全文获取类型
收费全文 | 3265篇 |
免费 | 112篇 |
国内免费 | 260篇 |
专业分类
3637篇 |
出版年
2024年 | 3篇 |
2023年 | 15篇 |
2022年 | 32篇 |
2021年 | 39篇 |
2020年 | 38篇 |
2019年 | 58篇 |
2018年 | 37篇 |
2017年 | 49篇 |
2016年 | 48篇 |
2015年 | 48篇 |
2014年 | 81篇 |
2013年 | 106篇 |
2012年 | 53篇 |
2011年 | 133篇 |
2010年 | 75篇 |
2009年 | 180篇 |
2008年 | 196篇 |
2007年 | 207篇 |
2006年 | 177篇 |
2005年 | 150篇 |
2004年 | 158篇 |
2003年 | 119篇 |
2002年 | 70篇 |
2001年 | 62篇 |
2000年 | 64篇 |
1999年 | 76篇 |
1998年 | 82篇 |
1997年 | 62篇 |
1996年 | 59篇 |
1995年 | 62篇 |
1994年 | 72篇 |
1993年 | 71篇 |
1992年 | 72篇 |
1991年 | 74篇 |
1990年 | 61篇 |
1989年 | 53篇 |
1988年 | 60篇 |
1987年 | 74篇 |
1986年 | 102篇 |
1985年 | 76篇 |
1984年 | 90篇 |
1983年 | 34篇 |
1982年 | 63篇 |
1981年 | 54篇 |
1980年 | 52篇 |
1979年 | 43篇 |
1978年 | 9篇 |
1977年 | 16篇 |
1976年 | 12篇 |
1974年 | 4篇 |
排序方式: 共有3637条查询结果,搜索用时 15 毫秒
941.
942.
We investigated whether changes in sucrose-phosphate synthase (EC 2.4.1.14, SPS) activity could alter N remobilization during leaf senescence. Transgenic rice (Oryza sativa L. cv. Nipponbare) with low SPS activities and wild-type rice plants were grown with basal N (1.0 mM NH4NO3) until the late vegetative stage. Subsequently, half of the plants were transferred to a low N (0.1 mM NH4NO3) condition to accelerate leaf senescence, and the others were continuously grown with basal N. With low N supply, the amounts of chlorophyll and soluble protein in flag leaf blades decreased after anthesis in both the low SPS plants and wild-type plants, although the decrease was less in the low SPS plants. Panicle weights were significantly lower in the low SPS plant than in the wild-type plant. These results suggest that the remobilization of N from flag leaves was diminished by suppressing the development of reproductive sinks in the low SPS plant. 相似文献
943.
We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest:
evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation
(CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed
seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg−1 yr−1) and lowest in CFP (0.4 ± 0.1 mg-N kg−1 yr−1). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg−1 yr−1), but lowest in SS (0.02 ± 0.01 mg-N kg−1 yr−1). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively),
soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%,
respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations
of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N
ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration
of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass.
In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and
total C concentration and C:N ratio of forest floor. 相似文献
944.
Chris D. Evans Christine L. Goodale Simon J. M. Caporn Nancy B. Dise Bridget A. Emmett Ivan J. Fernandez Chris D. Field Stuart E. G. Findlay Gary M. Lovett Henning Meesenburg Filip Moldan Lucy J. Sheppard 《Biogeochemistry》2008,91(1):13-35
Dissolved organic carbon (DOC) concentrations have risen in upland waters across large areas of Europe and North America.
Two proposed drivers of these increases are (1) deposition of atmospheric pollutant nitrogen (N) with consequent effects on
plant and decomposer carbon dynamics, and (2) soil recovery from acidification associated with decreasing sulphur deposition.
Examination of 12 European and North American field N addition experiments showed inconsistent (positive, neutral, and negative)
responses of DOC to N addition. However, responses were linked to the form of N added and to resulting changes in soil acidity.
Sodium nitrate additions consistently increased DOC, whereas ammonium salts additions usually decreased DOC. Leachate chemistry
was used to calculate an index of “ANC forcing” of the effect of fertilization on the acid-base balance, which showed that
DOC increased in response to all de-acidifying N additions, and decreased in response to all but three acidifying N additions.
