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   764篇
  免费   51篇
林业   15篇
农学   24篇
  113篇
综合类   96篇
农作物   25篇
水产渔业   41篇
畜牧兽医   389篇
园艺   42篇
植物保护   70篇
  2023年   8篇
  2022年   8篇
  2021年   8篇
  2020年   15篇
  2019年   8篇
  2018年   6篇
  2017年   14篇
  2016年   14篇
  2015年   17篇
  2014年   16篇
  2013年   42篇
  2012年   43篇
  2011年   47篇
  2010年   14篇
  2009年   23篇
  2008年   55篇
  2007年   40篇
  2006年   51篇
  2005年   41篇
  2004年   56篇
  2003年   52篇
  2002年   33篇
  2001年   10篇
  2000年   8篇
  1999年   5篇
  1998年   9篇
  1997年   8篇
  1994年   7篇
  1993年   10篇
  1992年   5篇
  1989年   8篇
  1985年   6篇
  1984年   5篇
  1983年   5篇
  1981年   9篇
  1980年   5篇
  1978年   6篇
  1977年   3篇
  1976年   6篇
  1975年   4篇
  1974年   5篇
  1973年   9篇
  1972年   5篇
  1970年   9篇
  1969年   3篇
  1968年   3篇
  1965年   3篇
  1958年   3篇
  1954年   3篇
  1953年   4篇
排序方式: 共有815条查询结果,搜索用时 15 毫秒
71.
Among Aboriginal peoples in Canada, mercury pollution has had an impact on social and cultural well being, both at the community level and among individual members within the community, even when there are no provable direct health effects. This paper explores the link between mercury pollution and social and cultural disruption in Canadian Aboriginal communities. It demonstrates how factors such as Aboriginal holistic concepts of environment and health, perceptions, and difficulties in communication can contribute to indirect impacts of mercury pollution on health and well being. It is argued that, given the blood mercury levels among Canadian Aboriginal peoples, social and cultural impacts of mercury pollution present a much greater problem than do the direct clinical effects of exposure.  相似文献   
72.
The endemic Seram cockatoo, Cacatua moluccensis, was placed on Appendix I of CITES in response to declining trade statistics but in the absence of population data. We conducted population surveys and collected data on habitat structure at seven sites on Seram in 1998. Cockatoo densities ranged from 0.93 to 17.25 birds/km2 and averaged 7.9 birds/km2 across sites. We classified habitat into three types but found that cockatoo densities did not correspond closely to habitat differences across sites. Cockatoo abundance was significantly associated with presence of potential nest trees and strangling figs. Analysis of forest cover and landuse indicates that while most of the island is still covered in lowland forest, only 14% of these forests are protected and almost half the island is classified as logging concession. Conservation recommendations include better information and enforcement of laws protecting Seram cockatoos and resolution of boundary conflicts between parks and logging concessions.  相似文献   
73.
In the mid 1980s, Asian elephant (Elephas maximus) populations were believed to persist in 44 populations on the Indonesian island of Sumatra. Twelve of these populations occurred in Lampung Province, but our surveys revealed that only three were extant in 2002. Causal factors underlying this decline include human population growth, changes in land use, and human-elephant conflict. Nevertheless, our surveys in the Province’s two national parks, Bukit Barisan Selatan and Way Kambas, produced population estimates of 498 (95% CI = [373, 666]) and 180 (95% CI = [144, 225]) elephants, respectively. The estimate for Bukit Barisan Selatan is much larger than previous estimates; the estimate for Way Kambas falls between previous estimates. The third population was much smaller and may not be viable. These are the first estimates for Southeast Asian elephant populations based on rigorous sampling-based methods that satisfied the assumptions of the models used, and they suggest that elephant numbers in these parks are of international importance. While our results suggest that Sumatra’s remaining elephant populations may be larger than expected, they also suggest that the future for these animals is bleak. Human-elephant conflict was reported around all three areas in Lampung and their elephant populations are currently threatened by habitat loss and poaching. Local solutions are possible, but will require much greater commitment by all stakeholders.  相似文献   
74.
Water, Air, &; Soil Pollution - The quality of the indoor environment has become a major health consideration, since urban-dwellers spend 80-90% of their time indoors, where air pollution can be...  相似文献   
75.
