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   919篇
  免费   61篇
林业   108篇
农学   68篇
基础科学   13篇
  218篇
综合类   57篇
农作物   54篇
水产渔业   48篇
畜牧兽医   323篇
园艺   15篇
植物保护   76篇
  2023年   11篇
  2022年   13篇
  2021年   18篇
  2020年   19篇
  2019年   19篇
  2018年   44篇
  2017年   47篇
  2016年   35篇
  2015年   25篇
  2014年   38篇
  2013年   64篇
  2012年   53篇
  2011年   62篇
  2010年   40篇
  2009年   25篇
  2008年   39篇
  2007年   50篇
  2006年   26篇
  2005年   36篇
  2004年   33篇
  2003年   31篇
  2002年   29篇
  2001年   14篇
  2000年   14篇
  1999年   18篇
  1998年   19篇
  1997年   17篇
  1996年   15篇
  1995年   15篇
  1994年   10篇
  1993年   12篇
  1992年   10篇
  1991年   12篇
  1990年   11篇
  1989年   15篇
  1988年   10篇
  1987年   1篇
  1986年   4篇
  1985年   3篇
  1984年   7篇
  1983年   2篇
  1982年   7篇
  1980年   1篇
  1978年   1篇
  1977年   1篇
  1976年   2篇
  1973年   2篇
排序方式: 共有980条查询结果,搜索用时 15 毫秒
971.
An organo-zeolite was prepared by loading hexadecyltrimethylammonium (HDTMA) onto clinoptilolite and was used to remove ceftriaxone sodium and paracetamol in aqueous solutions. Batch experiments were conducted to perform kinetics and sorption isotherms at 25 °C and 100 rpm. The results indicate that the equilibration times were 24 h for ceftriaxone sodium and 9 h for paracetamol. Furthermore, sorption capacities were 0.7288 and 0.0058 mg/g, respectively. The data were treatment with different models including pseudo first order, second order, and Elovich, the results suggested a chemical adsorption mechanism, and the adsorption equilibrium data for the two drugs show that they follow a linear trend, indicating a partitioning mechanism. Physicochemical properties such as solubility, log Kow, and pka play an important role in the adsorption process. Finally, the values obtained for zero charge point (ZPC) for zeolitic materials were 6.90, 6.94, and 6.90 for natural zeolite (ZN), sodium zeolite (ZNa), and zeolite modified surface at 30 mM HDTMA (ZMS-30), respectively.  相似文献   
972.
973.
Pomegranate bacterial blight disease caused by Xanthomonas axonopodis pv. punicae is one of the most serious threat to the pomegranate industry in India. So far, the chemicals used to control this pathogen alone are less effective. In this article, an attempt was made to supplement chemical control through improving the systemic acquired resistance of plant using nutrients such as nitrogen (N) and salicylic acid (SA) as elicitor. The study revealed that foliar application of SA at the rate of 300 ppm increased above-ground dry mass of plant by 64.97%, improved chlorophyll content of leaves and resulted higher concentration of macro-elements and micro-elements, particularly Mn and Zn, in leaves. It also significantly increased the activities of nitrate reductase (NR), superoxide dismutase (SOD), catalase and peroxidase. However, the maximum response of SA was recorded when it was sprayed after soil application of N at 100% of the recommended dose. As a consequence, this combination recorded the least bacterial blight infection with average severity of 11.58% followed by without-N application under challenge inoculation of the pathogen. However, sustainable production cannot be realized without N application. The integration of this strategy for activation of defence mechanism of plant with the bactericide streptocycline resulted in further reduction of bacterial blight disease incidence and severity under field condition to very low levels (3.84% and 18.57% respectively).  相似文献   
