全文获取类型
收费全文 | 1146篇 |
免费 | 63篇 |
国内免费 | 96篇 |
专业分类
1305篇 |
出版年
2024年 | 2篇 |
2023年 | 16篇 |
2022年 | 16篇 |
2021年 | 12篇 |
2020年 | 24篇 |
2019年 | 37篇 |
2018年 | 27篇 |
2017年 | 44篇 |
2016年 | 30篇 |
2015年 | 34篇 |
2014年 | 51篇 |
2013年 | 142篇 |
2012年 | 35篇 |
2011年 | 48篇 |
2010年 | 37篇 |
2009年 | 43篇 |
2008年 | 42篇 |
2007年 | 54篇 |
2006年 | 52篇 |
2005年 | 50篇 |
2004年 | 43篇 |
2003年 | 48篇 |
2002年 | 43篇 |
2001年 | 49篇 |
2000年 | 25篇 |
1999年 | 30篇 |
1998年 | 12篇 |
1997年 | 15篇 |
1996年 | 17篇 |
1995年 | 20篇 |
1994年 | 25篇 |
1993年 | 20篇 |
1992年 | 29篇 |
1991年 | 23篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 15篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1975年 | 1篇 |
排序方式: 共有1305条查询结果,搜索用时 0 毫秒
1.
The present study was undertaken to comparatively investigate the attachment capacities of Azospirillum brasilenseSp245 and its lipopolysaccharide-defective Omegon-Km mutants KM018 and KM252, as well as their activities with respect to the alteration of the morphology of wheat seedling root hairs. The adsorption dynamics of the parent Sp245 and mutant KM252 strains of azospirilla on the seedling roots of the soft spring wheat cv. Saratovskaya 29 were similar; however, the attachment capacity of the mutant KM252 was lower than that of the parent strain throughout the incubation period (15 min to 48 h). The mutation led to a considerable decrease in the hydrophobicity of the Azospirillumcell surface. The lipopolysaccharides extracted from the outer membrane of A. brasilenseSp245 and mutant cells with hot phenol and purified by chromatographic methods were found to induce the deformation of the wheat seedling root hairs, the lipopolysaccharide of the parent strain being the most active in this respect. The role of the carbohydrate moiety of lipopolysaccharides in the interaction of Azospirillumcells with plants is discussed. 相似文献
2.
For three acid soils from Santa Catarina, Brazil, lime application and time of incubation with lime had little effect on the
adsorption of added phosphorus. In two soils with high contents of exchangeable aluminium, solution P and isotopically exchangeable
P were decreased by incubating with lime for 1 month: phosphorus was probably adsorbing on freshly precipitated aluminium
hydrous oxides. In one soil with less exchangeable aluminium, P in solution was increased by liming. After 23 months lime
increased solution and exchangeable P possibly due to crystallization of aluminium hydrous oxides reducing the number of sites
for P adsorption. All these changes were however small.
In a pot experiment, lime and phosphorus markedly increased barley shoot and root dry matter and P uptake. Although liming
reduced P availability measured by solution P, isotopically exchangeable P and resin extractable P, it increased phosphorus
uptake by reducing aluminium toxicity and promoting better root growth. The soil aluminium saturation was reduced by liming,
but the concentration of aluminium in roots changed only slightly. The roots accumulated aluminium without apparently being
damaged. 相似文献
3.
Peat,zeolite and basalt as adsorbents of ammoniacal nitrogen during manure decomposition 总被引:4,自引:0,他引:4
The effectiveness of sphagnum peat, zeolite (clinoptilonite) and basalt in reducing ammonia losses during aerobic manure decomposition was determined in an incubation experiment. Peat placed in the spent air-stream adsorbed all of the ammonia volatilized during the first 8 days of decomposition, and reduced overall ammonia losses by 59%. Zeolite reduced total ammonia losses by 16%, and basalt by 6%.All adsorbents were considerably less effective in reducing ammonia losses when mixed with the manure. Reductions in ammonia losses of 24% and 1.5% were obtained with the peat and zeolite, respectively. The addition of basalt increased losses.Ammonia and ammonium adsorption isotherms were determined for the three materials. The adsorption capacities and affinity terms of the adsorbents calculated from the isotherms, reflected their ability to reduce ammonia losses in the incubation experiment. Zeolite had both the highest affinity for ammonium and the highest ammonium adsorption capacity. The peat had a very high affinity for ammonia and a high adsorption capacity (23.4 mg NH3–N g–1), whereas zeolite and basalt had a much lower adsorption capacity (1.8 and 0.05 mg NH3–N g–1, respectively) compared with their capacity to adsorb ammonium (18.1 and 0.18 mg NH4–N g–1). 相似文献
4.
The hemolymph of the adult female Manduca sexta was fractionated by cascade-mode multi-affinity chromatography (CASMAC) on a main-line tandem column chain containing Zn(2+)-TED, T-gel, Ni(2+)-DPA, and phenylsepharose and a side-line column containing Zn(2+)-DPA. The technique separated some of the previously described major hemolymph proteins, and yielded a number of fractions with simple composition. Some of these fractions contained only less abundant proteins of Manduca hemolymph. Thus, it appears that CASMAC would be a very useful fractionation technique for purification and characterization of the minor proteins of insect hemolymph. 相似文献
5.
