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   5371篇
  免费   456篇
  国内免费   4篇
林业   429篇
农学   195篇
基础科学   32篇
  1418篇
综合类   186篇
农作物   281篇
水产渔业   493篇
畜牧兽医   2241篇
园艺   91篇
植物保护   465篇
  2024年   8篇
  2023年   50篇
  2022年   58篇
  2021年   111篇
  2020年   113篇
  2019年   65篇
  2018年   267篇
  2017年   296篇
  2016年   261篇
  2015年   193篇
  2014年   241篇
  2013年   388篇
  2012年   444篇
  2011年   423篇
  2010年   206篇
  2009年   174篇
  2008年   306篇
  2007年   317篇
  2006年   241篇
  2005年   233篇
  2004年   235篇
  2003年   224篇
  2002年   174篇
  2001年   140篇
  2000年   134篇
  1999年   106篇
  1998年   29篇
  1997年   16篇
  1996年   19篇
  1995年   12篇
  1994年   13篇
  1993年   11篇
  1992年   23篇
  1991年   23篇
  1990年   25篇
  1989年   40篇
  1988年   13篇
  1987年   29篇
  1986年   14篇
  1985年   16篇
  1984年   10篇
  1983年   12篇
  1980年   8篇
  1979年   8篇
  1977年   7篇
  1976年   12篇
  1972年   7篇
  1968年   7篇
  1967年   8篇
  1966年   9篇
排序方式: 共有5831条查询结果,搜索用时 468 毫秒
81.
Postmortem changes in blue shrimp (Litopenaeus stylirostris) muscle were studied on the basis of biochemical, chemical, physical, and microbiological changes during an 18 day storage period at 0°C. Adenosine 5′-triphosphate (ATP) content, breakdown products, K-value, pH, trimethylamine nitrogen (TMA-N), total volatile basic nitrogen (TVB-N), water holding capacity (WHC), color, and texture (shear force) changes were examined. Also, total mesophilic and psychrophilic bacterial counts were measured. K-value increased linearly (r2 = 0.98) from an initial value of 1.37 ± 0.59 to 59.42 ± 6.05% at Day 18. Spoilage indicators TVB-N and TMA-N increased from 29.56 ± 1.33 and 0.69 ± 0.25 to 39.04 and 2.04 ± 0.59 mg of N/100 g at Day 18, respectively; meanwhile, the total viable counts of mesophilic and psychrophilic bacteria increased from 3.48 ± 0.44 and 2.61 ± 0.29 log CFU/g to 6.27 ± 0.21 and 7.14 ± 0.39 log CFU/g, respectively, which indicated spoilage at the end of the storage period. The pH, texture, WHC, and color were affected (p < 0.05) during the storage period. Overall, results indicate that blue shrimp muscle quality was maintained for 12 days of storage in ice.  相似文献   
82.
Liver, kidney, gill and olfactory epithelium cytosolic fractions of rainbow trout (Oncorhynchus mykiss) were examined for glutathione S-transferase (GST) contents. Proteins retained on a glutathione (GSH)-affinity matrix were separated as monomers by reversed-phase HPLC and characterized by immunoblotting, mass spectrometry and partial amino acid sequence. For each organ concerned, a specific pattern of these proteins was determined and appeared similar for liver and kidney on one hand, and for gill and olfactory epithelium on the other hand. It was confirmed that the prominent hepatic GST is a class enzyme, also constitutively expressed as a major isoform in the four organs studied. Moreover, a class variant and two new class GST subunits were characterized in minor fractions. An unknown protein, which was found major in gills and olfactory epithelium, exhibited some characteristics of class GSTs. Occurrence of possible GSH-adduct formation observed on two distinct monomers in specific experimental conditions is discussed. These results and methods were used to investigate the effect of 3,3,4,4-tetrachlorobiphenyl (TCB), a polychlorinated biphenyl (PCB), on GST expression in trout liver. From HPLC-profiling, significant co-induction of the major class and the two minor class GST subunits was observed in trout after waterborne exposure to TCB which was followed by a slight increase in 1-chloro-2,4-dinitrobenzene (CDNB) activity. The present work allows qualitative evaluation of the specific detoxification potential of rainbow trout. The use of HPLC-profiling of GSTs as a possible tool for the biomonitoring of polluted aquatic environment is suggested.  相似文献   
83.
Two experimental modules with different stocking densities (M1 = 70 and M2 = 120 shrimp /m2) were examined weekly over a culture cycle in tanks with low‐salinity water (1.9 g/L) and zero water exchange. Results showed survival rates of 87.7 and 11.9% in M1 and M2, respectively. Water temperature, pH, dissolved oxygen, electrical conductivity and chlorophyll a were not significantly (p > .05) different between modules. In contrast, the concentrations of nitrogen compounds were significantly (p < .05) different between modules, except nitrite‐N (M2 were 2.31 ± 1.38 mg/L N‐TAN, 0.18 ± 0.49 mg/L N‐NO2? and 6.83 ± 6.52 mg/L N‐NO3?; in M1: 0.97 ± 0.73 mg/L N‐TAN, 0.05 ± 0.21 mg/L N‐NO2? and 0.63 ± 0.70 mg/L N‐NO3?). When waters of both modules reached higher levels of ammonia and nitrite, histological alterations were observed in gills. The histological alterations index (HAI) was higher in M2 (5‐112) than in M1 (2‐22).  相似文献   
84.
The molecular cloning of a cDNA encoding the pejerrey brain cytochrome P450 aromatase (P450aromB) is described. This form shares higher identity to other brain aromatases than with their respective ovarian counterparts and the self ovarian aromatase. Tissue-specific expression of both aromatases was examined in pejerrey by RT-PCR. The immunocytochemical distribution of P450aromB was described.  相似文献   
85.
