全文获取类型
收费全文 | 1948篇 |
免费 | 129篇 |
国内免费 | 2篇 |
专业分类
林业 | 166篇 |
农学 | 36篇 |
基础科学 | 12篇 |
358篇 | |
综合类 | 328篇 |
农作物 | 60篇 |
水产渔业 | 146篇 |
畜牧兽医 | 781篇 |
园艺 | 63篇 |
植物保护 | 129篇 |
出版年
2024年 | 4篇 |
2023年 | 17篇 |
2022年 | 28篇 |
2021年 | 54篇 |
2020年 | 56篇 |
2019年 | 78篇 |
2018年 | 66篇 |
2017年 | 59篇 |
2016年 | 62篇 |
2015年 | 48篇 |
2014年 | 63篇 |
2013年 | 84篇 |
2012年 | 150篇 |
2011年 | 179篇 |
2010年 | 85篇 |
2009年 | 60篇 |
2008年 | 141篇 |
2007年 | 110篇 |
2006年 | 146篇 |
2005年 | 117篇 |
2004年 | 102篇 |
2003年 | 113篇 |
2002年 | 91篇 |
2001年 | 11篇 |
2000年 | 18篇 |
1999年 | 12篇 |
1998年 | 15篇 |
1997年 | 12篇 |
1996年 | 4篇 |
1995年 | 14篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1957年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有2079条查询结果,搜索用时 0 毫秒
991.
Mum-Keng Wong Paramananthan Selliah Meor Hakif Amir Hassan Eric Van Ranst Kazuyuki Inubushi 《Soil Science and Plant Nutrition》2020,66(1):214-224
ABSTRACT In Malaysia, soils derived from sedimentary rocks are extensively used for agricultural purposes with oil palm and rubber being the main dwellers. In order to understand the environmental impact of these perennial crops planting, the variability of physicochemical properties of 25 representative soils derived from sedimentary rocks under different ecosystems (agriculture land and natural forest) at six study sites spread across Malaysia was examined. Among the soil physicochemical properties, total soil organic carbon, total nitrogen, and fertility level were found to be generally higher in the forest ecosystems followed by rubber plantation and finally oil palm plantations. Likewise, projection of principal component analysis showed an associative relationship between soil physicochemical properties and microhabitats. Finally, this study showed that soils from different agricultural and natural sites, but derived from similar sedimentary rocks, had distinctive weathering conditions and soil properties. Therefore, site-specific field management according to soil type, soil management techniques as well as fertilizer strategies are required to maximize crop production and to sustain ecosystem services. The output of this study will enable farmers to improve their crop yield via the selection of suitable crops cultivation based on soil characteristics. 相似文献
992.
We analyze the status of the Angel de la Guarda deer mouse (Peromyscus guardia), a species endemic to Mexico, based on our own fieldwork, bibliographic records and information from colleagues. This species, with different subspecies on three islands and a population of an undetermined subspecies on a fourth, was apparently common until the mid-1960s. Currently the species is critically endangered, if not already extinct. One of the subspecies and the undetermined population are presumed extinct. Of the two other subspecies, one is at least in critical condition, and the other at least reduced. The demise of the species can be attributed to the introduction of domestic cats. The case of P. guardia is a good example of the vulnerability of other taxa of endemic rodents on islands in the Gulf of California. 相似文献
993.
Enhanced resistance to Helicoverpa zea in tobacco expressing an activated form of maize ribosome- inactivating protein 总被引:5,自引:0,他引:5
Dowd PF Zuo WN Gillikin JW Johnson ET Boston RS 《Journal of agricultural and food chemistry》2003,51(12):3568-3574
Progeny of two transgenic tobacco (Nicotiana tabacum L.) lines that expressed an activated form of maize (Zea mays L.) ribosome-inactivating protein (RIP) had varying resistance to the insect species tested. A subset of R(2) plants from the two lines appeared to be more resistant to larvae of the cigarette beetle, Lasioderma serricorne (F.), and the tobacco hornworm, Manduca sexta (L.) than the wild type plants. Progeny (R(3)) of the more resistant R(2) plants were tested more extensively for insect resistance. Resistance to the corn earworm, Helicoverpa zea (Boddie), was most consistent, with significantly decreased feeding often accompanied by increased mortality and reduced weights of survivors fed on leaf disks of the two transgenic lines compared to the wild type. The amount of damage by H. zea was significantly inversely correlated with levels of RIP. Resistance of RIP-producing plants to H. zea was greater than expected on the basis of prior in vitro results using diet-incorporated maize RIP. The R(3) transgenic plant leaf disks were also often more resistant to feeding by larvae of L. serricorne compared to wild type plants. Although reduced feeding by M. sexta was noted when they were fed leaf disks from transgenic compared to wild type plants the first day of exposure, differences were not significant. This information provides further support for maize RIP having a role in resistance to maize-feeding insects. 相似文献
994.
