全文获取类型
收费全文 | 535篇 |
免费 | 55篇 |
出版年
2023年 | 4篇 |
2022年 | 3篇 |
2021年 | 7篇 |
2020年 | 3篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 5篇 |
2016年 | 14篇 |
2015年 | 27篇 |
2014年 | 16篇 |
2013年 | 38篇 |
2012年 | 35篇 |
2011年 | 33篇 |
2010年 | 22篇 |
2009年 | 24篇 |
2008年 | 31篇 |
2007年 | 30篇 |
2006年 | 24篇 |
2005年 | 29篇 |
2004年 | 36篇 |
2003年 | 26篇 |
2002年 | 21篇 |
2001年 | 9篇 |
2000年 | 6篇 |
1999年 | 4篇 |
1998年 | 11篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 16篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1969年 | 1篇 |
1953年 | 1篇 |
1938年 | 3篇 |
1937年 | 2篇 |
1932年 | 1篇 |
1927年 | 1篇 |
排序方式: 共有590条查询结果,搜索用时 31 毫秒
581.
582.
Mikania grazielae afforded two new germacranolides which were identified as 2α-acetoxyeupatolide and 2α-acetoxylaurenobiolide. 相似文献
583.
Esther C. H. E. T. Lucassen Alfons J. P. Smolders Leon P. M. Lamers Jan G. M. Roelofs 《Plant and Soil》2005,269(1-2):109-115
Nitrate leaching from agricultural land leads to oxidiation of FeS
x
in FeS
x
-containing subsoils resulting in SO
4
2−
mobilisation. Pollution of the groundwater with SO
4
2−
causes a higher availability of o-PO
4
3−
, eutrophication and loss in biodiversity in groundwater fed fens with stagnating surface water. Under natural conditions,
fens along the river Meuse are continuously fed by groundwater that besides SO
4
2−
mostly also contains high concentrations of NO
3
−
and bivalent cations (Ca2+ and mg2+). During summer groundwater input is restricted resulting in periodic drought. Under these conditions no SO
4
2−
induced o-PO
4
3−
eutrophication occurs. Periodic drought and a high discharge of NO
3
−
, have a strong effect on S and P biogeochemistry in sulphate-rich fens. NO
3
−
inhibits SO
4
2−
reduction and concomitant o-PO
4
3−
mobilisation in fen sediments by being an energetically more favourable electron acceptor. In addition, NO
3
−
is capable of oxidising reduced Fe compounds, including FeS
x
, increasing the amount of oxidised Fe in the sediment capable of binding o-PO
4
3−
. Periodic drought is important in reincreasing the concentration of oxidised Fe in the top layer of S-rich sediments preventing
o-PO
4
3−
mobilisation and an undesirable vegetation development. Damming of surface water, in order te restore desiccated sulphate-rich
fens, prevents periodic drought and decreases groundwater input. This leads to NO
3
−
depletion, stimulation of SO
4
2−
reduction, Fe depletion, o-PO
4
3−
mobilisation and, in contrast to what was hoped for, in massive growth of algae, lemnids and fast growing wetland grasses.
Therefore discharge of NO
3
−
– rich groundwater and the fluctuation of the water table are vital for succesful restoration of desiccated sulphate-rich
fens. Successful rewetting of these type of fens, without causing stagnation of surface water and without preventing periodic
drought, can be achieved by raising the water table to levels below the potential groundwater table using a controllable dam. 相似文献
584.
