首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   10130篇
  免费   681篇
  国内免费   45篇
林业   814篇
农学   403篇
基础科学   98篇
  2606篇
综合类   413篇
农作物   593篇
水产渔业   1016篇
畜牧兽医   3916篇
园艺   223篇
植物保护   774篇
  2024年   15篇
  2023年   71篇
  2022年   129篇
  2021年   229篇
  2020年   227篇
  2019年   243篇
  2018年   385篇
  2017年   416篇
  2016年   404篇
  2015年   326篇
  2014年   403篇
  2013年   667篇
  2012年   740篇
  2011年   780篇
  2010年   441篇
  2009年   426篇
  2008年   601篇
  2007年   596篇
  2006年   483篇
  2005年   430篇
  2004年   448篇
  2003年   406篇
  2002年   314篇
  2001年   251篇
  2000年   250篇
  1999年   222篇
  1998年   67篇
  1997年   54篇
  1996年   44篇
  1995年   53篇
  1994年   31篇
  1993年   29篇
  1992年   46篇
  1991年   42篇
  1990年   43篇
  1989年   60篇
  1988年   30篇
  1987年   45篇
  1986年   32篇
  1985年   35篇
  1984年   23篇
  1983年   28篇
  1980年   15篇
  1979年   25篇
  1978年   18篇
  1976年   19篇
  1973年   18篇
  1968年   14篇
  1967年   15篇
  1966年   16篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
This study focuses on the examination of the total anthocyanin content (TAC) and spectrum of winter barley cultivars with blue caryopsis. Preparation of grain samples was simplified, and extraction was performed with 85% methanol acidified by 1% trifluoroacetic acid. Compared with our previous tests performed with 1% hydrochloric acid in aqueous methanol, the extraction process resulted in higher total anthocyanin content with more acylated derivatives. The TAC measured by spectrophotometry varied in two blue Hungarian cultivars and two lines from 46.9 to 84.4 mg/kg. The anthocyanin spectrum was analyzed by electrospray ionization tandem mass spectrometry. The predominant anthocyanins were delphinidin‐3‐malonylglucoside and cyanidin‐3‐malonylglucoside, followed by delphinidin‐3‐glucoside and cyanidin‐3‐glucoside. The caryopsis retains the anthocyanins in long‐term storage. The TAC values of the crops over the last 10 years were definable retroactively. The relative abundance of the main anthocyanin components in grains of different harvest years (2000 and 2010) was similar. Statistical analysis revealed a relatively strong negative correlation (r = –0.7439, P < 0.05) between two variables: TAC and rainfall during the grain‐ripening period.  相似文献   
992.
Characterization of a cost-efficient analytical method based on alkaline sample digestion with KOH and NaOH, followed by aqueous phase phenylation derivatization with NaBPh4 and solid phase microextraction (SPME) for the determination of methylmercury in typical fish-containing food samples commercially available in Hungary, is reported. The sample preparation procedure along with the applied SPME-GC-pyrolysis-AFS system was validated by measuring certified reference materials (CRM) BCR-464, TORT-2, and a candidate CRM BCR 710. To carry out an estimation of average Hungarian methylmercury exposures via marine fish and/or fish-containing food consumption, 16 commercially available products and 3 pooled representative seafood samples of-according to a previous European survey--the three most consumed fish species in Hungary, herring, sardines, and hake, were analyzed. Methylmercury concentrations of the analyzed samples were in the range 0.016-0.137 microg of MeHg g(-1) dry weight as Hg.  相似文献   
993.
Table grape is a nonclimacteric fruit that shows a rapid loss of quality during storage and is very susceptible to colonization by fungi, especially Botrytis cinerea, which is considered the most important disease of this commodity. To solve this problem, synthetic fungicides have been used, although legal restrictions and consumer's concern demand the search of other safe means. In the present paper, and as an alternative of synthetic fungicides, an active packaging to improve MAP effectiveness on preserving table grape (cv. Crimson Seedless) quality and safety was developed by the addition of 0.5 mL of eugenol, thymol, or menthol inside the packages. Packages were stored at 1 degree C for 35 days. The final gas composition inside the packages was 1.4-2.0 and 10.0-14.5 kPa of CO(2) and O(2), respectively, with no significant slight differences among treatments. Results showed that the addition of eugenol, thymol, or menthol improved the beneficial effect of MAP in terms of delaying weight loss and color changes, retarding degrees Brix/acidity ratio evolution, and maintaining of firmness. Thereafter, these treatments showed additional benefit in terms of delayed rates of rachis deterioration and berry decay. Finally, the total viable counts for both mesophilic aerobics and especially yeast and molds were significantly reduced in the grapes packaged with the natural antimicrobial compounds. All of the above effects led to maintenance of table grape quality and safety for longer storage periods (3 additional weeks as compared to controls under MAP only).  相似文献   
994.
