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71.
72.
Kilimanjaro ice core records: evidence of holocene climate change in tropical Africa 总被引:1,自引:0,他引:1
Thompson LG Mosley-Thompson E Davis ME Henderson KA Brecher HH Zagorodnov VS Mashiotta TA Lin PN Mikhalenko VN Hardy DR Beer J 《Science (New York, N.Y.)》2002,298(5593):589-593
Six ice cores from Kilimanjaro provide an approximately 11.7-thousand-year record of Holocene climate and environmental variability for eastern equatorial Africa, including three periods of abrupt climate change: approximately 8.3, approximately 5.2, and approximately 4 thousand years ago (ka). The latter is coincident with the "First Dark Age," the period of the greatest historically recorded drought in tropical Africa. Variable deposition of F- and Na+ during the African Humid Period suggests rapidly fluctuating lake levels between approximately 11.7 and 4 ka. Over the 20th century, the areal extent of Kilimanjaro's ice fields has decreased approximately 80%, and if current climatological conditions persist, the remaining ice fields are likely to disappear between 2015 and 2020. 相似文献
73.
Prabhakar S Visel A Akiyama JA Shoukry M Lewis KD Holt A Plajzer-Frick I Morrison H Fitzpatrick DR Afzal V Pennacchio LA Rubin EM Noonan JP 《Science (New York, N.Y.)》2008,321(5894):1346-1350
Changes in gene regulation are thought to have contributed to the evolution of human development. However, in vivo evidence for uniquely human developmental regulatory function has remained elusive. In transgenic mice, a conserved noncoding sequence (HACNS1) that evolved extremely rapidly in humans acted as an enhancer of gene expression that has gained a strong limb expression domain relative to the orthologous elements from chimpanzee and rhesus macaque. This gain of function was consistent across two developmental stages in the mouse and included the presumptive anterior wrist and proximal thumb. In vivo analyses with synthetic enhancers, in which human-specific substitutions were introduced into the chimpanzee enhancer sequence or reverted in the human enhancer to the ancestral state, indicated that 13 substitutions clustered in an 81-base pair module otherwise highly constrained among terrestrial vertebrates were sufficient to confer the human-specific limb expression domain. 相似文献
74.
Brown KN Chen S Han Z Lu CH Tan X Zhang XJ Ding L Lopez-Cruz A Saur D Anderson SA Huang K Shi SH 《Science (New York, N.Y.)》2011,334(6055):480-486
The neocortex contains excitatory neurons and inhibitory interneurons. Clones of neocortical excitatory neurons originating from the same progenitor cell are spatially organized and contribute to the formation of functional microcircuits. In contrast, relatively little is known about the production and organization of neocortical inhibitory interneurons. We found that neocortical inhibitory interneurons were produced as spatially organized clonal units in the developing ventral telencephalon. Furthermore, clonally related interneurons did not randomly disperse but formed spatially isolated clusters in the neocortex. Individual clonal clusters consisting of interneurons expressing the same or distinct neurochemical markers exhibited clear vertical or horizontal organization. These results suggest that the lineage relationship plays a pivotal role in the organization of inhibitory interneurons in the neocortex. 相似文献
75.
David A. Keith Tara G. Martin Eve McDonald-Madden Carl Walters 《Biological conservation》2011,(4):1175-1178
Adaptive management of natural resources is widely supported, but in biodiversity conservation there have been few practical applications of the approach in its entirety. Some of the contributions to this special publication examine progress in the implementation of adaptive approaches into conservation policy, while others explore novel theoretical and modeling approaches that seek to accommodate the complexities of real-world applications. Several of the papers address the treatment of uncertainty in adaptive management through innovative approaches to experimentation and monitoring, use and characterisation of expert knowledge and reconciliation of differences of opinion about parameters or systems. Drawing on these contributions, we discuss the major impediments to implementing adaptive management, why adaptive management has been slow to be implemented and how this can be redressed. 相似文献
76.
Turnipseed SB Storey JM Clark SB Miller KE 《Journal of agricultural and food chemistry》2011,59(14):7569-7581
A quadrupole time-of-flight (Q-TOF) liquid chromatography-mass spectrometry (LC-MS) method was developed to analyze veterinary drug residues in milk. Milk samples were extracted with acetonitrile. A molecular weight cutoff filter was the only cleanup step in the procedure. Initially, a set of target compounds (including representative sulfonamides, tetracyclines, β-lactams, and macrolides) was used for validation. Screening of residues was accomplished by collecting TOF (MS(1)) data and comparing the accurate mass and retention times of found compounds to a database containing information for veterinary drugs. The residues included in the study could be detected in samples fortified at the levels of concern with this procedure 97% of the time. Although the method was intended to be qualitative, an evaluation of the MS data indicated a linear response and acceptable recoveries for a majority of target compounds. In addition, MS/MS data were also generated for the [M + H](+) ions. Product ions for each compound were identified, and their mass accuracy was compared to theoretical values. Finally, incurred milk samples from cows dosed with veterinary drugs, including sulfamethazine, flunixin, cephapirin, or enrofloxacin, were analyzed with Q-TOF LC-MS. In addition to monitoring for the parent residues, several metabolites were detected in these samples by TOF. Proposed identification of these residues could be made by evaluating the MS and MS/MS data. For example, several plausible metabolites of enrofloxacin, some not previously observed in milk, are reported in this study. 相似文献
77.
