全文获取类型
收费全文 | 7676篇 |
免费 | 56篇 |
国内免费 | 14篇 |
专业分类
林业 | 781篇 |
农学 | 235篇 |
基础科学 | 49篇 |
926篇 | |
综合类 | 336篇 |
农作物 | 506篇 |
水产渔业 | 479篇 |
畜牧兽医 | 3706篇 |
园艺 | 200篇 |
植物保护 | 528篇 |
出版年
2022年 | 15篇 |
2021年 | 14篇 |
2018年 | 124篇 |
2017年 | 189篇 |
2016年 | 182篇 |
2015年 | 244篇 |
2014年 | 212篇 |
2013年 | 167篇 |
2012年 | 463篇 |
2011年 | 313篇 |
2010年 | 114篇 |
2009年 | 81篇 |
2008年 | 72篇 |
2007年 | 58篇 |
2006年 | 54篇 |
2005年 | 782篇 |
2004年 | 660篇 |
2003年 | 454篇 |
2002年 | 120篇 |
2001年 | 134篇 |
2000年 | 40篇 |
1999年 | 151篇 |
1992年 | 118篇 |
1991年 | 145篇 |
1990年 | 169篇 |
1989年 | 147篇 |
1988年 | 173篇 |
1987年 | 115篇 |
1986年 | 144篇 |
1985年 | 117篇 |
1984年 | 91篇 |
1983年 | 82篇 |
1982年 | 33篇 |
1981年 | 17篇 |
1980年 | 23篇 |
1979年 | 133篇 |
1978年 | 96篇 |
1977年 | 91篇 |
1976年 | 77篇 |
1975年 | 137篇 |
1974年 | 129篇 |
1973年 | 116篇 |
1972年 | 138篇 |
1971年 | 118篇 |
1970年 | 88篇 |
1969年 | 102篇 |
1968年 | 113篇 |
1967年 | 125篇 |
1966年 | 102篇 |
1965年 | 70篇 |
排序方式: 共有7746条查询结果,搜索用时 0 毫秒
141.
Edward?L.?BooneEmail author Keying?Ye Eric?P.?Smith 《Journal of Agricultural, Biological & Environmental Statistics》2005,10(2):131-147
Understanding the relationship between an index of biological community state and habitat is important for policy makers and
environmental managers. A common approach to modeling this relationship is to use regression. However, this simple method
becomes complicated when the data are clustered and have both within-cluster and between-cluster spatial correlation. This
article proposes a Bayesian hierarchical model that incorporates both types of spatial correlation. This model yields both
an understanding of the within-cluster relationships as well as an overall relationship between these variables. We apply
this method to evaluate the relationship between the index of biotic integrity (a common measure of fish condition) and the
qualitative habitat evaluation index (a common measure of habitat quality). This method allows us to show that there is a
relationship between the biological community state and habitat and that this relationship varies across river basins, while
accounting for the within- and between-spatial correlations. 相似文献
142.
基于多年MODIS NDVI 分级的河北平原农田生产力评价 总被引:2,自引:0,他引:2
根据中等分辨率航天成像光谱仪(MODIS)遥感数据计算的归一化植被指数(NDVI)被广泛用于作物长势监测和产量预报, 但由于NDVI 数值在不同年份的同一时期变化较大, 直接用于评价农田生产力会有较大误差。本文以河北平原所在的北纬37°~39°之间连续种植的冬小麦农田为研究区域, 通过对多年冬小麦MODISNDVI 数据进行比较和分级, 尝试用每季NDVI 在区域内的高低级别评估区域农田生产力。Landsat 卫星数据用于对不同时相MODIS 图像进行精确配准, 从而实现像素尺度上长时间序列数据的统计分析。首先, 对区域内2000~2008 年间每年作物返青期到成熟期的NDVI 平均值及各生育阶段NDVI 平均值分别进行高低分级, 以了解河北平原农田生产力的空间变异, 结果显示其中高水平农田分布在太行山山前平原, 指数等级水平并没有完全按南北走向趋势分布, 表明该研究方法受纬度差异的影响较小。不同年份分析结果显示, 2008 年东部地区也出现了较高等级的田块。其次, 利用NDVI 分级结果计算出9 年间NDVI 等级的变异系数, 对采用不同生育期NDVI 可能带来的误差进行了分析, 结果显示不同小麦生育期NDVI 等级的变异系数不同, 返青期和成熟期变异系数较大, 且具有一定的地理差异。最后, 利用GIS 空间分析方法以9 年NDVI 分级结果为基础制作了以县为单元的麦田生产力等级图, 结果显示河北平原农田生产力高低分区, 同时也表明中低水平区块有较大提升空间, 为河北县级土地管理和耕地质量管理提供理论依据。 相似文献
143.