Exceptions occurred at two sites where N additions caused tree mortality, and one experiment located on an older, unglaciated
soil with high anion adsorption capacity. We conclude that collectively these experiments do not provide clear support for
the role of N deposition as the sole driver of rising DOC, but are largely consistent with an acidity-change mechanism. It
is however possible that the unintended effect of acidity change on DOC mobility masks genuine effects of experimental N enrichment
on DOC production and degradation. We suggest that there is a need, more generally, for interpretation of N manipulation experiments
to take account of the effects that experimentally-induced changes in acidity, rather than elevated N per se, may have on
ecosystem biogeochemistry. 相似文献
945.
Alternative strategies to sustain N-fertility in acid and calcaric beech forests: Low microbial N-demand versus high biological activity 总被引:1,自引:0,他引:1
A.M. Kooijman M.M. Kooijman-Schouten G.B. Martinez-Hernandez 《Basic and Applied Ecology》2008,9(4):410-421
To challenge the “conventional wisdom” that rates of net N-mineralization increase with pH, we measured net N-mineralization, respiration and/or microbial C and N in four Luxembourg beech forests with similar litter input, but different soil types, using laboratory incubation experiments. Litter input and fungal/bacterial colony ratios were also measured. To test whether the results could be explained by existing theoretical models, equations of C and N dynamics were reformulated to allow estimation of microbial growth efficiency, gross C and N release and microbial uptake, based on measured values of net N-mineralization, respiration and C:N ratios of substrate and microbes.Instead of an increase, net N-mineralization rates showed a significant sevenfold decrease from acid to calcaric soil in the organic layer, and a fourfold decrease in the mineral topsoil. At the same time, microbial N-demand increased with pH, as indicated by the significant decrease in net N-mineralization per unit microbe or unit C respired. These results could be explained by theoretical models. In organic layer and mineral topsoil, despite high gross N-release, net N-mineralization rates decreased with pH because of higher microbial immobilization. Increase in microbial N-demand was associated with a decrease in fungal/bacterial colony ratio: the more the bacteria, the higher the microbial N-demand.Acid and calcaric soils seem to have different strategies to sustain ecosystem N-fertility. In calcaric soil, N-availability to the vegetation seems indeed supported by high biological activity and gross N-release, which is needed to compensate for the potentially high immobilization by bacteria. In acid soil, however, despite low gross N-release, N-availability to the vegetation may not be lower than in calcaric soil, due to high amounts of fungi and low microbial N-demand. 相似文献
946.
Net N input and riverine N export from Illinois agricultural watersheds with and without extensive tile drainage 总被引:3,自引:2,他引:3
Some of the largest riverine N fluxes in the continental USA have been observed in agricultural regions with extensive artificial subsurface drainage, commonly called tile drainage. The degree to which high riverine N fluxes in these settings are due to high net N inputs (NNI), greater transport efficiency caused by the drainage systems, or other factors is not known. The objective of this study was to evaluate the role of tile drainage by comparing NNI and riverine N fluxes in regions of Illinois with similar climate and crop production practices but with different intensities of tile drainage. Annual values of NNI between 1940 and 1999 were estimated from county level agricultural production statistics and census estimates of human population. During 1945–1961, riverine nitrate flux in the extensively tile drained region averaged 6.6kgNha–1year–1 compared to 1.3 to 3.1kgNha–1 for the non-tile drained region, even though NNI was greater in the non-tile drained region. During 1977–1997, NNI to the tile-drained region had increased to 27kgNha–1year–1 and riverine N flux was approximately 100% of this value. In the non-tile-drained region, NNI was approximately 23kgNha–1year–1 and riverine N flux was between 25% and 37% of this value (5 to 9kgNha–1year–1). Denitrification is not included in NNI and, therefore, any denitrification losses from tile-drained watersheds must be balanced by other N sources, such as depletion of soil organic N or underestimation of biological N fixation. If denitrification and depletion of soil organic N are significant in these basins, marginal reductions in NNI may have little influence on riverine N flux. If tile drained cropland in Illinois is representative of the estimated 11 million ha of tile drained cropland throughout the Mississippi River Basin, this 16% of the drainage area contributed approximately 30% of the increased nitrate N flux in the Lower Mississippi River that occurred between 1955 and the 1990s. 相似文献
947.
