全文获取类型
收费全文 | 543篇 |
免费 | 36篇 |
专业分类
林业 | 43篇 |
农学 | 13篇 |
基础科学 | 3篇 |
107篇 | |
综合类 | 27篇 |
农作物 | 24篇 |
水产渔业 | 24篇 |
畜牧兽医 | 258篇 |
园艺 | 17篇 |
植物保护 | 63篇 |
出版年
2024年 | 1篇 |
2023年 | 7篇 |
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 15篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 13篇 |
2016年 | 20篇 |
2015年 | 22篇 |
2014年 | 24篇 |
2013年 | 28篇 |
2012年 | 47篇 |
2011年 | 59篇 |
2010年 | 28篇 |
2009年 | 45篇 |
2008年 | 44篇 |
2007年 | 29篇 |
2006年 | 27篇 |
2005年 | 27篇 |
2004年 | 28篇 |
2003年 | 18篇 |
2002年 | 17篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1978年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有579条查询结果,搜索用时 15 毫秒
31.
Use of decision support systems (DSS) has thus far been framed as a social process of adoption or technical process of usability. We analyze the development of a DSS as a process of institutionalization of new as well as drift of existing practices. We write an Actor-Network-Theory (ANT) account, i.e. an interpretive study, that follows the traces left by both human and non-human actors (e.g. technology, methodologies, etc.) to understand how a DSS development project institutionalizes DSS technology in several forest management organizations in the German state of Rheinland Pfalz. The research has an innovative value since it uses ANT in the design of a DSS, hence affecting it, while commonly ANT has been used to understand why networks work or do not. Moreover, we use a new technology (PREZITM, www.prezi.com) for the visualization of the whole actor network coherent to the ANT methodology, i.e. “keeping the social flat.” As a result, the development of the ANT account proposed in the present paper, even if still partial, supports the design of new technologies being introduced in current practice and generates an important learning effect thanks to the underpinning interpretative approach. 相似文献
32.
Luc Denys 《水产资源保护:海洋与淡水生态系统》2003,13(3):191-211
- 1. Diatom assemblages of man‐made coastal dune wetlands between Blankenberge and Heist (Belgium), dating from 1852 to 1929 and sampled from herbarium specimens of macrophytes, were compared with more recent samples collected in the remaining calcareous dune marshes and pools in this area.
- 2. Overall, nutrient conditions inferred from the reference assemblages were fairly eutrophic for phosphorus. Only a minority of the historical assemblages pointed to presumably nitrogen‐limited conditions.
- 3. Significant alterations in general assemblage composition were observed, including a marked decline of epiphytic species, and a decrease in the compositional variation in sediment diatom assemblages. These changes can be attributed mainly to an increased availability of nutrients and degradable organic matter since the mid 1970s. No changes in the salinity range seem to have occurred, suggesting fairly stable hydrological conditions.
- 4. Possible causes for eutrophication include increased atmospheric deposition of nutrients, but also more site‐related phenomena such as guanotrophication, angling and, perhaps, effects of nature management on soil–nutrient cycling. Their relative importance needs to be established and further monitoring is necessary.
- 5. Measures are required to reduce nutrient levels of both permanently and periodically inundated sites and to promote small‐scale habitat differentiation. Due to physical constraints, the latter will be possible only by mimicking the processes that act upon more natural dune systems in management practice.
33.