?ukasz?J.??uczajEmail author Ingvar?Svanberg Piotr?K?hler 《Genetic Resources and Crop Evolution》2011,58(5):783-793
The aim of this article is to study the geographical distribution and historical patterns of use of a little known root crop
native to Europe—marsh woundwort Stachys palustris L. The species grows in wet grasslands and arable fields. It produces edible tubers. Both ethnographic literature and archival
sources were searched. Seventeen reliable references concerning the consumption of S. palustris in southern and south–eastern Poland were found. The tubers were usually dried and powdered, and then added to soups or to
bread dough. They were also eaten as raw snacks. The plant was used mainly during food shortages in spring, until the turn
of the nineteenth and twentieth century, and later only as an occasional raw snack until the 1970s. Marsh woundwort was also
eaten in western Ukraine (one reference). There are many references to the edibility of marsh woundwort in the eighteenth
and nineteenth century European economic botany literature, particularly in Sweden and Great Britain. These publications tried
to popularize the use of S. palustris as food. However, there is no firm evidence from these countries that marsh woundwort was used as food there. Marsh woundwort
was also used, throughout northern and central Europe, as pig fodder and as a medicinal plant, particularly for healing wounds.
Further studies on the nutritive value of this forgotten crop should be undertaken, particularly that there is little knowledge
of the chemical composition of both marsh woundwort and its Asian relative Stachys affinis widely cultivated in China as a vegetable. 相似文献
144.
Martin Mihaljevi? Vojt��ch Ettler Ond?ej ?ebek Ondra Sracek Bohdan K?��bek Tom��? Kyncl Vladim��r Majer Franti?ek Veselovsky 《Water, air, and soil pollution》2011,216(1-4):657-668
The composition of tree rings and soils was studied at several locations affected by smelting and transportation in the vicinity of Kitwe (Copperbelt, Zambia). The contents of cobalt (Co), copper (Cu), manganese (Mn), and lead (Pb) and the 206Pb/207Pb isotope ratios in the tree rings were interpreted in relation to potential sources of contamination such as smelter production, acidification of the environment, soil composition, raw material processing, and atmospheric suspended particulate matter (SPM). The highest Co contents in the tree rings correspond to maximum ore production in the mid-1970s. Acidification through SO2 emissions is documented in the increased Mn contents from the mid-1980s. The isotopic composition of the tree rings of the studied tree species varies in the interval 1.16?C1.34 and the youngest parts of all the studied trees exhibit a low 206Pb/207Pb ratio (<1.17). The soil isotope composition varies in the range 206Pb/207Pb?=?1.18?C1.35. The Pb isotope composition in the soils and tree rings was formed by a combination of lithogenic Pb (206Pb/207Pb????1.3), Pb in processed ores (206Pb/207Pb????1.2), and SPM (automobile) Pb (206Pb/207Pb??1.1). As the soils in the distant region have high 206Pb/207Pb ratios (>1.3) in the whole profile and simultaneously the youngest parts of the tree rings of tree species growing in this soil have a low 206Pb/207Pb ratio (<1.17), it can be assumed that the Pb in the youngest parts of the tree species is derived from absorption of SPM Pb through the bark rather than root uptake. The absence of Pb with a low 206Pb/207Pb ratio in soils in the distant area is probably affected by fires in the herbaceous and bush undergrowth and plant litter, which prevents Pb from biomass from entering the soil and mobilize it back into the atmosphere. 相似文献
145.
Many studies have reported impeded root growth in topsoil under reduced tillage or direct drilling, but few have quantified the effects on the least limiting water range for root growth. This study explored the effects of tillage intensity on critical soil physical conditions for root growth in the topsoil. Samples were taken from a 7-year tillage experiment on a Danish sandy loam at Foulum, Denmark (56°30′ N, 9°35′ E) in 2008. The main crop was spring barley followed by either dyer's woad (Isatis tinctoria L.) or fodder radish (Raphanus sativus L.) cover crops as subtreatment. The tillage treatments were direct drilling (D), harrowing 8-10 cm (H), and ploughing (P) to 20 cm depth. A chisel coulter drill was used in the H and D treatments and a traditional seed drill in the P treatment. Undisturbed soil cores were collected in November 2008 at soil field moisture capacity from the 4-8 and 12-16 cm depths.We estimated the critical aeration limit from either 10% air-filled porosity (εa) or relative gas diffusivity (D/D0) of 0.005 or 0.02 and found a difference between the two methods. The critical limit of soil aeration was best assessed by measuring gas diffusivity directly. Root growth was limited by a high penetration resistance in the D and H soils (below tillage depth). Poor soil aeration did not appear to be a significant limiting factor for root growth for this sandy loam soil, irrespective of tillage treatment. The soil had a high macroporosity and D/D0 exceeded 0.02 at field capacity. Fodder radish resulted in more macropores, higher gas diffusivity and lower pore tortuosity compared to dyer's woad. This was especially important for the H treatment where compaction was a significant problem at the lower depths of the arable layer (10-20 cm depth). Our results suggest that fodder radish could be a promising tool in the amelioration of soil compaction. 相似文献
146.