M. Salomé Duarte Andreia F. Salvador Ana J. Cavaleiro Alfons J. M. Stams M. Alcina Pereira M. Madalena Alves 《Environmental microbiology》2020,22(9):3650-3659
Anaerobic degradation of long-chain fatty acids (LCFA) involves syntrophic bacteria and methanogens, but facultative anaerobic bacteria (FAB) might have a relevant role as well. Here we investigated oleate degradation by a syntrophic synthetic co-culture of Syntrophomonas zehnderi (Sz) and Methanobacterium formicicum (Mf) and FAB (two oleate-degrading Pseudomonas spp. I1 + I2). Sz + Mf were first cultivated in a continuous bioreactor under strict anaerobic conditions. Thereafter, I1 + I2 were inoculated and microaerophilic conditions were provided. Methane and acetate were the main degradation products by Sz + Mf in anaerobiosis and by Sz + Mf + I1 + I2 in microaerophilic conditions. However, acetate production from oleate was higher in microaerophilic conditions (5% O2) with the four microorganisms together (0.41 ± 0.07 mmol day−1) than in anaerobiosis with Sz + Mf (0.23 ± 0.05 mmol day−1). Oleate degradation in batch assays was faster by Sz + Mf + I1 + I2 (under microaerophilic conditions) than by Sz + Mf alone (under strict anaerobic conditions). I1 + I2 were able to grow with oleate and with intermediates of oleate degradation (hydrogen, acetate and formate). This work highlights the importance of FAB, particularly Pseudomonas sp., in anaerobic reactors treating oleate-based wastewater, because they accelerate oleate conversion to methane, by protecting strict anaerobes from oxygen toxicity and also by acting as alternative hydrogen/formate and acetate scavengers for LCFA-degrading anaerobes. 相似文献
585.
In this note, we reported the decline of weasel populations in NE Spain between the end of the last century and present. Our results suggested a delayed numerical response of weasels to wood mice abundance during the first study period (1995–1998), but no response at all in the second and longer period (2008–2015). Small mammals’ outbreaks triggered weasel’ populations in the first period, but they did not in the second period. Other factors may be invoked to explain the recent lack of numerical responses of weasels to mice. Population declines of small mammals have been noted in the study area in recent years, whereas an increase of casual predation records of weasels by generalist raptors has been documented. Both trends seemed to be related to the effects of the process of natural afforestation experienced by Mediterranean areas due to land abandonment. Top-down increased predation may add to bottom-up decreasing prey availability to explain current weasel declines. 相似文献
586.
Tzachor Asaf Smidt-Jensen Asger Ramel Alfons Geirsdóttir Margrét 《Marine biotechnology (New York, N.Y.)》2022,24(5):991-1001
Marine Biotechnology - Spirulina algae (Spirulina platensis) cultivated in geothermally powered photobioreactors is here proposed as a potentially resource efficient, zero-carbon, and nutritious... 相似文献
587.
Stefanie J.W.H. Oude Elferink Piet N.L. Lens Cor Dijkema Alfons J.M. Stams 《FEMS microbiology letters》1996,142(2-3):237-241
Abstract The isomerization of butyrate and isobutyrate was investigated for the sulfate reducer Desulforhabdus amnigenus . Nuclear magnetic resonance (NMR) studies with 13 C-labelled butyrate showed that isobutyrate was formed by migration of the carboxyl group, in conformity with the butyrate isomerization reaction reported for methanogenic consortia. In addition to D. amnigenus , several other butyrate-degrading sulfate reducers ( Desulfobacterium vacuolatum, Desulfoarculus baarsii and Desulfotomaculum sp.) were capable of butyrate isomerization. 相似文献
588.
Ferdinand Bohlmann Alfons Adler Angeliga Schuster Rajinder K. Gupta Robert M. King Harold Robinson 《Phytochemistry》1981,20(8):1899-1902
The investigation of four further Mikania species afforded several diterpenes, four geranylgeraniol derivatives and three kaurane epoxides. The structures were elucidated by spectroscopic methods and a few chemical transformations. 相似文献
589.
590.
“How Could I Live Here and Not Be a Member?”: Economic Versus Social Drivers of Participation in Namibian Conservation Programs 总被引:1,自引:0,他引:1
Economic goals form a critical component of community-based natural resource management (CBNRM) programs, but many studies question whether these initiatives actually deliver economic benefits to local communities. This presents a puzzle regarding why rural residents remain in CBNRM programs, and raises the possibility that non-economic incentives also influence participation. We address this question in a study of two Namibian communal conservancies, analyzing survey and interview data collected between 2009 and 2011. We find that economic incentives explain participation in one conservancy, whereas social motivations take precedence in the other. Our findings indicate that strong attachment to place and preferences for social cohesion can motivate people to comply with CBNRM even when economic incentives fail to materialize. 相似文献