全文获取类型
收费全文 | 4661篇 |
免费 | 170篇 |
出版年
2023年 | 31篇 |
2020年 | 76篇 |
2019年 | 69篇 |
2018年 | 131篇 |
2017年 | 138篇 |
2016年 | 167篇 |
2015年 | 115篇 |
2014年 | 116篇 |
2013年 | 1282篇 |
2012年 | 131篇 |
2011年 | 155篇 |
2010年 | 145篇 |
2009年 | 108篇 |
2008年 | 126篇 |
2007年 | 101篇 |
2006年 | 77篇 |
2005年 | 93篇 |
2004年 | 81篇 |
2003年 | 39篇 |
2002年 | 57篇 |
2001年 | 71篇 |
2000年 | 56篇 |
1999年 | 68篇 |
1998年 | 46篇 |
1997年 | 48篇 |
1996年 | 43篇 |
1995年 | 33篇 |
1994年 | 40篇 |
1993年 | 44篇 |
1992年 | 45篇 |
1991年 | 35篇 |
1989年 | 37篇 |
1988年 | 36篇 |
1987年 | 46篇 |
1986年 | 32篇 |
1985年 | 52篇 |
1984年 | 40篇 |
1983年 | 66篇 |
1982年 | 48篇 |
1981年 | 56篇 |
1980年 | 54篇 |
1979年 | 39篇 |
1978年 | 40篇 |
1977年 | 58篇 |
1976年 | 42篇 |
1975年 | 45篇 |
1974年 | 31篇 |
1973年 | 31篇 |
1972年 | 29篇 |
1971年 | 33篇 |
排序方式: 共有4831条查询结果,搜索用时 15 毫秒
101.
Acid sulphate soils form when iron pyrite oxidises to sulphuric acid on atmospheric exposure, lowering soil pH below 4. In the Richmond River estuary, northern New South Wales, flood mitigation, drainage works and floodplain excavations have augmented acid sulphate soil formation by increasing pyrite availability for oxidation. These engineering works have facilitated the transport of acidified water and have impeded recovery from tributary acidification. The increased frequency and duration of acidification are associated with elevated concentrations of inorganic monomeric aluminium and dissolved iron, and major iron hydroxide precipitation, with deleterious effects on exposed aquatic biota and habitat. Fish kills and an outbreak of epizootic ulcerative syndrome (EUS) have been recorded from acidified sites in the estuary. 相似文献
102.
G.J.R. LINGE 《Geographical Research》1995,33(1):116-132
The sovereignty of the bleak Kurile islands, off northern Japan, has long been the subject of a bitter stand-off between Russia and Japan. This has not only prevented their signing a post-1945 Peace Treaty but has retarded the flow of official financial assistance and corporate investment funds across the few hundred kilometre-wide Sea of Japan. As a result the reconstruction of the Russian Far East has been made more difficult, thus exacerbating the economic and social hardship there. Examples like this should cause us to reflect on the dangers of making simplistic explanations about the spatial organisation of economic activity as well as the need to rediscover political geography. 相似文献
103.
104.
105.
106.
107.
108.
109.
Taavitsainen Jussi-Pekka Simola Heikki Grönlund Elisabeth 《Journal of World Prehistory》1998,12(2):199-253
We have investigated the environmental history of human occupation and the development of agriculture in the eastern interior Lake District of Finland. The material consists of archaeological data, which is reviewed in topographical and agrogeological context, and pollen analytical evidence of agricultural indices from eight precisely dated (varved) lake sediment sequences. Before the Viking Age, archaeological evidence, consisting of stray finds, dwelling sites, and graves, is very scarce. Iron Age finds are clearly confined to the lowland environs with silty and clayey soils. During the Viking Age, the number of stray finds multiplies and the first cemeteries are established. Comparison between Viking and Crusade Period finds reveals a topographic shift toward higher locations and morainic soils. Most of the cup-stones are located on upland sites—that is, not in connection with known Iron Age sites. These are interpreted as medieval indicators of slash-and-burn farming of the fertile but stony supraaquatic morainic soils. There is pollen analytical evidence of sporadic cultivation in the area from the Bronze Age onward. Afterca. AD 700, the occurrence of cereal pollen grains becomes regular but remains discontinuous at each site until after the turn of the millennium. There is then an exponential rise in the cereal pollen rain, indicating a fully agricultural population. 相似文献
110.