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   454篇
  免费   17篇
林业   85篇
农学   2篇
  206篇
综合类   42篇
农作物   15篇
水产渔业   8篇
畜牧兽医   84篇
园艺   7篇
植物保护   22篇
  2023年   6篇
  2021年   4篇
  2020年   11篇
  2019年   6篇
  2018年   6篇
  2017年   10篇
  2016年   15篇
  2015年   12篇
  2014年   14篇
  2013年   24篇
  2012年   29篇
  2011年   37篇
  2010年   20篇
  2009年   19篇
  2008年   35篇
  2007年   29篇
  2006年   30篇
  2005年   21篇
  2004年   15篇
  2003年   13篇
  2002年   19篇
  2001年   10篇
  2000年   2篇
  1999年   5篇
  1998年   6篇
  1997年   3篇
  1996年   6篇
  1995年   3篇
  1994年   4篇
  1993年   2篇
  1977年   2篇
  1952年   1篇
  1950年   1篇
  1949年   1篇
  1941年   2篇
  1940年   3篇
  1939年   3篇
  1936年   1篇
  1935年   1篇
  1934年   1篇
  1932年   2篇
  1931年   3篇
  1928年   3篇
  1927年   4篇
  1926年   7篇
  1925年   1篇
  1922年   2篇
  1905年   2篇
  1903年   1篇
  1898年   4篇
排序方式: 共有471条查询结果,搜索用时 156 毫秒
111.
137Cs activities in mosses collected from four sites in Macedonia, N. Greece were measured for regular periods of time during the last decade following the reactor accident in Chernobyl. Samples taken in 1985 allow to determine the 137Cs contamination before the accident. A clear increase of the cesium interception was observed for about two years after the accident. Later a gradual decrease was detected which depended on various factors such as moss species, location, growth rate and substrate. Higher 137Cs amounts were observed in epilithic (Grimmia) in comparison to epiphytic or epigeic ones ( Encalypta, Tortula). From the studied biotopes Mt. Vermion presented the highest 137Cs activities. The high 137Cs values found 10 yr after Chernobyl suggest that mosses are effective and inexpensive biological detectors of the distribution and burden of fallout radionuclides.  相似文献   
112.
Temperature, drying, and rewetting are important climatic factors that control microbial properties. In the present study we looked at the respiration rates, adenosine 5′‐triphosphate (ATP) content, and adenylate energy charge (AEC) as a measure for energy status of microbial biomass in the upper 5 cm of mineral soils of three beech forests at different temperatures and after rewetting. The soils differed widely in pH (4.0 to 6.0), microbial biomass C (92 to 916 μg (g DW)—1) and ATP content (2.17 to 7.29 nmol ATP (g DW)—1). The soils were incubated for three weeks at 7 °C, 14 °C, and 21 °C. After three weeks the microbial properties were determined, retaining temperature conditions. The temperature treatment did not significantly affect AEC or ATP content, but respiration rates increased significantly with increasing temperature. In a second experiment the soils were dried for 12 hours at 40 °C. Afterwards the soils were rewetted and microbial properties were monitored for 72 hours. After the drying, respiration rates dropped below the detection limit, but within one hour after rewetting respiration rates increased above control level. Drying reduced AEC by 16 % to 44 % and ATP content by 47 % to 78 %, respectively. Rewetting increased AEC and ATP content significantly as compared to dry soil, but after 72 hours the level of the controls was still not reached. The level of AEC values indicated dormant cells, but ATP content increased. These results indicate that the microbial carbon turnover was not directly linked to microbial growth or microbial energy status. Furthermore our results indicate that AEC may describe an average energy status but does not reflect phases of growing, dormant, or dying cells in the complex microbial populations of soils.  相似文献   
113.
The efficacy of a homologous series of biodegradable rapeseedoil derivatives (triglyceride ethoxylates; Agnique RSO® series, Cognis, Düsseldorf, Germany) with an average of 5, 10, 30, and 60 units of ethylene oxide (EO) as adjuvants for foliar‐applied calcium chloride (CaCl2) was evaluated in Malus domestica L. Borkh. Previous experiments revealed that the surfactants used are not phytotoxic. All the surfactants applied decreased contact angles of spray solutions. The lipophilic surfactants RSO 5 and RSO 10 reduced contact angles more than the hydrophilic ones. Drying time of spray droplets was most prolonged when RSO 5 was added. Our data provide evidence that the most lipophilic surfactant RSO 5 can enhance Ca penetration, which was shown in isolated cuticles of tomato fruits (Lycopersicon esculentum cv. Panovy) as a model system. The surfactant effect was most pronounced during the first 4 h and reached a plateau after 24 h. Rainfastness of Ca on surfaces of Malus domestica leaves was improved when RSO 5 was added to CaCl2 spray solutions. All surfactants applied improved rainfastness of Ca on ’︁Braeburn’ fruit surfaces compared to the treatment with CaCl2 alone. Under field conditions, leaf/fruit application of CaCl2/RSO 5 led to an increased Ca content in ’︁Braeburn’ apple fruits and a reduced bitter pit incidence.  相似文献   
114.
