全文获取类型
收费全文 | 6494篇 |
免费 | 354篇 |
国内免费 | 10篇 |
专业分类
林业 | 325篇 |
农学 | 345篇 |
基础科学 | 67篇 |
975篇 | |
综合类 | 440篇 |
农作物 | 281篇 |
水产渔业 | 256篇 |
畜牧兽医 | 3328篇 |
园艺 | 144篇 |
植物保护 | 697篇 |
出版年
2021年 | 43篇 |
2020年 | 105篇 |
2019年 | 88篇 |
2018年 | 118篇 |
2017年 | 141篇 |
2016年 | 123篇 |
2015年 | 95篇 |
2014年 | 126篇 |
2013年 | 344篇 |
2012年 | 262篇 |
2011年 | 329篇 |
2010年 | 253篇 |
2009年 | 217篇 |
2008年 | 306篇 |
2007年 | 270篇 |
2006年 | 252篇 |
2005年 | 250篇 |
2004年 | 228篇 |
2003年 | 202篇 |
2002年 | 226篇 |
2001年 | 196篇 |
2000年 | 186篇 |
1999年 | 170篇 |
1998年 | 75篇 |
1997年 | 62篇 |
1996年 | 65篇 |
1995年 | 51篇 |
1994年 | 62篇 |
1993年 | 72篇 |
1992年 | 108篇 |
1991年 | 116篇 |
1990年 | 118篇 |
1989年 | 109篇 |
1988年 | 83篇 |
1987年 | 75篇 |
1986年 | 67篇 |
1985年 | 74篇 |
1984年 | 81篇 |
1983年 | 62篇 |
1982年 | 63篇 |
1981年 | 45篇 |
1980年 | 50篇 |
1979年 | 51篇 |
1978年 | 54篇 |
1977年 | 49篇 |
1976年 | 53篇 |
1975年 | 43篇 |
1974年 | 52篇 |
1971年 | 49篇 |
1969年 | 53篇 |
排序方式: 共有6858条查询结果,搜索用时 187 毫秒
101.
Species-specific earthworm population responses in relation to flooding dynamics in a Dutch floodplain soil 总被引:3,自引:0,他引:3
Earthworms dominate the animal biomass in moist floodplain soils. They are known to survive long periods in aerated water, but little is known about earthworm population dynamics in floodplain systems with changing inundation frequencies. This study determined earthworm population dynamics in a floodplain system, in relation to frequency and duration of flooding events. From October 2000 to May 2003 earthworms were hand sorted in the ‘Afferdensche en Deestsche Waarden’, a floodplain on the south bank of the river Rhine, near Druten, The Netherlands. Earthworm numbers and biomasses per age class (adult, subadult, juvenile) were recorded. Numbers and biomasses tend to decrease during flooding. Lumbricus terrestris was found in high numbers (>10/m2) only at the end of a flooding period. Allolobophora chlorotica was hardly affected by flooding; their biomass remained stable during the year. Aporrectodea caliginosa showed fluctuating numbers and biomasses during the sampling period that did not correlate with flooding frequency. Numbers and biomasses of Lumbricus rubellus were strongly reduced at the end of each flooding event, but their population densities fully recovered until next flooding event. Earthworm populations in floodplains fluctuate in time, depending on the season and on the time, duration and frequency of flooding. Different earthworm species react differently towards these flooding dynamics. 相似文献
102.
Joke Geets Brigitte Borremans Jaco Vangronsveld Ludo Diels Dani?l van der Lelie 《Journal of Soils and Sediments》2005,5(3):149-163
Background, Aims and Scope Sulfate-reducing bacteria (SRB) are known for their capacity to reduce and precipitate heavy metals (HM) as metal sulfides,
offering the opportunity to create an in situ reactive zone for the treatment of heavy metal-contaminated groundwater, a process
called in situ metal precipitation (ISMP). The applicability of the ISMP technology first has to be investigated at a laboratory
scale before going into an on site application. The evaluation and optimization of the ISMP process is facilitated when physical/chemical
analysis techniques are combined with molecular tools that specifically monitor the abundance, diversity and dynamics of the
indigenous sulfate reducing microbial community. In this study, batch experiments were conducted in order to investigate the
feasibility of ISMP as a groundwater remediation strategy for an industrial site contaminated with elevated levels of Zn,
Cd, Co and Ni.