The influence of forage conservation method on microbial and chemical composition in forage and on colon content and faeces in horses fed with the forages was investigated. Hay (815 g DM/kg), haylage (548 g DM/kg) and silage (343 g DM/kg) was produced from the same grass crop and fed to four fistulated horses in a crossover study. Horses were sampled in right ventral colon and of faeces after a feeding period of 21 days (for each forage). Fermentation kinetics of right ventral colon was also studied on two subsequent days in each period, with sampling times immediately before and 2, 4, 8 and 12 h after the morning meal. Forage conservation method influenced microbial and chemical composition of the forages. Silage differed from haylage and hay in more variables than haylage differed from hay. However, the different forage types had similar influence on microbial and chemical composition of right ventral colon content and faeces in horses at Day 21, with the exception of counts of Streptococci which were highest when horses were fed with hay. However, interaction among forage types and sampling locations (right ventral colon and faeces) was also present for Streptococci as well as for pH. In general, pH was lower and counts of lactate utilizing and lactate producing bacteria were higher in faeces compared to right ventral colon. Fermentation kinetics in colon varied little among forage types and sampling times, and no interaction between forage types and sampling times were found, indicating that silage, haylage and hay had very similar fermentation kinetics in right ventral colon of horses. In conclusion, silage, haylage and hay produced from the same grass crop tended to produce similar responses in microbial and chemical composition in right ventral colon and faeces in horses fed with the forages.  相似文献   
995.
The National Health Interview Survey (NHIS) is a multipurpose household survey of the U.S. civilian non-institutionalized population conducted annually since 1957. From 1986 to 1994, over 450,000 U.S. workers, age 18 years and older, participated in a probability sampling of the entire non-institutionalized U.S. population; variables collected included a range of measures of acute and chronic disability. The objective of the present study was to assess predictors of health status, and acute and chronic disability for farmers and pesticide applicators (pesticide-exposed workers) compared to all other U.S. workers using the 1986-1994 NHIS. After adjustment for sample weights and design effects using SUDAAN, several measures of acute and chronic disability and health status were modeled with multiple logistic regression. Farmers (n = 9576) were significantly older compared to all other U.S. workers (n = 453,219) and pesticide applicators (n = 180). Farmers and pesticide applicators had a higher proportion of males, whites, and Hispanics and were less educated. After adjusting for age, gender, race-ethnicity, and education, compared to all other workers, farmers were significantly less likely to report acute and chronic disability and health conditions, while pesticide applicators were more likely to report chronic disability, health conditions, and poor health. Given the cross-sectional nature of the data and the significant job demands of farming, both leading to a relative healthy worker effect, the present results indicate that at any point in time, farmers report less acute and chronic disability, compared to other U.S. workers, whereas pesticide applicators report similar or poorer health.  相似文献   
996.
In this work the quantitative determination and analytical speciation of arsenic were undertaken in different types of nuts, randomly purchased from local markets. The hardness of the whole nuts and high lipid content made the preparation of this material difficult for analysis. The lack of sample homogeneity caused irreproducible results. To improve the precision of analysis, arsenic was determined separately in nut oil and in the defatted sample. The lipids were extracted from the ground sample with the two portions of a mixture of chloroform and methanol (2:1). The defatted material was dried and ground again, yielding a fine powder. The nut oil was obtained by combining the two organic extracts and by evaporating the solvents. The two nut fractions were microwave digested, and total arsenic was determined by inductively coupled plasma mass spectrometry (ICP-MS). The results obtained for oils from different types of nuts showed element concentration in the range 2.9-16.9 ng g(-)(1). Lower levels of arsenic were found in defatted material (<0.1 ng g(-)(1) with the exception of Brazil nuts purchased with and without shells, 3.0 and 2.8 ng g(-)(1) respectively). For speciation analysis of arsenic in nut oils, elemental species were extracted from 2 g of oil with 12 mL of chloroform/methanol (2:1) and 8 mL of deionized water. The aqueous layer, containing polar arsenic species, was evaporated and the residue dissolved and analyzed by ion chromatography-ICP-MS. The anion exchange chromatography enabled separation of As(III), dimethylarsinic acid (DMAs(V)), monomethylarsonic acid (MMAs(V)), and As(V) within 8 min. Several types of nuts were analyzed, including walnuts, Brazil nuts, almonds, cashews, pine nuts, peanuts, pistachio nuts, and sunflower seeds. The recovery for the speciation procedure was in the range 72.7-90.6%. The primary species found in the oil extracts were As(III) and As(V). The arsenic concentration levels in these two species were 0.7-12.7 and 0.5-4.3 ng g(-)(1), respectively. The contribution of As in DMAs(V) ranged from 0.1 +/- 0.1 ng g(-)(1) in walnuts to 1.3 +/- 0.3 ng g(-)(1) in pine nuts. MMAs(V) was not detected in almonds, peanuts, pine nuts, sunflower seeds, or walnuts, and the highest concentration was found in pistachio nuts (0.5 +/- 0.2 ng g(-)(1)).  相似文献   
997.
