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   910篇
  免费   101篇
林业   54篇
农学   61篇
基础科学   6篇
  181篇
综合类   32篇
农作物   95篇
水产渔业   123篇
畜牧兽医   325篇
园艺   24篇
植物保护   110篇
  2023年   8篇
  2022年   17篇
  2021年   25篇
  2020年   28篇
  2019年   65篇
  2018年   46篇
  2017年   58篇
  2016年   53篇
  2015年   35篇
  2014年   42篇
  2013年   71篇
  2012年   52篇
  2011年   69篇
  2010年   35篇
  2009年   28篇
  2008年   38篇
  2007年   30篇
  2006年   29篇
  2005年   31篇
  2004年   17篇
  2003年   25篇
  2002年   16篇
  2001年   19篇
  2000年   9篇
  1999年   12篇
  1998年   9篇
  1997年   8篇
  1996年   8篇
  1995年   5篇
  1994年   7篇
  1993年   12篇
  1992年   6篇
  1991年   8篇
  1990年   5篇
  1988年   8篇
  1987年   4篇
  1984年   5篇
  1983年   6篇
  1982年   4篇
  1980年   2篇
  1979年   11篇
  1978年   5篇
  1977年   4篇
  1976年   6篇
  1975年   4篇
  1973年   2篇
  1971年   4篇
  1967年   5篇
  1966年   3篇
  1965年   2篇
排序方式: 共有1011条查询结果,搜索用时 31 毫秒
991.
The use of azo dyes is popular in different branches of industry. Discharge of colourants to surface water cause harmful environmental effects. The aim of the present study was evaluation of effectiveness of diazo Evans blue decolourization by two Pseudomonas strains and estimation of process byproducts toxicity. In static conditions, both tested strains removed more than 85?% of dye after 48?h and completely decolorized samples after 120?h. Agitation had negative impact on Evans blue removal (less than 70?% of dye removed after 120?h). Ecotoxicological effects were different for both studied strains beside comparable decolourization effectiveness. Increase of zootoxicity was noticed for strain Sz6 and decrease from IV to III class was noticed for strain SDz3. Optimization of process conditions for the most promising strain SDz3 should be deeply examined.  相似文献   
992.
Barley is considered a healthy food because of its high content of β‐glucan and phenolic antioxidants. In the current study, 28 black, blue, and yellow barleys were investigated in terms of their composition of free and bound phenolic acids and 2,2‐diphenyl‐1‐picrylhydrazyl radical scavenging capacity. Free phenolics were based on aqueous methanol extraction, whereas bound phenolics were extracted following alkaline hydrolysis. Phenolics were then separated and quantified by liquid chromatography and the Folin–Ciocalteu method. Significant differences were observed between the three barley color groups, and within each color group a wide range of phenolics concentrations existed. Ferulic acid was the predominant phenolic acid in free and bound extracts, followed by p‐coumaric acid in all the barleys investigated. Total phenols content and individual phenolic acids strongly correlated with free radical scavenging capacity of barley. Black and blue barley were found to be related and distinct from yellow barley. The results showed significant variations in phenolics among barleys, with a potential for the development of barley grains with high content of phenolic compounds as antioxidant potential.  相似文献   
993.
Habitat connectivity is important for the survival of species that occupy habitat patches too small to sustain an isolated population. A prominent example of such a species is the European bison (Bison bonasus), occurring only in small, isolated herds, and whose survival will depend on establishing larger, well-connected populations. Our goal here was to assess habitat connectivity of European bison in the Carpathians. We used an existing bison habitat suitability map and data on dispersal barriers to derive cost surfaces, representing the ability of bison to move across the landscape, and to delineate potential connections (as least-cost paths) between currently occupied and potential habitat patches. Graph theory tools were then employed to evaluate the connectivity of all potential habitat patches and their relative importance in the network. Our analysis showed that existing bison herds in Ukraine are isolated. However, we identified several groups of well-connected habitat patches in the Carpathians which could host a large population of European bison. Our analysis also located important dispersal corridors connecting existing herds, and several promising locations for future reintroductions (especially in the Eastern Carpathians) that should have a high priority for conservation efforts. In general, our approach indicates the most important elements within a landscape mosaic for providing and maintaining the overall connectivity of different habitat networks and thus offers a robust and powerful tool for conservation planning.  相似文献   
994.

Worldwide population is in permanent growth, leading to an over-presser on the food resources. This demand leads to an over-exploitation of land and water resources. Recently, the quinoa plant has attracted attention as alternative crop with high nutritive value and high tolerance to the marginal conditions. In this study, a field experiment was conducted during two successive seasons to investigate the effect of deficit irrigation on the agronomic performance. Four treatments of deficit irrigation (100%, 50%, 33% ETc and rainfed) were applied to one variety in the first season and to four genotypes (two varieties and two lines). The results were evaluated by measuring biomass and seed quinoa yield, water-use efficiency, harvest index, seed size and 1000 seeds weight. Results show that the implementation of deficit irrigation is an appropriate strategy to reduce the use of agricultural water and maintain relatively high yields. On the other hand, the results of the economic quality reflected by the size and weight of seed yield, indicates that quinoa can be considered a well-adapted to the conditions of water scarcity culture. According to agronomic parameters, L143 line followed by the line L11 showed a high potential of adaptation under the different treatments of stress, while the “Puno” variety presented the best performance under the favorable conditions of irrigation (100% ETc).