Minna-Liisa?RantalainenEmail author Hannu?Fritze Jari?Haimi Oili?Kiikkil? Taina?Pennanen Heikki?Set?l? 《Biology and Fertility of Soils》2004,39(3):200-208
Due to their high abundance and ubiquitous existence, microbes are considered to be efficient colonisers of newly established habitats. To shed light on the dispersal mechanisms of soil microbes, a controlled microcosm experiment was established. In these microcosms, the dispersal of microbes from a source humus patch to originally sterile humus patches (embedded in a mineral soil matrix) was followed for 16 months, applying 16S and 18S ribosomal DNA-based PCR-DGGE molecular methods. Specifically, the role of enchytraeid worms and habitat (humus) corridors as possible facilitators of microbe dispersal was studied. The results showed that enchytraeid worms function efficiently as vectors for horizontal dispersal of saprophytic fungi in soil. Some of the fungi also proved to disperse through the corridors by vegetative growth, although this dispersal was inefficient as compared to dispersal with the enchytraeids. Virtually no saprophytic fungi were able to disperse through the mineral soil matrix in the absence of both enchytraeid worms and corridors. Unlike soil fungi, the dispersal of soil bacteria was not affected by any of the studied factors. The results of the present experiment provide direct evidence of the crucial role of soil fauna in aiding the horizontal dispersal of soil fungi. The role of enchytraeids as a functionally important species in boreal forest soils is further emphasized, since bringing microbes into contact with new resources is likely to enhance the rate of decomposition in soils. 相似文献
147.
A greenhouse experiment was conducted with a specially designed apparatus consisting of an upper and lower chamber where the treatment with rice was carried out (treatment 1). The apparatus also had a single chamber where treatment 2, without rice plants, was carried out. The scope of this study was to elucidate the influence of rice plant growth on gaseous N losses as N2 and N2O produced by nitrification-denitrification in a flooded soil fertilized with (NH4)2SO4 (with 56.50 atom% 15N). Gas samples were withdrawn weekly and analyzed for (N2 + N2O)-15N losses by mass spectrometer and for N2O by gas chromatograph. The gaseous (N2 + N2O)-15N losses of the treatment with rice plants were significantly (P =0.01) higher than those of the treatment without rice plants, as were the amounts of N2O emitted. Rice plants facilitate the efflux of N2 and N2O from soil to atmosphere, as about half of the total gaseous 15N loss as N2 and N2O was found in the upper chamber. The proportion of N2O-15N to (N2 + N2O)-15N in the upper chamber was 10.56%, much higher than that of the lower chamber in treatment 1 and the headspace of treatment 2. 相似文献
148.
149.
Land use changes threaten agricultural land. If agricultural land is going to be preserved, the social and economic causes
of conversion must be understood. However, analyzing the causes of agricultural conversion is complex because trends need
to be documented before analyzing the causes. One of the leading uses of agricultural land is for residential purposes. This
paper projects residential development in a Hudson River Valley watershed within Dutchess County in New York State using an
integrated modeling framework consisting of an econometric model, a geographic information system (GIS), and Monte Carlo simulation.
The econometric model is used to project residential development, providing parcel-specific probabilities of residential development.
The GIS is employed to extract socio-economic and county-level tax parcel data to be used in conjunction with bio-geophysical
attributes, such as slope, soil, and location, to calculate and project growth trends on a residential level for undeveloped
land parcels. Monte Carlo simulation is used to distribute these projections into the GIS to display outcomes of scenario
analyses to provide policy-makers a demonstration of how policies would likely affect the agricultural landscape of the watershed.
John M. Polimeni is an Assistant Professor of Economics in the Department of Humanities and Social Sciences at the Albany College of Pharmacy
in Albany, New York. Dr. Polimeni received a PhD in Ecological Economics and a BS in Mathematics at Rensselaer Polytechnic
Institute. His professional interests include: quantitative economics, spatial and regional economics, land use change, economic
development, and graduate education in Ecological Economics. He is currently working on linking ecological economics with
the laws of thermodynamics and neuroeconomics into one unified model. 相似文献
150.
Kay?HamerEmail author Astrid?Hadeler Thomas?Muschalla Jürgen?Schr?ter Günter?Timmer 《Journal of Soils and Sediments》2003,3(4):284-291