Adsorption and mobility of the fungicide metalaxyl were studied in 16 vineyard soils from the La Rioja region (Spain), with organic matter (OM) contents in the 0.31--1.37% range, and in 7 natural soils with OM contents in the 3.30--8.24% range. Adsorption isotherms were obtained using the batch equilibrium technique, and mobility was studied by soil thin-layer chromatography (soil-TLC). In all cases, the adsorption isotherms fit the Freundlich equation. The values of the K(f) constants were low in the vineyard soils (0.01--0.64) and increased in the natural soils (1.05--2.83). The n(f) values were in general lower than unity. K(f) constants were significantly correlated (p < 0.001) with the OM content when all of the soils were considered. According to the determination coefficient, r(2), OM would account for 88% of the variance in adsorption. When the vineyard soils alone were considered, a significant correlation was seen between K(f) and the OM and clay contents; both parameters, varying simultaneously, explain 80% of the variance in adsorption. Study of the mobility of metalaxyl with soil-TLC indicated that in vineyard soils the fungicide has the potential for being highly mobile in 19% of the soils and mobile in 81% of them. In natural soils, the fungicide has the potential for being moderately mobile or mobile in 86 and 14% of the soils, respectively. This type of behavior of metalaxyl indicates that in vineyards soils of the La Rioja region (Spain) with low OM contents, where application of the compound is continuous, a leaching of the fungicide from the soil to groundwaters could potentially occur. These results should be borne in mind when metalaxyl is to be used in the soils of this region.  相似文献   
998.
This work introduces the development and application of a multi-goal evaluation procedure for assessing sets of remedial options that have previously been found technically suitable for managing a certain contaminated site. The construction of this decision support tool, in the form of a balanced scorecard, has followed a multi-attribute conception of the contaminated sites problem, identifying the most critical aspects considered by all stakeholders involved in contaminated sites management decisions. The developed tool has been checked in its formulation and application against an export benchmark of an Austrian remediation project. The performed evaluation allowed to identify and suggest a remedial option that best fitted the expectations of all stakeholders involved in the project. Furthermore, through the accomplishment of a sensitivity analysis it has been demonstrated that the system is stable and robust in terms of the chosen elicitation schemes, and that the preferences of the different stakeholders are influencing factors on the final result. The results and conclusions of this work present the benefits of implementing the developed balanced scorecard as a tool for supporting decision making in contaminated sites remediation projects.  相似文献   
999.
The most widely used Iron (Fe) fertilizer in calcareous soils is the synthetic chelate Fe(o,o‐EDDHA). However, humic substances are occasionally combined with Fe chelates in drip irrigation systems in order to lower costs. We investigated the effect of various mixtures of Fe(o,o‐EDDHA) and a commercially available humic substance on Fe availability in a calcareous soil from Murcia, Spain (in vitro experiment) and on leaf Fe content and fruit‐quality attributes of Citrus macrophylla (field experiment). In the in vitro experiment, a calcareous soil was incubated for 15 d with solutions of sole Fe(o,o‐EDDHA) and humic substance and of a mixture of humic substance and Fe(o,o‐EDDHA) to determine the dynamics of available Fe. While the mixture did not significantly increase the available soil Fe, it did decrease the rate of Fe retention in the surface soil compared to sole Fe(o,o‐EDDHA). In the field experiment, the substitution in the application solution of 67% of Fe(o,o‐EDDHA) by commercial humic substance increased leaf P in lemon trees from 0.19% with sole Fe(o,o‐EDDHA) to 0.30% and leaf Fe from 94 mg kg–1 to 115 mg kg–1. Some quality parameters like vitamin C content and peel thickness were also improved with a partial substitution of Fe(o,o‐EDDHA) by humic substances. We conclude that a partial substitution of commercial Fe chelates by humic substance can improve crop Fe uptake and may thus be economically attractive. The underlying physiological mechanisms and ecological implications require further studies.  相似文献   
1000.
In nature, iron (Fe) occurs in abundance and ranks fourth among all elements on Earth’s surface. Still, its availability to plants is reduced, once this element is in the form of hydrated oxides, which can limit plant productivity and biomass production. On the other hand, in high concentrations, this essential micronutrient for the plants can become a toxic agent, increasing the environmental contamination. Fe is necessary for the maintenance of essential processes like respiration and photosynthesis, participating in the electron transport chain and in the conversion between Fe2+ and Fe3+, being a key element for carbon dioxide (CO2) fixation and, therefore, important for crop production of cultivated or natural species. The balance of Fe should be strictly controlled, because both its deficiency and its toxicity affect the physiological process of plants. In aerated soils Fe is present in the form of Fe3+, which is the oxidized form and is less available to plants, so these organisms have developed different strategies for absorption, transport and storage of Fe. Deficiency and excess of Fe correlate with local soil conditions and with the care adopted in plant nutrition during the phenological phases and/or in the course of its cultivation. In situations of excessive accumulation of Fe in tissues, an enhancement of hydroxyl radical generation (OH?) occurs by Fenton reaction. Here, we review the nutritional, genetic and ecophysiological aspects of uptake, translocation and accumulation of Fe ions in plants growing under conditions of deficiency or toxicity of this metal.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号