying the Agricultural Contribution to Eutrophication’ as it deals with diffuse nutrient sources from agriculture, and aids more targeted use of practical strategies to mitigate nutrient losses to minimise the potential for the eutrophication of receiving waters. 相似文献
80.
Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand 总被引:8,自引:2,他引:8
David Gaumont-Guay T. Andrew Black Tim J. Griffis Alan G. Barr Rachhpal S. Jassal Zoran Nesic 《Agricultural and Forest Meteorology》2006,140(1-4):220
Continuous half-hourly measurements of soil CO2 efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (Rs) on soil temperature (Ts) and water content (θ). Daily mean Rs varied from a minimum of 0.1 μmol m−2 s−1 in February to a maximum of 9.2 μmol m−2 s−1 in mid-July. Daily mean Ts at the 2-cm depth was the primary variable accounting for the temporal variation of Rs and no differences between Arrhenius and Q10 response functions were found to describe the seasonal relationship. Rs at 10 °C (Rs10) and the temperature sensitivity of Rs (Q10Rs) calculated at the seasonal time scale were 3.8 μmol m−2 s−1 and 3.8, respectively. Temperature normalization of daily mean Rs (RsN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of Rs during the growing season. Daily RsN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θfc, 0.30 m3 m−3): (1) RsN was strongly reduced when θ decreased below θfc, which reflected a reduction in microbial decomposition, and (2) RsN slightly decreased when θ increased above θfc, which reflected a restriction of CO2 or O2 transport in the soil profile.Diurnal variations of half-hourly Rs were usually out of phase with Ts at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime Rs10 and Q10Rs parameters calculated from half-hourly nighttime measurements of Rs and Ts at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m−2 s−1 and 5.5 to 1.3, respectively. On average, daily nighttime Rs10 (4.5 μmol m−2 s−1) and Q10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low Rs10 and Q10Rs values at low θ. Overall, the use of seasonal Rs10 and Q10Rs parameters led to an overestimation of daily ranges of half-hourly Rs (ΔRs) during drought conditions, which supported findings that the short-term temperature sensitivity of Rs was lower during periods of low θ. The use of daily nighttime Rs10 and Q10Rs parameters greatly helped at simulating ΔRs during these periods but did not improve the estimation of half-hourly Rs throughout the year as it could not account for the diurnal hysteresis effect. 相似文献