Ana Alves Antonio Santos Philippe Rozenberg Luc E. Paques Jean-Paul Charpentier Manfred Schwanninger José Rodrigues 《Wood Science and Technology》2012,46(1-3):157-175
Wood density is defined as the ratio of mass to volume and therefore in principle it should be possible to calculate a unique partial least squares regression (PLS-R) model for several species. PLS-R models for wood density based on X-ray microdensity data were calculated for each species Pinus pinaster and Larix × eurolepis and for both species together. After cross-validation and test set validation the data sets were combined and final models were calculated. The common model gave a residual prediction deviation (RPD) of 3.1, a range error ratio (RER) of 11.7, and a SEP/SEC of 1.06. The single models for Pinus pinaster and Larix?×?eurolepis gave RPD’s of 3.5 and 3.2, RER’s of 13 and 11, and a SEP/SEC of 1.2. To the best knowledge of the authors all obtained PLS-R models are the first ones that fulfil the requirements according to AACC Method 39-00 (AACC in AACC Method, 39-00:15, 1999) to be used at least for screening (RPD?≥?2.5). Although this method and the defined limits were developed for the analysis of grains they can be used as a rough rule of thumb until limits for wood are available. The improvement of the PLS-R models, compared to published results, might be due to three facts (1) the higher number of scans collected for a single spectrum, (2) that the samples were better represented by the NIR spectra and X-ray microdensity values, and (3) that the sites for the measurement of NIR spectra and X-ray microdensity were coincided as strictly as possibly. 相似文献
34.
35.
The turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with 14C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of 14C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg 14C‐labelled glucose kg?1 and 200 mg NH4+‐N kg?1, and incubated aerobically. Production of 14CO2 and CO2, dynamics of NH4+, NO2– and NO3–, and microbial biomass 14C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because > 90 mg 14C kg?1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH4+ and NO3– in the NH4+‐amended soil could not be simulated unless an immobilization of NH4+ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH4+ was added to alkaline saline soils. 相似文献
36.
37.
Xiaoxi Zhang Zengwen Liu Bochao Zhu Yuanhao Bing Nhu Trung Luc Liangzhen Du Zhenhua Zhu 《林业研究》2016,27(3):525-532
The productivity of Robinia pseudoacacia (R.p.) pure forest usually declines at the late growth stage, and reforming it into mixed forests could be a promising way to resolve this problem. When choosing a suitable tree species that can be mixed with R.p., the interspecific relationship is an important issue. Therefore, we gathered the autumn litter fall from R.p. and 10 other species from the Loess Plateau of China were mixed in dual species litterbags (R.p. + each other species) and buried them in soil for a 345 days lab decay incubation. We measured the litter mass loss and nutrient contents to determine whether the nutrient release was affected by mixed species litter decomposition. The impacts of mixed litter decomposition on macro-elements release were more obvious than on micro-elements. The litters with similar substrate quality might show variable impacts on nutrients release in mixed decomposition. The C loss and release of nutrient was improved by descending order when R.p. litter was mixed with Hippophae rhamnoides, Ulmus pumila, Populus simonii, Larix principis-rupprechtii and Quercus liaotungensis (Q.l.). But, except for Q.l., only the other species were recommended as suitable mix-plants for R.p. since promoting a high turnover of the nutrient in the litter compartment and a rapid availability for tree. 相似文献
38.
Nathalie Porters Tim Bosmans Mariëlla Debille Hilde de Rooster Luc Duchateau Ingeborgh Polis 《Veterinary anaesthesia and analgesia》2014,41(1):90-96
ObjectiveTo compare sedation and antinociception after oral transmucosal (OTM) and intramuscular (IM) administration of a dexmedetomidine-buprenorphine combination in healthy adult cats.Study designRandomized, ‘blinded’ crossover study, with 1 month washout between treatments.AnimalsSix healthy neutered female cats, weighing 5.3–7.5 kg.MethodsA combination of dexmedetomidine (40 μg kg?1) and buprenorphine (20 μg kg?1) was administered by either the OTM (buccal cavity) or IM (quadriceps muscle) route. Sedation was measured using a numerical rating scale, at baseline and at various time points until 6 hours after treatment. At the same time points, analgesia was scored using a dynamic and interactive visual analogue scale, based on the response to an ear pinch, and by the cat’s response to a mechanical stimulus exerted by a pressure rate onset device. Physiological and adverse effects were recorded, and oral pH measured. Signed rank tests were performed, with significance set at p < 0.05. Data are presented as median and range.ResultsThere were no differences in sedation or antinociception scores between OTM and IM dosing at any of the time points. Nociceptive thresholds increased after both treatments but without significant difference between groups. Buccal pH remained between 8 and 8.5. Salivation was noted after OTM administration (n = 2) and vomiting after both OTM (n = 4), and IM (n = 3) dosing.Conclusions and clinical relevanceIn healthy adult cats, OTM administration of dexmedetomidine and buprenorphine resulted in comparable levels of sedation and antinociception to IM dosing. The OTM administration may offer an alternative route to administer this sedative-analgesic combination in cats. 相似文献
39.
