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   70901篇
  免费   3972篇
  国内免费   32篇
林业   3226篇
农学   2529篇
基础科学   477篇
  8698篇
综合类   11418篇
农作物   2609篇
水产渔业   3641篇
畜牧兽医   36630篇
园艺   907篇
植物保护   4770篇
  2018年   1053篇
  2017年   1130篇
  2016年   1056篇
  2015年   908篇
  2014年   1136篇
  2013年   2629篇
  2012年   2058篇
  2011年   2461篇
  2010年   1683篇
  2009年   1751篇
  2008年   2592篇
  2007年   2363篇
  2006年   2326篇
  2005年   2125篇
  2004年   2193篇
  2003年   2166篇
  2002年   2036篇
  2001年   2366篇
  2000年   2375篇
  1999年   1925篇
  1998年   912篇
  1997年   821篇
  1996年   782篇
  1995年   861篇
  1994年   843篇
  1993年   767篇
  1992年   1523篇
  1991年   1542篇
  1990年   1662篇
  1989年   1507篇
  1988年   1450篇
  1987年   1340篇
  1986年   1384篇
  1985年   1336篇
  1984年   1128篇
  1983年   1025篇
  1982年   628篇
  1979年   1004篇
  1978年   795篇
  1977年   717篇
  1976年   676篇
  1975年   719篇
  1974年   766篇
  1973年   830篇
  1972年   803篇
  1971年   748篇
  1970年   729篇
  1969年   744篇
  1967年   658篇
  1966年   631篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Soil-surface CO2 efflux (FS; ‘soil respiration’) accounts for ≥50% of the CO2 released annually by the terrestrial biosphere to the atmosphere, and the magnitude and variability of this flux are likely to be sensitive to climate change. We measured FS in nine permanent plots along a 5.2 °C mean annual temperature (MAT) gradient (13-18.2 °C) in Hawaiian tropical montane wet forests where substrate type and age, soil type, soil water balance, disturbance history, and canopy vegetation are constant. The objectives of this study were to quantify how the (i) magnitude, (ii) plot-level spatial variability, and (iii) plot-level diel variability of FS vary with MAT. To address the first objective, annual FS budgets were constructed by measuring instantaneous FS monthly in all plots for one year. For the second objective, we compared plot-level mean instantaneous FS in six plots derived from 8 versus 16 measurements, and conducted a power analysis to determine adequate sample sizes. For the third objective, we measured instantaneous FS hourly for 24 h in three plots (cool, intermediate and warm MATs). The magnitude of annual FS and the spatial variability of plot-level instantaneous FS increased linearly with MAT, likely due to concomitant increases in stand productivity. Mean plot-level instantaneous FS from 8 versus 16 measurements per plot yielded statistically similar patterns. The number of samples required to estimate plot-level instantaneous FS within 10% and 20% of the actual mean increased with MAT. In two of three plots examined, diel variability in instantaneous FS was significantly correlated with soil temperature but minimal diel fluctuations in soil temperature (<0.6 °C) resulted in minimal diel variability in FS. Our results suggest that as MAT increases in tropical montane wet forests, FS will increase and become more spatially variable if ecosystem characteristics and functioning undergo concurrent changes as measured along this gradient. However, diel variation in FS will remain a minor component of overall plot-level variation.  相似文献   
982.
Partial defoliation has been shown to affect the water relations and transpiration (gas exchange) of plants. Over one growing season, the water relations in response to partial (∼45%) defoliation were examined in four-year-old Eucalyptus globulus trees in southern Australia. Daily maximum transpiration rates (Emax), maximum canopy conductance (GCmax), and diurnal patterns of tree water-use were measured over a period of 215 days using the heat-pulse technique in adjacent control (non-defoliated) and defoliated trees. Sap-flux measurements were used to estimate canopy conductance and soil-to-leaf hydraulic conductance (KP); leaf water potential (Ψ) and climate data were also collected. Following the removal of the upper canopy layer, defoliated trees exhibited compensatory responses in transpiration rate and canopy conductance of the remaining foliage. Defoliated E. globulus had similar predawn but higher midday Ψl, transpiration rates (E), canopy conductance (GC) and KP compared to the non-defoliated controls, possibly in response to increased water supply per unit leaf area demonstrated by higher midday Ψl. Higher E in defoliated E. globulus trees was the result of higher GC in the morning and early afternoon. This paper also incorporates the cumulative effect of defoliation, in a phenomenological model of maximum canopy conductance of E. globulus. These results contribute to a mechanistic understanding of plant responses to defoliation, in particular the often observed up-regulation of photosynthesis that also occurs in response to defoliation.  相似文献   
983.
984.
Our main goal was to investigate if robust chemical fingerprints could be developed for three Argentinean red wines based on organic, inorganic, and isotopic patterns, in relation to the regional soil composition. Soils and wines from three regions (Mendoza, San Juan, and Co?rdoba) and three varieties (Cabernet Sauvignon, Malbec, and Syrah) were collected. The phenolic profile was determined by HPLC-MS/MS and multielemental composition by ICP-MS; (87)Sr/(86)Sr and δ(13)C were determined by TIMS and IRMS, respectively. Chemometrics allowed robust differentiation between regions, wine varieties, and the same variety from different regions. Among phenolic compounds, resveratrol concentration was the most useful marker for wine differentiation, whereas Mg, K/Rb, Ca/Sr, and (87)Sr/(86)Sr were the main inorganic and isotopic parameters selected. Generalized Procrustes analysis (GPA) using two studied matrices (wine and soil) shows consensus between them and clear differences between studied areas. Finally, we applied a canonical correlation analysis, demonstrating significant correlation (r = 0.99; p < 0.001) between soil and wine composition. To our knowledge this is the first report combining independent variables, constructing a fingerprint including elemental composition, isotopic, and polyphenol patterns to differentiate wines, matching part of this fingerprint with the soil provenance.  相似文献   
985.