Trenches excavated across fault scarps on a 13 ka delta surface at Brigham City, Utah, USA, document four types of soil-catena phenomena. First, the relict summit soil is eroded at the scarp crest, and its components incorporated into the accumulating colluvial wedge at the scarp base. Second, the pre-faulting soil weakens laterally as it is traced scarpward beneath the colluvial wedge from a relict position beyond the scarp toe. Third, soils developed on individual colluvial wedges and on the underlying, pre-faulting surface merge downslope to form a cumulative soil profile at the scarp toeslope. Fourth, soil horizons thicken downslope, probably due to the combined affect of more available moisture, water infiltration, and sediment accumulation.Two soil catenas were used to evaluate the applicability of a continuity approach to paleoearthquake dating, by which percentages of total development time represented by each soil at a fault scarp footslope could be estimated. The continuity approach sets total soil development at the scarp footslope, including any buried pre-faulting soil plus the sum development of all colluvial wedge soils corrected by a slope factor (F), equal to total soil development at the scarp summit. Soil development was characterized by Profile Development Index (PDI) and pedogenic clay parameters, using facies models to estimate parent material values for genetically-different colluvial units, and recognizing inherited soil components in colluvial wedges as parent material properties. The F slope factor, used to correct colluvial wedge soils for development attributable to slope position rather than age, was checked by using a nearby fluvial scarp as an independent control. Our soil data, when compared to independent age estimates, imply that rates of soil development have not been constant over the last 13 ka at Brigham City, but rather were considerably higher than average between 13 ka and 8.3 ka. Higher rates are probably due to a relatively high influx of eolian dust during the late Pleistocene to early Holocene following the lowering of Lake Bonneville, a large paleolake in the western USA. These variable rates of soil development preclude using the continuity approach for paleoearthquake dating at Brigham City. However, promising results showing that generally consistent F factors can be calculated for fault and fluvial scarps leave open the possibility that in areas where soil development rates have been relatively constant over the time period in question, the continuity approach may be a broadly applicable method for paleoearthquake dating.  相似文献   
76.
Spraying programmes for apple scab and mildew control sometimes had residual effects for one or even two years. Thus, benomyl (0.025% a.i.) and pyridinitril (0.035% a.i.) drastically reduced the subsequent incidence of cankers caused by Nectria galligena on young wood. Apple scab fruit infection, caused by Venturia inaequalis, was also less on trees on which benomyl had been used in the previous season. In the bark of such trees, toxic residues equivalent to 1 to 4 μg benomyl/g fresh wt. were detected by bioassay 7 months after spraying.  相似文献   
77.
In laboratory tests with 115 species of dicotyledons, the concentration of pronamide in water for 50% inhibition of root elongation ranged from 0.03 to >8 parts/million. With certain exceptions, species within a family responded similarly. Polygonaceae and Caryophyllaceae were the most susceptible and Compositae the most tolerant, as they were also to chlorpropham. There was good correspondence between the test results and the relative susceptibility of species to pre-emergence application in the field.  相似文献   
78.
Bromoxynil octanoate labelled with 14C in the ring or in the cyano-group was applied to wheat seedlings at the two-leaf or fully-tillered stage and at rates equivalent to up to 16 oz a.i./acre. The plants were grown either in environmental chambers under controlled conditions for up to 28 days, or outdoors under field conditions for various periods up to harvest. Initially, elimination of radioactivity occurred more rapidly with bromoxynil-cyano-[14C]-octanoate than with bromoxynil-ring-[14C]-octanoate, indicating metabolic attack on the cyano group. Under outdoor conditions with ring-[14C]-herbicide applied at the two-leaf stage, only 12% of the radioactivity was retained after 28 days, principally in the treated leaves. When application was made at fully-tillered stage, about 33% of the 14C was retained after 56 days, almost entirely in the treated senescent leaves at the base of the plant. There was very little translocation of the herbicide or of any major metabolite. The level of radioactivity in harvested grain and in straw more than 7.5 cm above the ground was very low, even after very late application of ring-[14C]-labelled herbicide. The amount of bromoxynil octanoate, together with any metabolite retaining part of the aromatic ring, did not collectively exceed the equivalent of approx. 0.01 parts/million bromoxynil octanoate.  相似文献   
79.
80.
The yields of inflorescences, corms and cormlets, and the inflorescence quality of winter-grown gladioli in south-east Queensland, were studied under 2 daylength regimes, 3 plant densities (150 000, 300 000 and 450 000 corms ha?1) and 3 plant arrangements (bed, double row and single row). The daylength treatments were the natural daylength (12.3–14.5 hours) and a 24-hour photoperiod treatment in which natural daylengths were extended using incandescent light of an intensity of 150 lux. Two cultivars were used.Extending the photoperiod to 24 hours delayed flowering by approximately 15 days, and increased the number of inflorescences harvested from low, medium and high density treatments by 20, 91 and 169%, respectively, when compared to the inflorescence yield from these density treatments under natural daylengths. The quality of the inflorescences from the high-density treatment receiving the 24-hour photoperiod was similar or superior in all quality characteristics to that of inflorescences grown under the most favourable density treatment under natural daylengths (150 000 corm ha?1). Plant arrangement had little effect on the number of days to flowering or inflorescence yield but inflorescence quality was improved when plants were grown in a double-row arrangement compared to those from the bed or single-row arrangements. Extension of the photoperiod had no effect on the number of new corms per plot. However, the average weight of new corms and the weight of cormlets per plot and per corm were reduced by approximately 32, 71 and 63%, respectively, when compared to the results obtained from plants grown under natural daylengths. These results suggest that flowers compete for available photosynthates with corms and cormlet development.The economic feasibility of extending photoperiod for the commercial production of winter-grown gladioli in south-east Queensland is discussed.  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