974.
Currently, European farmers do not have access to sufficient numbers and diversity of crop species/varieties. This prevents them from designing cropping systems more resilient to abiotic and biotic stresses. Crop diversification is a key lever to reduce pest (pathogens, animal pests and weeds) pressures at all spatial levels from fields to landscapes. In this context, plant breeding should consist of: (1) increased efforts in the development of new or minor crop varieties to foster diversity in cropping systems, and (2) focus on more resilient varieties showing local adaptation. This new breeding paradigm, called here ‘breeding for integrated pest management (IPM)’, may boost IPM through the development of cultivars with tolerance or resistance to key pests, with the goal of reducing reliance on conventional pesticides. At the same time, this paradigm has legal and practical implications for future breeding programs, including those targeting sustainable agricultural systems. By putting these issues into the context, this article presents the key outcomes of a questionnaire survey and experts' views expressed during an EU workshop entitled ‘Breeding for IPM in sustainable agricultural systems’. © 2017 Society of Chemical Industry  相似文献   
975.
The extrusion process (EP) consists of heat and mechanical treatments under different conditions of moisture, shear, and pressure and rapidly causes structural alterations and changes in the functional properties of the extruded material. The aim of this study was to evaluate the effect of extrusion conditions and optimize the wheat bran extrusion conditions to achieve the greatest content of phenolic compounds and antioxidant activity using response surface methodology. The EP factors evaluated were feed moisture (FM) (25–33.54%) and final extrusion temperature (T) (140–180 °C). The properties evaluated in the extruded material were bound total phenol content (BTPC), total phenolic compounds and antioxidant activity (AOX). Analysis of variance (ANOVA) and response surface methodology were used in the evaluation. The determination coefficients, (FM)2 and (T)2, very significantly affected the BTPC and bound 2,2-diphenyl-1-picrylhydrazyl content (BDPPHC). The optimization was performed by overlaying two contour plots to predict the best combination regions. The optimized extrusion conditions were the following: FM?=?30% and T?=?140 °C, which provided BTPC?=?3547.01 μgGAE/g (predicted: 3589.3 μgGAE/g) and BDPPHC?=?9.5 μmolTE/g (predicted: 10.4 μmolTE/g); and FM?=?30% and T?=?180 °C, which provided BTPC?=?3342.3 μgGAE/g (predicted: 3727.7 μgGAE/g) and BDPPHC?=?9.5 μmolTE/g (predicted: 9.3 μmolTE/g). The EP increased the phenolic compounds and AOX, and enhancement of these properties in wheat bran products could make them functional foods.  相似文献   
976.