A fixed-bed photocatalytic reactor equipped with a cylindrical parabolic light concentrator was studied to remove organic dyes from water using natural volcanic ashes particles and nanostructured titania supported on volcanic ashes as photocatalytic materials. The influences of flow rate, photocatalyst and photocatalytic material adsorption capacity were studied. A fixed-bed photocatalytic reactor was designed and built in the laboratory; a methylene blue aqueous solution, used as model compound for dye contaminated water, was fed into the reactor. Methylene blue destruction efficiencies were monitored spectrophotometrically. Combined effects of dye adsorption and photodecomposition on photocatalyst were studied and compared by infrared spectroscopy.  相似文献   
986.
Partial rootzone drying (PRD) is a water‐saving irrigation technology that may affect apple (Malus domestica Borkh cv. Golden Delicious/Malling7)‐tree nutrition if applied for an extended period. The objective of this study was to test the hypothesis that long‐term application of PRD causes seasonal changes in macro‐ and micronutrients of apple leaves. The irrigation treatments were: (1) commercial irrigation as control (CI) and (2) PRD. After 3 years of evaluation, PRD irrigation had saved about 3240 m3 of water per hectare. Leaf xylem water potential was slightly lower in the PRD treatment than in CI. The seasonal concentration of macro‐ and micronutrients was comparable between treatments, although significant differences were found at times. The macronutrient concentrations were within the normal range in PRD apple leaves. All micronutrient concentrations were slightly above the normal range except for Zn, which was slightly below the normal range. No physiological disorders associated with plant nutrition were observed on leaves or fruits. Therefore, data suggest that PRD did not alter apple‐tree nutrition during the 3‐year trial. Thus, PRD may be feasible for apple production in Central Mexico. However, further studies need to be conducted in those regions where groundwater is the main water source for irrigation and rain is negligible, particularly during the growing season.  相似文献   
987.
Soil applications of recalcitrant biochar offer the possibility of mitigating climate change effects through long‐term carbon sequestration and potentially also by reducing emissions of the potent greenhouse gas nitrous oxide (N2O). This laboratory study examined the effect of combining a fast‐pyrolysis biochar at small (1% by mass) and large (3%) concentrations with anaerobically digested slurry on soil N2O and carbon dioxide (CO2) emissions over a period of 55 days. The results showed that fast‐pyrolysis biochar applied on its own increased N2O emissions from soil. However, when biochar was applied together with slurry, the larger biochar concentration decreased N2O emissions by 47%, relative to those from the slurry treatment with the smaller biochar concentration. Reduced N2O emissions coincided with enhanced soil microbial activity and immobilization of nitrogen. A combined application of biochar and anaerobic digested slurry could therefore be beneficial for cropping systems in terms of soil nitrogen retention while concurrently mitigating N2O fluxes and sequestering carbon in soil.  相似文献   
988.
Capparis decidua (Forsk.) Edgew. commonly known as “Kair”, is an important shrub widely distributed in arid and semi-arid regions. The species has been used for making pickle and vegetables and number of other uses that include medicine, fuel wood and fodder. It is tolerant to temperature, salt and drought stresses and helps in arresting wind erosion and improving soil fertility. This paper is an attempt to provide scientific information pertaining to distribution, morphology, propagation, phenological characteristics, diversity and uses of the species for stimulating interest to promote its domestication and commercialization.  相似文献   
989.
The requirement of mint oil and menthol in India has been estimated to be around 100 tons a year which is increasing consistently year after year (47). Till recently, almost all the quantity of mint oil and menthol in this country was imported. At present, a portion of it is being met by indigenous production and Japanese mint is credited to be the sole source for that. It may, however, be mentioned that Japanese mint was introduced in India in early fifties from Japan through the good offices of UNESCO (25). This plant has found the new habitat quite suitable making possible its successful cultivation in the variousp arts of the country on commercial scale (7, 12, 13, 14, 18, 19, 29, 32, 34, 35, 41, 45, 46). Its oil which is equally comparable to Japanese and Drazillinn grown mint oil contains more than 70 per cent of menthol, 25–30 per cent of which can easily be isolated on cooling (25, 37).  相似文献   
990.
An estimated 40% of arable soils worldwide contain phytotoxic levels of aluminum (Al). Recent evidence indicates that Al‐stress‐induced low molecular weight proteins may bind Al in Al‐tolerant plants. The objective of this study was to investigate protein patterns in young roots of two Al‐sensitive and two Al‐tolerant alfalfa (Medicago sativa L.) clones grown at 0 and 111 μmol Al in pH 4.5 nutrient solution. Based on SDS‐PAGE of supernatant, Al stress resulted in an increase in detectable root proteins in all clones and results are consistent with results reported for other plant species. A proliferation of new low‐molecular‐weight proteins in the tolerant clones could be related to Al tolerance. One protein (18.7 kD) was produced in both tolerant clones yet not detected in the sensitive clones. Protein levels were more often reduced than enhanced under Al stress and reduction was more prevalent in sensitive than in tolerant clones. Aluminum stress may initiate the production of some of the same proteins in alfalfa and wheat (Triticum aestivum L. Thell.). General stress proteins could be produced in reaction to a variety of chemical, environmental, and pathological stresses.  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