The residues and metabolites of radiolabelled imidacloprid [1-(6-chloropyridin-3-ylmethyl)-N-nitroimidazolidin-2-ylideneamine], formulated as a wettable powder containing 250 g kg-1 active ingredient diluted with water and administered to tobacco plants, were studied in sidestream and mainstream smoke, in the ash and butts after smoking cigarettes. An almost complete recovery of radioactivity (93·5%) was achieved. The highest amounts of radioactivity were found in the butts and sidestream smoke. The two dominant compounds identified after smoking were unchanged parent compound and carbon dioxide. A total of 76% of the recovered radioactivity was identified. © 1998 SCI.  相似文献   
115.
Managing fisheries resources to maintain healthy ecosystems is one of the main goals of the ecosystem approach to fisheries (EAF). While a number of international treaties call for the implementation of EAF, there are still gaps in the underlying methodology. One aspect that has received substantial scientific attention recently is fisheries‐induced evolution (FIE). Increasing evidence indicates that intensive fishing has the potential to exert strong directional selection on life‐history traits, behaviour, physiology, and morphology of exploited fish. Of particular concern is that reversing evolutionary responses to fishing can be much more difficult than reversing demographic or phenotypically plastic responses. Furthermore, like climate change, multiple agents cause FIE, with effects accumulating over time. Consequently, FIE may alter the utility derived from fish stocks, which in turn can modify the monetary value living aquatic resources provide to society. Quantifying and predicting the evolutionary effects of fishing is therefore important for both ecological and economic reasons. An important reason this is not happening is the lack of an appropriate assessment framework. We therefore describe the evolutionary impact assessment (EvoIA) as a structured approach for assessing the evolutionary consequences of fishing and evaluating the predicted evolutionary outcomes of alternative management options. EvoIA can contribute to EAF by clarifying how evolution may alter stock properties and ecological relations, support the precautionary approach to fisheries management by addressing a previously overlooked source of uncertainty and risk, and thus contribute to sustainable fisheries.  相似文献   
116.
Near-infrared reflectance spectroscopy (NIRS) is known for its inexpensiveness, rapidity and accuracy and may become a useful tool for the assessment of soil quality. Objectives were (i) to evaluate the ability of NIRS to predict several chemical and biological properties of organically managed arable soils as well as the properties of grain yield from winter cereals for a closed population and (ii) to test whether the use of field-moist and pre-treated (quick-freezing followed by freeze-drying and grinding) samples will generate similar results. One hundred and sixteen soil samples from nine organically managed farms from Germany sampled in 2005 and 2006 were used for this investigation. Spectra of the near-infrared region (including the visible range, 400–2500 nm) from field-moist (<2 mm) or pre-treated soil samples were recorded. A modified partial least-square regression method and cross-validation were used to develop an equation over the whole spectrum (first–third derivation). For the pre-treated soils, good predictions were obtained for pH, contents of organic C, total N, plant-available P (Olsen) and exchangeable K (calcium-acetate-lactate (CAL)), contents of microbial biomass C and N (Cmic and Nmic) and ergosterol, basal respiration, metabolic quotient, the ratio of organic C/total N, the grain yield of winter cereals and grain nitrogen uptake. The RSC (the ratio of standard deviation of laboratory results to standard error of cross-validation) was greater than 2.0, the correlation coefficients (r) of a linear regression (measured against predicted values) were greater than or equal to 0.9 and the regression coefficients (a) ranged from 0.9 to 1.1. Similar good predictions were obtained if field-moist samples were used, with the exception of P (Olsen), K (CAL), metabolic quotient, grain yield of winter cereals and grain nitrogen uptake (satisfactory predictions) and ergosterol content (unsatisfactory prediction). Good predictions of the contents of Mg (CaCl2) and microbial biomass P (Pmic) were achieved for field-moist but not for pre-treated samples. Despite sample preparation, only satisfactory predictions were obtained for the ratios of Cmic/Nmic and ergosterol/Cmic and grain nitrogen content (1.4RSC2.0, r0.8 and 0.8a1.2). However, unsatisfactory predictions for field-moist and pre-treated samples were achieved for the content of P (CAL), the nitrogen mineralisation rate and the ratios of Cmic/Pmic and basal respiration/nitrogen mineralisation rate. Our results demonstrate that biological soil properties can be predicted with NIRS for closed populations in both sample states. The pre-treatment should be used if samples have to be stored prior to infrared measurements for periods longer than a month.  相似文献   
117.