Methods The potential of different types of carbon source/ electron donor (lactate, acetate, methanol, ethanol, Hydrogen Release
Compound?, molasses) to stimulate the sulfate reduction and metal precipitation activity of the naturally present (or indigenous)
SRB community was explored. In addition, the effect of amending vitamin B12 and yeast extract was evaluated. The ISMP process
was monitored by combining analytical analyzes of process parameters (SO42-concentration, heavy metal concentrations,
pH, Eh) with molecular tools such as SRB subgroup and genus specific PCR, denaturing gradient gel electrophoresis (DGGE),
and phylogenetic analysis of clone sequences, based on either the 16S rRNA or the dsr (dissimilatory sulfite reductase) gene.
Results and Discussion The efficiency of different carbon-sources to stimulate the ISMP process followed the order HRC 〉 molasses 〉 methanol
〉 lactate 〉 ethanol 〉 acetate. Within 10 weeks, the highest sulfate and metal removal efficiencies ranged from 85% to 99%.
Addition of yeast extract boosted the ISMP process, whereas vitamin B12 negligibly affected SRB activity. Analysis of the
sulfate reducing population by SRB subgroup and genus specific PCR demonstrated that members of the genus Desulfosporosinus
dominated in all batch tests, while 16S rDNA DGGE profiles additionally revealed the presence in the microbial communities
of non-sulfate reducing bacteria within the family Clostridium and the -proteobacteria. The dsrB-based DGGE profiles
allowed us to assess the diversity and dynamics of the sulfate reducing community and added to a better understanding of the
effects of different batch conditions on the ISMP process. Remarkably, all dsrB sequences affiliated with the dsrB gene sequence
cluster found in Desulfotomaculum, which received their xenologous dsrB gene from the -proteobacteria.
Conclusions The batch experiments, which aimed at stimulating the activities of the indigenous SRB communities, demonstrated that these
communities were present and that their activities could be used to obtain efficient in situ precipitation of the contaminating
heavy metals. This opens the possibility to test this concept in the future as an on site demonstration as part of the groundwater
strategy for the heavy metal contaminated site. Although batch setups are suitable for preliminary feasibility studies for
ISMP, they do not reflect the in situ situation where sulfate and heavy metal and metalloid polluted groundwater are supplied
continuously. A sulfate reducing strain JG32A was isolated from whose 16S rRNA gene affiliated with the genus Desulfosporosinus,
while its dsrB gene sequence clustered with Desulfotomaculum dsrB gene sequences, which received their xenologous dsr genes
from -proteobacteria. Therefore we hypothesize that the batch experiments enrich members of the Desulfosporosinus
genus that possess a non-orthologous dsrB gene.
Recommendation and Perspective The next step towards an on site pilot test for ISMP will be the setup of a series of column experiments, with process conditions
that are selected based on the above mentioned results. This will allow to define optimal ISMP process conditions and to test
its long-term efficacy and sustainability before going into an on site bioremediation application. By applying the described
molecular tools together with physical-chemical analyzes, it can be investigated whether the same SRB community is enriched
and which type of C-source is most effective in promoting and sustaining its growth and sulfate-reduction activity. 相似文献
103.
Zhang GF Maudens KE Storozhenko S Mortier KA Van Der Straeten D Lambert WE 《Journal of agricultural and food chemistry》2003,51(27):7872-7878
A procedure involving chemical conversion of all forms of folate present in plant material into para-aminobenzoic acid (PABA) and a liquid chromatographic-fluorimetric determination with on-line postcolumn derivatization is reported. All folates are cleaved with liberation of PABA by hydrogen peroxide followed by acid hydrolysis using concentrated hydrochloric acid (37%) at 110 degrees C for 6 h. The reaction yield for individual folates conversion to PABA ranged from 44.4 to 97.3%. PABA could be determined sensitively by on-line postcolumn derivatization with fluorescamine, the detection limit for PABA being 3.02 nM. On the basis of this principle, a method for the determination of total folate in plant material, including a purification step on an affinity column, is presented, which offers a sufficient sensitivity and selectivity for routine analysis of total folate in natural samples. The total folate contents of tomatoes, carrots, white cabbage, and spinach were determined, and the results were quite comparable to the data reported. The recovery of PABA and the comparison of total folate analysis in spinach on different occasions (over 6 months) are also reported. The method is reliable, universal for all folates, including polyglutamate and monoglutamate forms, and eliminates the need for a deconjugation step and multiple conversion reactions. 相似文献
104.