Inferring the phylogenetic position of the Drosophila flavopilosa group: Incongruence within and between mitochondrial and nuclear multilocus datasets 下载免费PDF全文
948.
Phosphite dehydrogenase represents a new enzymatic system for regenerating reduced nicotinamide cofactors for industrial biocatalysis. We previously engineered a variant of phosphite dehydrogenase with relaxed cofactor specificity and significantly increased activity and stability. Here we performed one round of random mutagenesis followed by comprehensive saturation mutagenesis to further improve the enzyme thermostability while maintaining its activity. Two new thermostabilizing mutations were identified. These, along with the 12 mutations previously identified, were subjected to saturation mutagenesis using the parent enzyme or the engineered thermostable variant 12x as a template, followed by screening of variants with increased thermostability. Of the 12 previously identified sites, 6 yielded new variants with improved stability over the parent enzyme. Several mutations were found to be context-dependent. On the basis of molecular modeling and biochemical analysis, various mechanisms of thermostabilization were identified. Combining the most thermostabilizing mutation at each site resulted in a variant that showed a 100-fold increase in half-life at 62 degrees C over the 12x mutant. The final mutant has improved the half-life of thermal inactivation at 45 degrees C by 23,000-fold over the parent enzyme. The engineered phosphite dehydrogenase will be useful in NAD(P)H regeneration. 相似文献
949.
Conservation of nitrogen increases with precipitation across a major grassland gradient in the Central Great Plains of North America 总被引:1,自引:0,他引:1
Regional analyses and biogeochemical models predict that ecosystem N pools and N cycling rates must increase from the semi-arid
shortgrass steppe to the sub-humid tallgrass prairie of the Central Great Plains, yet few field data exist to evaluate these
predictions. In this paper, we measured rates of net N mineralization, N in above- and belowground primary production, total
soil organic matter N pools, soil inorganic N pools and capture in resin bags, decomposition rates, foliar 15N, and N use efficiency (NUE) across a precipitation gradient. We found that net N mineralization did not increase across
the gradient, despite more N generally being found in plant production, suggesting higher N uptake, in the wetter areas. NUE
of plants increased with precipitation, and δ15N foliar values and resin-captured N in soils decreased, all of which are consistent with the hypothesis that N cycling is
tighter at the wet end of the gradient. Litter decomposition appeared to play a role in maintaining this regional N cycling
trend: litter decomposed more slowly and released less N at the wet end of the gradient. These results suggest that immobilization
of N within the plant–soil system increases from semi-arid shortgrass steppe to sub-humid tallgrass prairie. Despite the fact
that N pools increase along a bio-climatic gradient from shortgrass steppe to mixed grass and tallgrass prairie, this element
becomes relatively more limiting and is therefore more tightly conserved at the wettest end of the gradient. Similar to findings
from forested systems, our results suggest that grassland N cycling becomes more open to N loss with increasing aridity. 相似文献
950.
Edoardo Daly Sari Palmroth Paul Stoy Mario Siqueira A. Christopher Oishi Jehn-Yih Juang Ram Oren Amilcare Porporato Gabriel G. Katul 《Biogeochemistry》2009,94(3):271-287
Profiles of subsurface soil CO2 concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and
2006 in 16 locations at the free air CO2 enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO2 concentration levels (ambient [CO2]a, ambient + 200 ppmv, [CO2]e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 gN m−2 year−1). The combination of these measurements permitted indirect estimation of belowground CO2 production and flux profiles in the mineral soil. Adjacent to the soil CO2 profiles, direct (chamber-based) measurements of CO2 fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the
measured soil CO2 profiles, neither [CO2]e nor N fertilization had a statistically significant effect on seasonal soil CO2, CO2 production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects
on CO2 concentration depending on the depth. Variations in CO2 were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface
(within the first 10 cm), where CO2 pulses were induced by rainfall events. The soil effluxes were equal to the CO2 production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from
the CO2 profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests
that a better parameterization of the soil CO2 diffusivity is required for these soil moisture extremes. 相似文献