To improve soil structure and take advantage of several accompanying ecological benefits, it is necessary to understand the underlying processes of aggregate dynamics in soils. Our objective was to quantify macroaggregate (> 250 μm) rebuilding in soils from loess (Haplic Luvisol) with different initial soil organic C (SOC) contents and different amendments of organic matter (OM) in a short term incubation experiment. Two soils differing in C content and sampled at 0–5 and 5–25 cm soil depths were incubated after macroaggregate destruction. The following treatments were applied: (1) control (without any addition), (2) OM1 (addition of OM: preincubated wheat straw [< 10 mm, C : N 40.6] at a rate of 4.1 g C [kg soil]–1), and (3) OM2 (same as (2) at a rate of 8.2 g C [kg soil]–1). Evolution of CO2 released from the treatments was measured continuously, and contents of different water‐stable aggregate‐size classes (> 250 μm, 250–53 μm, < 53 μm), microbial biomass, and ergosterol were determined after 7 and 28 d of incubation. Highest microbial activity was observed in the first 3 d after the OM application. With one exception, > 50% of the rebuilt macroaggregates were formed within the first 7 d after rewetting and addition of OM. However, the amount of organic C within the new macroaggregates was ≈ 2‐ to 3‐fold higher than in the original soil. The process of aggregate formation was still proceeding after 7 d of incubation, however at a lower rate. Contents of organic C within macroaggregates were decreased markedly after 28 d of incubation in the OM1 and OM2 treatments, suggesting that the microbial biomass (bacteria and fungi) used organic C within the newly built macroaggregates. Overall, the results confirmed for all treatments that macroaggregate formation is a rapid process and highly connected with the amount of OM added and microbial activity. However, the time of maximum aggregation after C addition depends on the soil and substrate investigated. Moreover, the results suggest that the primary macroaggregates, formed within the first 7 d, are still unstable and oversaturated with OM and therefore act as C source for microbial decomposition processes.  相似文献   
118.
The in vitro efficacy of tiamulin was compared to that of tylosin against 7 bovine, 7 ovine and 3 caprine mycoplasma strains isolated from various organs and belonging to different species, as well as 7 ureaplasma strains cultured from cattle, sheep, swine, chickens and turkeys. The minimal mycoplasmacidal concentrations of tiamulin varied between 0.01 and 10.0 μg ml?1, while tylosin proved to be active in concentrations of 0.5 and 100.0 μg ml?1. Five of mycoplasma strains showed identical sensitivities to both antibiotics while all other strains, including the ureaplasmas, were sensitive to tiamulin at concentrations 5–5000-times lower than tylosin.  相似文献   
119.
Inhalt: Auf Grand fünfjähriger systematischer Registrierung der angeborenen Anomalien bei allen Rinderrassen des Landes Hessen wird festgestellt, daß unter den Nachkommen von Besamungsbullen prozentual nicht mehr mißgebildete Kälber auftreten als bei den Nachkommen von Bullen, die im Natursprung eingesetzt waren. Das breitgestreute Spektrum der Organdefekte umfaßt etwa 43 Mißbildungskategorien und Mißbildungssyndrome; das Zentralnervensystem ist mit ca. 25% am häufigsten beteiligt. Die Aufstellung von "Teratograrnnaen" der Nachkommengruppen ermöglicht die verhältnismäßig frühzeitige Testung von Jungbullen, die für den Einsatz in der KB vorgesehen sind. Da zwischen Nachkommenzahl (Einsatzdauer) der einzelnen Bullen und Mißbildungshäufigkeit bei ihren Nachkommen positive Beziehungen bestehen, bedeuten signifikante Abweichungen von diesen "Normalwerten" der Mißbildungsfrequenzen innerhalb der Nachkommengrößenklassen besondere genetische Situationen der Väter. Diese können in der Anlage für besondere pathologische Merkmale bestehen oder sich in zufallsbedingten embryonalen Entwicklungs-storungen mannigfacher Art als Folge einer individuellen Genomlabilität der Embryonen äußern .  相似文献   
120.
The diagnosis of tapeworm infections in horses relies on copro-diagnostic methods, which are time-consuming and of limited sensitivity for determination of the exact prevalence. The development of serological tests has slightly improved the detection of tapeworm infections, but more sensitive methods are still required. A polymerase chain reaction (PCR)-based approach may constitute a valuable tool to improve tapeworm diagnosis. Nuclear ribosomal DNA (rDNA) is a useful target for species and/or strain markers. Partial 18S, the internal transcribed spacer 1 (ITS-1), the 5.8S, the internal transcribed spacer 2 (ITS-2), and partial 28S rDNA of the equine tapeworms Anoplocephala perfoliata and Anoplocephaloides mamillana were amplified and sequenced. The lengths and GC contents of the regions sequenced were 2087-2091bp and 49.35-49.69% for A. perfoliata, and 2110-2119bp and 49.15-49.32% for A. mamillana, respectively. Sequence alignment and comparison of both taxa showed 79.3-80.2% identity. The lowest identities were found in the ITS regions with 39.9-43.5% for the ITS-1 and 59.5-61.2% for the ITS-2. No matches of the ITS-2 of A. perfoliata and A. mamillana were found with other species by BLAST search. For this reason, ITS-2 sequences seemed appropriate as accurate species markers and A. perfoliata ITS-2 primers were developed. The ITS-2 PCR enabled the detection of genomic DNA as low as 0.5 pgs. First efforts on the practical application of the PCR-based approach were made. A 6-mg fragment of a tapeworm proglottid was detected in 0.5 and 1g of faeces.  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