R.P.T. Janssen M. G.M. Bruggenwert † & W. H. Van Riemsdijk 《European Journal of Soil Science》2003,54(2):335-345
Al hydroxide polymers (AlHO) can significantly influence the cation exchange behaviour of clays. We have determined the effect of synthesized AlHO on Ca–Na, Zn–Na and Pb–Na exchange for a series of exchanger compositions and two Al loadings at pH 6.0 and an ionic strength of 0.01 m . The preference for Ca on the siloxane surface of the clay–AlHO system (CAlHO) was greater than for the pure clay, and the average K V (Vanselow selectivity coefficient) was determined to be 2.16 and 1.24, respectively. The selectivity coefficients for the exchange reactions Zn–Na and Pb–Na were not directly determined in CAlHO systems, because heavy-metal ions bind as well to the clay surface as to the AlHO over a wide range of pH. We have estimated the effect of the presence of AlHO on the selectivity coefficients of Zn–Na and Pb–Na exchange by extrapolation of the experimental results of Ca–Na, Zn–Na and Pb–Na exchange for pure clay and Ca–Na exchange for CAlHO. The average K V was increased by the presence of the AlHO from 1.23 to 2.16 for Zn–Na exchange and from 1.59 to 2.77 for Pb–Na exchange. The increase in the preference for the divalent cations is probably caused by parallel alignment of clay platelets by sorption of AlHO. Increasing the amount of AlHO did not change the selectivity for Ca–Na exchange, and probably the structure of the system or the arrangement of the clay platelets and AlHO particles was not substantially changed. This was supported by the linear reduction of the cation exchange capacity with amount of AlHO present at pH 6.6. It seems likely that the selectivity coefficients for Ca–Na, Zn–Na and Pb–Na exchange that we found apply in naturally occurring montmorillonite–AlHO systems. 相似文献
105.
Gil A De La Fuente EB Lenardis AE López Pereira M Suárez SA Bandoni A Van Baren C Di Leo Lira P Ghersa CM 《Journal of agricultural and food chemistry》2002,50(10):2870-2877
The objective was to study the essential oil composition of coriander fruits in plants growing in environments differing in soil conditions and weediness level. Factorial field experiments were conducted in two locations from the Rolling Pampas, Argentina, and two coriander landraces (European and Argentinean) were tested under two levels of nitrogen fertilization and weediness. Data were evaluated with uni- and multivariate techniques. The variation in the oil composition was related to the relative proportion of the constituents and not to the presence/absence of a particular component. Weather conditions in 1997 favored linalool and camphor in both landraces. Location, fertilization, and weediness also affected the chemical profile. The European landrace showed a more stable concentration of the major components than the Argentinean landrace. These results, which show the relationships between some environmental conditions and the essential oil composition, are useful in the development of innovative strategies aimed to improve oil composition and to manage crop pests. 相似文献
106.
Van der Plancken I Van Loey A Hendrickx M 《Journal of agricultural and food chemistry》2007,55(1):127-135
This article addresses the effect of moisture content (0.8-9.9%) during dry-heating (80 degrees C) on selected physicochemical (solubility, turbidity, residual denaturation enthalpy, aggregation, surface hydrophobicity, and sulfhydryl content) and functional (foaming ability, foam density, and stability) properties of freeze-dried egg white (FDEW). Moisture content during dry-heating proved to be a parameter determining the functionality of the resulting egg white powder. The degree of conformational changes induced in the egg white proteins by dry-heating was strongly dependent on the amount of water present. Preferentially, dry-heating at 80 degrees C should be performed on egg white powder with a moisture content below 6.8%, as the loss of protein solubility above this value is extensive. In addition to insoluble aggregates, soluble, strongly stabilized aggregates were also formed, especially at higher moisture contents. The decrease in denaturation enthalpy, increase in surface hydrophobicity, and exposure of SH groups previously hidden in the protein core and their subsequent oxidation were more pronounced at prolonged dry-heating times and at higher moisture contents. These conformational changes resulted in improved foaming ability and foams with lower density. No effect of dry-heating on the foam stability was observed. 相似文献
107.