Diane Saint-Laurent Julien St-Laurent Patricia Duplessis Luc Lavoie 《Water, air, and soil pollution》2010,209(1-4):451-466
Current and past industrial pollution leaves many traces in the environment, in particular along rivers in industrial and urban areas. The isotopic analysis of the lead found in soils and tree rings offers a kind of environmental archive for presenting a portrait of the pollutant distribution in the environment in both spatial and temporal terms. This study is an attempt to identify and compare the source of contamination found in soils and tree rings located along two rivers affected by pollution over several years. Specifically, the focus is on the pattern of lead concentrations and lead isotopic signatures (206Pb/207Pb, 208Pb/206Pb, and 206Pb/204Pb) detected in soils and tree rings located on polluted floodplains. The concentration of Pb in overbank sediments does not rise with the increasing distance downstream from the point source (mining area), suggesting that significant fluvial transport of the pollutant particles over 80 km is involved. For the soil profiles, Pb concentration levels range between 12.32 and 149.13 mg/kg, with the highest concentrations found at the base of the profiles (>1 m). For the lead isotope ratios in the soil profiles, the values obtained range from 0.851 to 0.872 (206Pb/207Pb), 2.081 to 2.111 (208Pb/206Pb), and 0.547 to 0.562 (206Pb/204Pb). The tree ring analysis of red ash (Fraxinus pennsylvanica Marsh.) shows average lead concentrations of 0.63 μg/g, and the lead values of all the tree specimens range between 0.03 and 11.38 μg/g. Pb concentrations varied greatly between the specimens in selected sites and lead isotope ratios in the tree rings showed a strong variability in the time series, particularly from 1945 to 1970. The greater number of variations in the lead concentration rates and isotopic ratios suggest that many more events associated with pollution and contamination have in fact occurred in this area. The study demonstrates the utility of combining stable isotope analyses (soils and tree rings) to examine the source and dispersion of contaminant Pb in fluvial systems by providing reliable and robust indicators for the detection of environmental changes on a local and regional scale. 相似文献
40.
Verbeylen Goedele De Bruyn Luc Adriaensen Frank Matthysen Erik 《Landscape Ecology》2003,18(8):791-805
In determining isolation effects in fragmented populations, the landscape matrix is not often considered. Usually simple distance
measures are used to quantify degree of isolation. We tested the effect of the matrix on the presence of red squirrels in
354 wooded patches in the Brussels Region, by comparing several isolation measures. These were 1) distance to the nearest
source patch, 2) the Hanski-measure (a combination of distance to and size of all possible sources), 3) effective distances
calculated from different least cost models using the ArcView grid extension ‘Cost Distance’ (a combination of distance and
resistance of the landscape, with different resistances for different landscape types) and 4) some combinations of the Hanski-measure
and the effective distances. Size and quality of the target patches were always included in the tests of the predictive power
of different isolation measures on squirrel presence/absence. All variables examined (patch size, quality and isolation) significantly
influenced squirrel presence. Models using the effective distances gave the best results. Models including the Hanski-measure
improved significantly when Euclidean distance was replaced by effective distance, showing that parameterisation of matrix
resistance added significant additional explanatory power when modelling squirrel presence.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献