Threat syndromes and conservation of the Australian flora 总被引:1,自引:0,他引:1
The status of the Australian flora was reviewed by compiling published information on all critically endangered and endangered species listed federally in 2004. Threatening processes were categorised and their contributions to past, present and future declines were assessed. The information was cross-referenced against State agency information and field knowledge. Land clearance for agriculture (grazing and cropping) and urbanization have been the primary causes of range contractions and habitat loss in the past, responsible for the current status of the majority of threatened Australian plants. In the future, land clearance will remain important but new issues are emerging. Many species are now at risk from demographic and environmental uncertainty alone. Threats growing in importance include disease, salinity, invasive species and changed disturbance regimes. Many species are subject to common, landscape-level threats. A key issue to emerge from our analysis is that most species are threatened by a number of interacting factors - threat syndromes. Several future risks may be mitigated effectively by simple, low-cost changes in policy, such as more stringent controls on land clearance, strategic fire management, and firmer control on the importation of plant species. Other factors will require greater effort and new strategies to mitigate, including social and legal initiatives in urban landscapes and broad strategies for pathogens, climate change and other landscape-level processes. 相似文献
78.
This study investigates the episodic acidification of Reedy Creek, a wetland-influenced coastal plain stream near Richmond, Virginia. Primary objectives of the study were to quantify the episodic variability of acid-base chemistry in Reedy Creek, to examine the seasonal variability in episodic response and to explain the hydrological and geochemical factors that contribute to episodic acidification. Chemical response was similar in each of the seven storms examined, however, the ranges in concentrations observed were commonly greater in summer/fall storms than in winter/spring storms. An increase in SO inf4 sup2? concentration with discharge was observed during all storms and peak concentration occurred at or near peak flow. Small increases in Mg2+, Ca2+, K+ concentrations and dissolved organic carbon (DOC) were observed during most storms. At the same time, ANC, Na+ and Cl? concentrations usually decreased with increasing discharge. In summer/fall storms, the absolute increase in SO inf4 sup2? concentration was one-third to 15 times the increase observed in winter/spring storms; the decrease in ANC during summer/fall storms was usually within the range of the decrease observed in winter/spring storms. In contrast, the decrease in Na+ and Cl? concentrations during winter/spring storms was much greater than that observed during summer/fall storms. Data show that while base flow anion deficit was higher in summer/fall than in winter/spring, anion deficit decreased during most summer/fall storms. In contrast, base flow anion deficit was lower in spring and winter, but increased during winter/spring storms. Increased SO inf4 sup2? concentration was the main cause of episodic acidification during storms at Reedy Creek, but increased anion deficit indicates organic acids may contribute to episodic acidification during winter/spring storms. Changes in SO inf4 sup2? concentration coincident with the hydrograph rise indicate quick routing of water through the watershed. Saturation overland flow appears to be the likely mechanism by which solutes are transported to the stream during storm flow. 相似文献
79.
Steven W. Effler Emmet M. Owens Keith A. Schimel 《Water, air, and soil pollution》1986,27(3-4):247-258
Denstity stratification was characterized in ionically enriched Onondaga Lake for a 7 mo period of 1980, based on paired profiles of temperature and cloride collected at 1 m depth intervals from a single deep water location on 54 different occasions. The lake was both thermally and chemically stratified. The chemical component represented 38.5% of the density stratification for the study period. It was most often the dominant component in establishing the depth of the upper mixed layer, which was unusually shallow in the lake. Further, the presense of the chemical component prolonged the stratification period. The chemical component of stratification decreased progressively through the study. The altered stratification characteristics of the lake may have negative effects on the level of biomass in the upper waters and the oxygen resources of the lower waters. 相似文献
80.
Distributed erosion models are potentially good tools for locating soil sediment sources and guiding efficient Soil and Water Conservation (SWC) planning, but the uncertainty of model predictions may be high. In this study, the distribution of erosion within a catchment was predicted with a semi-empirical erosion model that combined a semi-distributed hydrological model with the Morgan, Morgan and Finney (MMF) empirical erosion model. The model was tested in a small catchment of the West Usambara Mountains (Kwalei catchment, Tanzania). Soil detachability rates measured in splash cups (0.48–1.16 g J− 1) were close to model simulations (0.30–0.35 g J− 1). Net erosion rates measured in Gerlach troughs (0.01–1.05 kg m− 2 per event) were used to calibrate the sediment transport capacity of overland flow. Uncertainties of model simulations due to parameterisation of overland flow sediment transport capacity were assessed with the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. The quality of the spatial predictions was assessed by comparing the simulated erosion pattern with the field-observed erosion pattern, measuring the agreement with the weighted Kappa coefficient of the contingency table. Behavioural parameter sets (weighted Kappa > 0.50) were those with short reinfiltration length (< 1.5 m) and ratio of overland flow power α to local topography power γ close to 0.5. In the dynamic Hortonian hydrologic regime and the dissected terrain of Kwalei catchment, topography controlled the distribution of erosion more than overland flow. Simulated erosion rates varied from − 4 to + 2 kg m− 2 per season. The model simulated correctly around 75% of erosion pattern. The uncertainty of model predictions due to sediment transport capacity was high; around 10% of the fields were attributed to either slight or severe erosion. The difficult characterisation of catchment-scale effective sediment transport capacity parameters poses a major limit to distributed erosion modelling predicting capabilities. 相似文献