Key message

We generate flexible management rules for black pine stands, adaptable to alternative stand management situations and entailing thinnings, final-felling, and salvage cuts, based on the results on 270 stand level optimizations.

Context

Forest management instructions often rely on the anticipated prediction of the stand development, which poses a challenge on variable economic and environmental conditions. Instead, an alternative approach to better adapt forest management decisions to changing conditions is defining flexible rules based on thresholds that trigger management operations.

Aims

This article develops rules for the adaptive management of P. nigra stands in Catalonia (Spain) addressing the risk of fire and post-fire forest management.

Methods

The stochastic version of the simulation-optimization system RODAL was used to optimize the management of forest stands in three sites under different fire probability levels. A total of 270 optimizations were done varying site fertility, fire probability, and economic factors. The results of the optimizations were used as the basis of flexible forest management rules for adaptive stand management.

Results

The developed management rules defined the basal area limit for thinning, the thinning intensity, the mean tree diameter at which regeneration cuttings should start, and the basal area below which a salvage cutting should be done. Fire risk was not a significant predictor of the models for thinning and final cutting rules.

Conclusion

The presented rules provide a flexible tool for forest management during the stand development and under changing conditions when the management objective is to maximize economic profitability of timber production.
  相似文献   
977.
A field trial comprising three levels of zinc (Zn) 0, 5 and 10 kg ha-1, three levels of iron (Fe) 0, 15 and 30 kg ha-1 and three levels of manganese (Mn) 0, 5 and 10 kg ha-1 was carried out during the rainy seasons of 200 8 and 2009 at Varanasi, to study their effects on macro and micro nutrients content, yields and quality of rice variety HUBR 2–1. The experiment was conducted in 33 partial confounding with two replications. Half doses of all the micronutrients were applied as basal and the rest half through foliar application at different intervals. Among the treatments, Zn at 10 kg ha-1, iron at 15 kg ha-1 and Mn at 5 kg ha-1 recorded the maximum yield of rice. A similar trend was observed in all the quality parameters of rice. Individually Zn, Fe and Mn registered, respectively, 12.05, 8.60 and 4.46% more yield than the control.  相似文献   
978.
Silvopastoral systems have great potential for storing carbon because of carbon assimilation in tree woody biomass, carbon input through litterfall and below-ground carbon turnover. In this study, we quantified and compared the carbon stocks at livestock ranches in Tabasco, Mexico, containing either scattered trees in grazing pastures (STP) or grass monocultures. Sampling plots were randomly established at each ranch where the above- and below-ground carbon stocks, carbon input from litterfall, grass production and arboreal biomass growth were measured. We found that silvopastoral systems stored an average of 257.45 Mg ha−1 of soil organic carbon (SOC) compared to 119.17 Mg SOC ha−1 at grass monoculture ranches (to 30 cm depth); silvopastoral systems also stored 44.64 Mg C ha−1 in wood biomass; and, grass monocultures had greater cumulative grass biomass production. Overall, it is concluded that livestock ranches in Tabasco, Mexico, with scattered trees in grazing pastures stored 58.8% more carbon than those grass monocultures, with carbon stocks of 327.01 Mg C ha−1and 134.47 Mg C ha−1, respectively. The results are useful for land management decision making for sustainable livestock systems framed in the Sustainable Development Goals (SDGs).  相似文献   
979.
The sodium hydroxide (NaOH) test for determining wheat color class depends on the observation that on soaking in NaOH, red wheat turns a darker red and white wheat turns straw yellow. To understand the mechanism of this test, Raman spectra of wheat bran, wheat starch, ferulic acid, and whole kernels of wheat, before and after NaOH soak, were studied. The major observable components in the whole kernel were that of starch, protein, and ferulic acid, perhaps esterified to arabinoxylan and sterols. When kernels are soaked in NaOH, spectral bands due to ferulic acid shift to lower energy and show a slightly reduced intensity that is consistent with deprotonation of the phenolic group and extraction of a portion of the ferulic acid into solution. Other phenolic acids, alkyl resorcinols, and flavonoids observed in the NaOH extracts of wheat by HPLC were not observed in the Raman spectra. Wheat bran accounts for most of the ferulic acid in the whole kernel, as indicated by the increased intensity of the doublet at 1,631 and 1,600 cm‐1 in the bran. The intense starch band at 480 cm‐1 in whole kernel wheat was nearly absent in the wheat bran.  相似文献   
980.
In spring lakes, which have homogeneous environmental characteristics, it is expected that there will be no significant changes in the composition and structure of communities over time, and fluctuations will generally be related to the ecological attributes of the species. We studied the fish community in a small subtropical spring lake, Lake Zacapu, for two different decades to analyse its structural characteristics. Sampling was performed every 3 months at four sites from January to October 1995 and from May 2019 to May 2020. We determined the temporal variation (seasonal and decadal) in the fish community structure with respect to composition, abundance, diversity and dominance. We evaluated the association between species and water quality. Our results showed spatial homogeneity in most physicochemical variables that remained in the same range across decades, although some nutrients (NO3) recently increased. All species identified in the early samples were present in the recent surveys (eleven native and two nonnative species), indicating that the fish community composition had significant qualitative stability. However, the dominant littoral cyprinid in 1995 was replaced by the more limnetic atherinopsid in 2019–2020, and the diversity and abundance of sensitive fish decreased in recent samplings. Consequently, the current status of critically endangered species is supported for some restricted and microendemic fish. The changes in the community structure attributes, which show a tendency for reduction in native species, emphasise the need to implement efforts to conserve freshwater fish diversity in small spring lakes.  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