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791.
In this study we show how substantial gains towards the goals of ecosystem-based fisheries management (EBFM) can be achieved by different single-species management. We show that fishing has much less impact on stocks if fish are caught after they have reached the size (Lopt) where growth rate and cohort biomass are maximum. To demonstrate our point we compare the impact of three fishing scenarios on 9 stocks from the North Sea and the Baltic. Scenario (1) is the current fishing regime, scenario (2) is a new management regime proposed by the European Commission, aiming for maximum sustainable yield obtained from all stocks, and scenario (3) is set so that it achieves the same yield as scenario (2), albeit with fishing on sizes beyond Lopt. Results show that scenarios (2) and (3) are significant improvements compared to current fishing practice. However, scenario (3) consistently shows least impact on the stocks, with seven-fold higher biomass of demersal fishes and an age structure similar to an unfished stock. This allows juveniles and adults to better fulfil their ecological roles, a major step towards the goals of ecosystem-based fisheries management. We give examples where scenario (3) is practiced in successful fisheries. We present a new interpretation of the relative yield per recruit isopleth diagram with indication of a new target area for fisheries operating within the context of EBFM. We present a new expression of the relative biomass per recruit isopleth diagram, which supports our analysis. We conclude that size matters for precautionary and ecosystem-based fisheries management and present a list of additional advantages associated with fishing at Lopt.  相似文献   
792.
Temporal and spatial variability of hydrological parameters of single soil aggregates The hydrological parameters determined for a single soil profile are usually not assumed to be variable and are often taken to represent a larger area and longer period of time, without consideration of spatial and temporal changes in the environment like cultivation or weather. Naturally structured and homogenized, subsequently restructuring aggregates from a “Stagno-haplic Luvisol” (FAO, 1991) with different possibilities of swelling were analysed with regard to the characteristics of the water retention curve and hydraulic conductivity. The experiments verified that the pattern of the water retention curve with respect to single aggregates was not constant but depended on the aggregate's initial intensity and frequency of desiccation. Additionally, effects on the hydraulic conductivity were determined. The measured values varied with the amount of swelling and shrinking, to which the aggregates were exposed. Thus, the calculation of hydraulic conductivity according to the model of Van Genuchten (1987) is doubtful. Therefore, the quantification of ecological characteristics from such parameters has to be questioned. The main cause for the described effects can be seen in the induced and only partly reversible structuring process, i.g. aggregate compaction due to shrinkage and rearrangement of particles inside the aggregate during repeated swelling and shrinkage. Additionally, physico-chemical hydrophobicity was supposed to have a major influence under certain circumstances.  相似文献   
793.
The effects of adding straw and bark compost at different nitrogen levels to fuel oil contaminated soil was investigated using the CHCl3 fumigation-extraction method to measure the soil microbial biomass, The addition of these substrates decreased the percentage of extractable fuel oil, increased the percentage of fuel oil mineralized to CO2, but also increased the fraction of unrecovered fuel oil residues. The different N levels had no significant effects on fuel oil decomposition in the presence of the compost substrates. The addition of the compost substrates increased markedly the content of microbial biomass in contrast to the fuel oil addition which had no effect on the biomass level indicating toxic properties.  相似文献   
794.
Although Brachyspira (B.) hyodysenteriae and Lawsonia (L.) intracellularis are widely distributed in pigs in Germany, there exists limited information on their clinical relevance.To get more insight into their potential role in swine diarrhoeal disease, in 2002 and 2003 faecal specimens from healthy pigs (n=1445) as well as from diarrhoeic pigs (n=2002) were tested by polymerase chain reaction (PCR) for the presence of both agents. Of the specimens from healthy pigs L. intracellularis and B. hyodysenteriae were detected in 7.3% and 6.7%, respectively. In contrast, of the diarrhoeic pigs the ratios of positive samples amounted to 19.4% for L. intracellularis and 17.9% for B. hyodysenteriae. Concerning the age of the diseased animals, in growing pigs the detection rates of L. intracellularis and B. hyodysenteriae were nearly identical (16.4% and 14.2%, respectively). In fattening pigs a significant higher number of animals were affected with B. hyodysenteriae (35.8%) than with L. intracellularis (28.2%). On the other hand, in sows L. intracellularis (35.6% positive samples) was dominant compared to B. hyodysenteriae (21.2% positive samples). Considering the nearly threefold higher percentage rates of L. intracellularis and B. hyodysenteriae in diarrhoeic pigs in comparison to healthy pigs, it is concluded that both agents play an important role in swine diarrhoeal disease. The results further indicated that in fattening pigs B. hyodysenteriae and in sows L. intracellularis have a dominant role, respectively.  相似文献   
795.
Wheeling induced compression and shearing forces are main stresses accounting for soil deformation and changes of hydraulic, gaseous and thermal properties. There are reports about the combined effects of compaction and subsequent shearing on soil hydraulic properties, but their consequences on soil strength properties (i.e., effective stress and shear strength) need to be further analysed. This study investigated the dynamics of soil mechanical properties as affected by pore water pressure ( u w $$ {u}_{\mathrm{w}} $$ ) during compaction and shearing. Soil samples from an A-horizon of Gleysols derived from glacial sediment and a Stagnic Luvisol from loess were analysed. The repacked and structured samples were compressed under static and cyclic loading and then sheared at two speeds (0.3 and 2.0 mm min−1) with three loading levels (50, 100, and 200 kPa). During each stress application, the u w $$ {u}_{\mathrm{w}} $$ , chi factor (χ) and effective stress ( σ $$ {\sigma}^{\prime } $$ ) were measured and calculated. The shear strength ( τ $$ \tau $$ ), angle of internal friction ( φ $$ \varphi $$ ) and cohesion ( c $$ c $$ ), were determined and fitted by the Mohr–Coulomb failure criterion. The results showed that compaction and shearing increased u w $$ {u}_{\mathrm{w}} $$ and χ in all homogenized soils while on structured soils this phenomenon only occurred when the applied loading stress exceeded the soil precompression stress. The increased u w $$ {u}_{\mathrm{w}} $$ resulted in soil hydraulic and mechanical stresses, which ultimately reduced the σ $$ {\sigma}^{\prime } $$ , especially at −6 kPa initial matric potential. Soils with finer texture, higher loading stresses and faster shear speed normally exhibited more reduced σ $$ {\sigma}^{\prime } $$ values. The structured soil had higher τ $$ \tau $$ values with higher φ $$ \varphi $$ and c $$ c $$ compared to the homogenized soils. The changes of u w $$ {u}_{\mathrm{w}} $$ at high loading stress (i.e., 200 kPa) may overlap the normal pattern of the Mohr–Coulomb failure line that results from the theoretical Mohr envelope. Thus, to minimize the destruction of soil structure and stability induced by wheeling, it is important to consider field water content, traffic loading and wheeling speed.  相似文献   
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