Influence of maize root mucilage on soil aggregate stability 总被引:9,自引:0,他引:9
This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected
from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg−1 dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting
cycle after 21 d. Evolved CO2 was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil.
In mucilage-amended soils CO2 evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase
in evolved CO2 up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation
of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent
of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate
stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work
supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the
newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of
these established organo-mineral bounds. 相似文献
6.
The adsorption of nonionic surfactants on hide powder previously treated with anionic surfactants has been studied. The adsorption of nonionic surfactants takes place through hydrophobic interactions. A mechanism has been proposed for this interaction, assuming that the nonionic surfactant has been fixed by means of secondary adsorption (hydrophobic interaction) after the primary adsorption of the anionic surfactant (ionic and hydrophobic interaction) which makes it possible. 相似文献
7.
Summary Many rocky shores are subject to periodic inundation by sand, which is often thought to reduce species richness by eliminating
organisms intolerant of sand scour or sand smothering. However, regular disturbance (e.g. inundation) should promote richness
by preventing the development of low diversity climax communities. A study of faunal richness on 10 regularly inundated shores
showed that inundation does promote richness, but by increasing habitat heterogeneity. Some species are excluded from parts
of the shore by sand, but because of the patchiness of sand deposits they are rarely excluded from the entire shore. Other
species are found only on rocks associated with sand, while typically sandy shore animals occur in the sand deposits themselves.
Total richness (281 species) was greater than for local noninundated shores and sandy beaches combined. 相似文献
8.
The phosphate adsorption maximum as calculated by the Langmuir equation was used to predict the fertilizer P requirement of
wheat (Triticum aestivium, cv. Caldwell) on both virgin and cultivated Decatur clay loam (clayey, kaolinitic, thermic, Rhodic Paleudult) and Hartsells
sandy loam soils (fine-loamy, siliceous, thermic, Typic Hapludult). Soils with higher adsorption maximum were found to require
more fertilizer P than soils with lower adsorption maximum. For soils 25% saturation of the adsorption maximum gave the optimum
dry matter yield. This corresponded to equilibrium P concentration of 0.45 mg L−1 for Decatur cultivated and 0.31 mg L−1 for Decatur and Hartsells virgin soils for optimum dry matter yield. These values are within the range of those reported
previously by other investigators working with different soils. 相似文献
9.
The adsorption of 5′-AMP onto precipitated calcium phosphate (CaPi) requires the presence of soluble calcium and this dependence
exhibits a Michaelian-like behavior. This result suggests that the formation of a complex between 5′-AMP and free Ca2+ (CaAMP) is a prelude to the adsorption of the nucleotide in the solid matrix. At concentrations one order of magnitude higher,
Mn2+ and Mg2+ can substitute for soluble Ca2+ in the adsorption of 5′-AMP onto solid CaPi. However, when added simultaneously with 5′-AMP to a heterogeneous mixture that
contains CaPi and soluble Ca2+, Mn2+ and Mg2+ inhibit the adsorption of 5′-AMP in a concentration-dependent manner. This suggests the formation of complexes that are much
less effective for 5′-AMP adsorption than the CaAMP complex. On the other hand, Mn2+ and Mg2+ cannot promote desorption of the nucleotide attached to the precipitate in the presence of soluble Ca2+ if they are added after adsorption has attained equilibrium. Although desorption of 5′-AMP can be obtained by a sequential
dilution of the soluble phase with buffer and no nucleotide in a process that obeys a Langmuir equation, the lack of effect
of Mn2+ or Mg2+ when adsorption has attained its maximal value suggests strong interactions between the CaAMP complex and the solid matrix
when adsorption equilibrium is reached. The divalent cations present in the matrix also participate with different selectivity
in the attachment of the CaAMP complex, indicating that a cation-exchange mechanism could have acted in the modulation of
adsorptive/desorptive processes involving biomonomers and phosphate surfaces in primitive aqueous environments.
Received: 11 December 1995 / Accepted: 5 April 1996 相似文献
10.
K. Mengel 《Plant and Soil》1996,178(1):83-93
The root development of barley seedlings grown for one week in an aerated nutrient solution was studied in the presence of dissolved organic matter from an aqueous chestnut leaf litter extract. In particular, the different effects of low and high molecular weight fractions (small molecules: molecular weight <1000; large molecules: >10,000) of the leaf litter extract were examined. In the presence of large molecules root growth was inhibited, an irregular root tip morphology was observed, and Ca and Mg concentrations in the shoots were lower than in control plants. These phytotoxic effects were not caused by the formation of an impermeable layer of large molecules on the root surfaces that lower accessibility for nutrient cations as inferred from voltammetric experiments. A germination assay using spruce seeds, however, indicated allelochemical effects of large molecules, which exhibit a higher aromaticity than the small molecules as indicated by spectroscopic characterisation. In the growth experiments with small molecules, no influence on the root development of barley was evident, but an increase of Ca and Mg in the shoots was detected. During these growth experiments, a large amount of the small molecules, mainly simple phenols and amino acids, disappeared from the nutrient solution. The loss of small molecules was most likely the effect of mineralisation.Abbreviations DOC
dissolved organic carbon
- DOM
dissolved organic matter
- LLE
leaf litter extract
- MW
molecular weight
- HMDE
hanging mercury drop electrode 相似文献