Long‐term effects of improved pasture establishment (with high proportion of legumes) on soil organic‐C status and N availability in Mediterranean cork oak (Quercus suber L.) woodlands were assessed. Soils were sampled beneath scattered crowns and in open areas, considering two systems: unmanaged and managed woodlands where improved pastures were installed 26 and 32 years ago. Total and labile C and N pools were measured and C and N mineralization were determined over 24 weeks laboratory incubation. Soils under improved pastures showed higher organic‐C, total‐N and net N mineralization than those under unmanaged pasture, mainly when established beneath trees. Potentially mineralizable C, C mineralization rate and microbial C were not statistically different between the unmanaged and improved pasture sites, but were higher closer to the tree than in the open area (1.8, 1.2 and 1.2 times, respectively). The qCO2 was higher in improved pastures (1.7 times). Labile pool of C and N extracted with hot water increased under improved pasture (3.4 and 1.7 times, respectively). Results indicate that soil quality amelioration by improved pastures is stronger in the presence of oak trees. Management systems that favour oak tree maintenance and regeneration should be taken into account to reverse soil degradation.  相似文献   
86.
This study evaluated the effect of the density at harvest on the performance and profitability of hatchery‐reared spotted rose snapper cultured in cages. The fish were stocked at harvest densities of 15, 20, and 22 kg/m3 in cages of 222 and 286 m3. More than 39,000 snapper fingerlings with an initial weight of 14 g were stocked. The fish were fed an extruded diet and cultured over a 360 d period. The thermal growth coefficient ranged from 0.04 to 0.05 and survival was 95% for all treatments, with the highest final weight (436.8 g) observed for fish reared at a density of 20 kg/m3. The allometric value b indicated that hatchery‐raised, cage‐cultured snapper were heavier than their wild counterparts. The major costs were feed (ranging from 44.7–45.9%), labor (22.4–32.6%), and seed costs (20.2–26.1%). The total production cost ranged from US$ 6.5 to US$ 7.5/kg. The baseline scenario was not economically feasible. However, a 10% increase in the sales price resulted in increases in the internal rate of return (183%) and net present value (US$ 97,628.9). These results suggest that L. guttatus has the potential for commercial production in cages.  相似文献   
87.
One alternative to contribute to the reduction of the pressure on coral reef ecosystems brought about by an increasing demand for marine aquarium ornamentals is to improve and further develop the culture of desirable species for trade. Peppermint shrimp from the genus Lysmata are among the most intensively traded decapod in the trade and while several studies have been conducted to improve its culture, individuals obtained from culture facilities are generally paler than those collected from the wild. Because color is a fundamental component of the price tag on most marine ornamental live species, poorly colored animals command a lower price, reducing competitiveness. In this study, we evaluate the influence of tank background color (reflected light) on the morphological coloration change of Lysmata boggessi. In a 15‐d experiment, 30 individuals were exposed either to reflected‐red or white light and the relative change of coloration between initial and final moments was quantified with photography (RGB color model). At the end of the experimental period, shrimp exposed to red‐reflected light presented a more intense red coloration than those exposed to white‐reflected light. These results demonstrate that a simple change in background tank color can enhance shrimp external coloration. Such a cheap‐to‐implement procedure can support the culture of more colorful, hence more valuable ornamental shrimp that can compete with those captured from the wild.  相似文献   
88.
89.
Water deficit is a serious problem for most agricultural crops, especially in arid–semiarid regions, and limits sustainable development. Production can be improved by reducing evapotranspiration and loss of infiltrated water by the use of a plastic cover. We monitored soil moisture fortnightly over 1 yr using a neutron probe near four trees in an olive grove (Olea europaea, var. Arbequina), two of which had a plastic cover buried in the topsoil around them. These trees were monitored using three neutron probe access tubes per tree to compare the behaviour of soil moisture over time with two other identically instrumented and nearby trees with no cover. Analysis was based on the resultant moisture profiles. The plastic cover retained moisture and increased soil water residence time. During the dry season, the amount of water retained in the plots was at most 15–20% greater in the mid‐season and at least 5–6% greater at the end of season than in the central part of the plots near the trunk. The plastic cover was effective to ca. 50 cm with maximum water content near the soil surface. During the wet season, the cover did not affect soil water. Soil moisture was greater near the tree trunk as a result of stemflow and throughfall.  相似文献   
90.
The impact of detritivorous earthworms (Eisenia andrei) on microbial community structure and function in grape marc, a lignocellulosic enriched plant residue, was investigated in a mesocosm experiment. Analysis of carbon and nitrogen pools was also carried out in order to evaluate how changes in microbial communities affect plant residue decomposition. The grape marc was completely processed after fifteen days as a result of the high density of earthworms present and the rapid gut transit time. Eisenia andrei had a large impact on the structure of the microbial community, as revealed by phospholipid fatty acid analysis. Earthworm activity reduced the abundance of both bacterial (except for Gram-negative bacteria) and fungal PLFA biomarkers relative to the control values. Decreases in microbial activity and in protease and cellulase activities were also attributable to the presence of earthworms. Moreover, earthworms strongly modified the substrate utilization patterns of microbial communities, as revealed by BIOLOG analysis. The presence of earthworms led to an increase in the utilization of some amino acids and polymers, which reached a higher substrate diversity value than that in the control mesocosm. The differences in microbial communities were accompanied by a reduction in the total C content and the labile C pool, relative to the control, although there were no significant differences in either cellulose or hemicellulose contents. However, total N content increased in both mesocosms – with and without earthworms – and the concentration of NH4+ was also enhanced by earthworm activity. The results indicate that detritivorous earthworms play a key role in decomposing fresh plant residues in the short term via their intensive interactions with microbial communities.  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