  相似文献   
995.
Ammonia emission is one of the most important pathways of nitrogen loss from agricultural cultivated field. In this paper, we report the measurement of ammonia emission from paddy rice field obtained by surface application of urea fertilizer with water management. The main objective of the present study were to assess the amount of NH3 emission and the loss of nitrogen from paddy field as affected by various N doses, i.e., 0 (control), 90 (N1), 180 (N2), 270 (N3) and 360 (N4) kg ha-1, following field surface application of urea fertilizer with water management. Ammonia emissions were measured by continuous airflow enclosure method from plots fertilized with the application of surface urea. Increase in urea-N dosage increased NH3 emission thatwas measured from paddy rice field. Ammonia emission started immediately and was almost complete within 12 days after top dressing of urea application to the soils. Ammonia emissions were nearly constant in all treatments from 12 days after fertilizer application. Highest ammonia emission rate was 28 g/day and total amount of ammonia emission was 56.21 kg ha-1 for 360 kg N ha-1 dose. No remarkable observation was found about temperature for ammonia emission. Due to proper water management practices less emission was observed throughout the experiment period. The results also show that N loss through NH3 emission accounted for 11 to 16% during the ricegrowing season. These magnitudes of loss of N appear to be most important for environmental point of view.  相似文献   
996.

Purpose

This study aimed to investigate the effect of initial soil pH and organic anion-to-acid ratio on changes in soil pH.

Materials and methods

Two soils (Podosol and Tenosol) along with two carboxylic acids (malic and citric acid) and their anions (sodium malate and citrate), commonly found in plant residues, were used in this study. Stock solutions of either malic acid and disodium malate or citric acid and trisodium citrate were added to pre-incubated soils at anion-to-acid ratios of 0:100, 10:90, 25:75, 50:50, 75:25, 90:10, 100:0 and at 0.25 g C kg?1 soil. Soils were adjusted to 80 % field capacity and mixed thoroughly, and three replicates of 50 g of each soil were transferred into individual plastic cores and incubated at 25 °C in the dark for 30 days. Soil pH, respiration, NH4 +, and NO3 ? were determined.

Results and discussion

Soil pH increased linearly with increasing organic anion-to-acid ratio. The addition of organic anions to soil resulted in net alkalinisation. However, the addition of organic acids immediately decreased soil pH. During subsequent incubation, soil pH increased when the organic anions were decomposed. Alkalinity generation was lower in the Podosol (initial pH 4.5) than in the Tenosol (initial pH 6.2), and was proportional to anion-to-acid ratio across all the treatments. Cumulative CO2-C release was approximately three times lower in the Podosol than the Tenosol at day 2 due to lower microbial activity in the low-pH Podosol.

Conclusions

Increasing anion-to-acid ratio of organic compounds increased soil pH. Increases in soil pH were mainly attributed to direct chemical reactions and decomposition of organic anions. Low pH decreased the amount of alkalinity generated by addition of organic compounds due to incomplete decomposition of the added compounds. This study implies that organic anion-to-acid ratio in plant residues plays an important role in soil pH change.  相似文献   
997.
We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.  相似文献   
998.
The aim of the study was to determine whether phenolic compounds in some varieties of buckwheat, winter and spring barley and peas can be used as factors which distinguish selected cultivars and varieties of plant material. It was observed that the content of total phenolics might be useful as a cultivar-distinguishing factor for all the plant materials analyzed, but it was a distinguishing factor for only some varieties. Individual cultivars and varieties were best distinguished by the content of syringic acid. The levels of syringic and vanillic acids were in reverse proportion to the total amount of phenolics soluble in methanol and a positive correlation between syringic and ferulic acid was observed. Moreover, the protein content of plant material was analyzed and a significant (p ≤ 0.05) correlation between this component and ferulic and vanillic acids was noted.  相似文献   
999.
A field investigation of two faba bean cultivars (cv.), Agabat and Silaim, showed that bean yellow mosaic virus (BYMV) infection reduced (p≤0.001) yield (Kg/ha), protein content and in vitro protein digestibility (IVPD) but increased (p≤0.05) tannin content (mg/100 ml). Nitrogen fertilization with viral infection significantly reduced yield and IVPD for cv. Silaim and increased (p≤0.05) protein and tannin contents. Nitrogen fertilization alone was found to increase (p≤0.05) yield, protein and tannin contents but slightly reduced IVPD.Rhizobium inoculation with viral infection significantly decreased yield per unit area, protein content and IVPD, but increased (p≤0.05) tannin content.Rhizobium inoculation alone significantly increased (p≤0.001) yield and tannin content and slightly increased protein content but decreased IVPD. The results indicated that nitrogen fertilization or nitrogen fixation increased yield, protein and tannin contents and decreased IVPD. Viral infection had an adverse effect on yield, protein content and IVPD but had no effect on tannin content.  相似文献   
1000.
Seaweeds are rich sources of dietary fiber which possess diverse physiological functions in the colon. In the present study, the modulating effect of feeding seatangle (Laminaria japonica), a fiber-rich seaweed, on initiation and postinitiation of rat colon carcinogenesis was investigated. Four groups of animals were exposed to two weekly injections of a chemical carcinogen azoxymethane (AOM). Animals were fed a fiber-free semisynthetic diet or a diet containing 15% seatangle (4.7 g dietary fiber/100 g diet) during initiation, postinitiation or initiation postinitiation phase of carcinogenesis. Study results showed that animals fed the seatangle-containing diet had a significantly decreased number of both aberrant crypts and aberrant crypt foci in the colon. Also, labeling indices and proliferation zones were signi- ficantly reduced in the colon of the rats fed a seatangle diet. All of the seatangle feeding regimens used in this study were effective in reducing aberrant crypt formation indicating anticarcinogenic effects are mediated through both the blocking of initiation and the suppression of cell proliferation in initiated cells. These results suggest that a diet containing seatangle may decrease the risk of developing colon cancer although the precise cellular mechanisms involved in anticarcinogenic effects of seatangle should be further examined.  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