Displacement imaging is a recent, powerful NMR method with which distributions of displacements can be acquired of e.g. fluids within a porous medium. Both motion parallel and perpendicular to the flow direction may be observed within a time window of a few milliseconds to several seconds. By combining displacement imaging with the line scan technique, one-dimensionally resolved measurements with a high temporal resolution ( < 1 min) of the spatial dependency of motion can be obtained. Here we present displacement images of flow through two simple model systems for soil: an unconsolidated glass bead water system and a sintered glass bead filter. It is demonstrated that the combination of displacement imaging and spatial resolution along a line is important to access both bulk displacement and local displacements in relation to the local porosity. 相似文献
108.
G. J. LEVY N. ALPEROVITCH A. J. van der MERWE I. SHAINBERG 《European Journal of Soil Science》1989,40(3):613-620
The hydrolysis of four kaolinitic soils from the Transvaal (RSA), and the effects of exchangeable Na, Ca and Mg were studied by monitoring the changes in the electrical conductivity of the soil suspension with time. The rate of the hydrolysis was found to be linearly related to the square root of time. The rates of the hydrolysis of the Half-Way House and Krugersdorp soils were appreciably lower than those of the Potchefstroom and Amsterdam soils, and rates reported in the literature for smectite soils. This was probably because of the low content of hydrolysable bases in the Half-Way House and Krugersdorp soils, resulting from the low cation exchange capacity of their clay fraction. The extensive hydrolysis observed in the Potchefstroom and Amsterdam soils could be ascribed to the presence of significant amounts of smectite in the former and high silt content in the latter; both factors are known to enhance the rate of weathering and hydrolysis. With the exception of the Potchefstroom soil, the rate of hydrolysis was not affected by the cationic composition of the adsorbed phase. Contrary to expectations, exchangeable Mg did not inhibit hydrolysis in the soils studied, probably because of the limited isomorphic substitutions in kaolinite which lead to very small amounts of octahedral Mg in the crystal. The hydrolysis of the Potchefstroom soil was enhanced in the presence of exchangeable Mg compared to exchangeable Ca; the reason for this is not fully understood. 相似文献
109.
110.
A. M. Van Tienhoven K. A. Olbrich R. Skoroszewski J. Taljaard M. Zunckel 《Water, air, and soil pollution》1995,85(4):2577-2582
South Africa is the most industrialised country in southern Africa and stands at some risk from negative pollution impacts. To the authors' knowledge, this paper presents the first attempt to apply the critical loads approach on the African continent; although sensitivity mapping has been performed for Africa and the rest of the world (Kuylerstierna et al, this conference). Actual sulphate and base cation deposition loads in Mpumalanga (formerly the Eastern Transvaal province of South Africa) were mapped from 16 monitoring sites. The region is characterised by long, dry periods with little rain, high evaporation (up to 8 mm per day) and low run-off (15% of MAP). Provisional critical load and exceedance maps were produced for the surface waters using the Steady-State Water Chemistry Model and the Diatom model. Maps of soil sensitivity to acid deposition, based on bedrock lithology, soil chemical characteristics and land cover, were produced. A weathering rate of 0.39–0.86 keq/ha/year was calculated for the most sensitive sites and taken as the critical load, based on the assumption that the weathering rate represents the buffering ability of the system. The critical loads were contrasted with measures of actual deposition to examine potential scenario's for critical load exceedances. Akey factor in refining the sensitivity maps, and allowing estimation of the critical loads, is the accurate calculation of weathering rates under the warmer and more arid environmental conditions prevalent in South Africa. In a developing country such as South Africa, where research resources are limited, the critical loads approach is a valuable means of assessing the risk of potential impacts of atmospheric deposition. 相似文献