Eric S. Menges Pedro F. Quintana Ascencio Orou G. Gaoue 《Biological conservation》2006,127(1):115-127
We use population viability analysis of an endangered Florida scrub mint, Dicerandra frutescens, to specify the optimal fire return intervals for its long-term persistence and for its specific habitat. We derived 83 population projection matrices from 13 years of demographic data from eight populations, 59 matrices from scrub populations and 24 from firelane or yard edges. Seed dormancy and germination transitions were inferred based on experimental data and verified by comparing modeled vs. observed population trajectories. Finite rates of increase in scrub sites were highest shortly after fire and declined steeply through 10 years postfire. The break-even value of λ = 1 was passed quickly, in about six years, suggesting that populations >6 years postfire were already facing decline. The decline is probably related to the rapid growth of competing shrubs in the habitat of D. frutescens. In long-unburned sites, finite rates of increase were nearly always <1 and declined the most in the long-unburned site with no foot trails or treefall gaps. Finite rates of increase in firelane populations also declined with years since fire or last disking. The yard edge population showed λ values both >1 and <1, with no temporal trend. Stochastic simulations in scrub sites suggested an optimal regular fire return interval of about 6-12 years. Regular fires at this interval were more favorable than stochastic fire regimes, but stochasticity reduced extinction percentages at longer fire return intervals. Stochastic fire return intervals implied a wider optimal fire return interval of 6-21 years. We suggest that prescribed fire in Florida scrub on yellow sand has occurred (and needs to occur) more frequently than previously recommended. 相似文献
995.
James M. Kaste Andrew J. Friedland Eric K. Miller 《Water, air, and soil pollution》2005,167(1-4):139-154
Anthropogenic emissions during the 20th century resulted in global lead (Pb) contamination of soils. Recent studies have demonstrated that the organic horizons of temperate montane Spodosols in the northeastern United States retain Pb on timescales of 50 to 150 years. The precise mechanism(s) of this strong Pb partitioning to organic-rich soil material remain elusive, but a detailed understanding of Pb retention by organic layers and mineral topsoils is critical for predicting the fate of pollutants deposited on ecosystems. Here we use selective extractions to quantify potentially mobile pools of Pb in the surface horizons of relatively remote montane Spodosols from New Hampshire and Vermont. Using 10 consecutive rinses of water, we extracted a total of 1 to 5% of the carbon, and 4 to 12% of the Pb. Dialysis equilibration experiments demonstrate that this Pb is >5,000 molecular weight, and not truly dissolved as Pb2+. When soil was extracted with a single rinse of 0.02 M HCl (pH 1.7), 5 to 11% of the Pb was mobilized. However, hydroxylamine hydrochloride in 0.02 M HCl (a reducing agent) extracted 30 to 40% of the Pb. Repeated rinses with sodium hydroxide and sodium pyrophosphate, which target organic matter but may extract other soil phases removed 16 to 75% and 60 to 100% of the Pb, respectively. We show that significant Fe, Pb, and Al can be released from soils under reducing conditions, and that this fraction can be underestimated if sodium pyrophosphate is used in a previous step for leaching the organic-metal phase, as is typically done in sequential extraction schemes. Our results indicate that inorganic phases play an important role in determining Pb mobility and bioavailability, even in surface soil horizons dominated by organic matter. 相似文献
996.
OBJECTIVES: To stress the importance of social and environment (nutritional) factors in determining the growth spurt during puberty and the risk of excessive adiposity, two contrasting adolescent populations, one from a rural area of Senegal (West Africa) and the other from Martinique (French West Indies), were compared. DESIGN: Cross-cultural comparison of contrasting populations. Adolescents from Senegal belonged to a cohort followed up since 1995. Adolescents from Martinique participated in a cross-sectional nutritional survey that covered the entire island. SUBJECTS: A total of 507 adolescents (mean age: 14.3+/-0.7 years) from Senegal (319 girls and 188 boys) and 703 adolescents from Martinique (351 boys and 352 girls) were surveyed. RESULTS: Differences in growth and maturation were striking: boys in Martinique were 22.7 kg heavier and 20.1 cm taller than boys in Senegal. Differences were less important for girls but still evident: 12.6 kg in weight and 10.5 cm in stature. In Senegal, there were virtually no overweight adolescents, but 18% of girls and 50% of boys could be considered as malnourished. In Martinique, 19% of girls and 23% of boys were overweight or obese. Adolescent girls from Martinique were also sexually more mature than adolescent girls from Senegal. When comparisons were repeated after Senegalese girls reached menarche, differences in weight and body mass index disappeared, but Senegalese girls were still shorter than girls from Martinique. CONCLUSIONS: Adolescents are extremely susceptible to nutritional changes and their particular situation needs to be incorporated into nutritional prevention programmes. 相似文献
997.
The salamander, Dicamptodon tenebrosus, is threatened by habitat loss and fragmentation associated with forest harvest activities. We used three microsatellite loci and 38 amplified fragment length polymorphisms (AFLPs) to quantify population structure and indirectly evaluate the impacts of forest harvesting on this species. We sampled two old growth sites, three second growth sites and three recently clearcut sites in British Columbia, Canada. Microsatellite allelic richness and the percentage of polymorphic AFLPs were positively correlated with the age of forest stands. Similarly, heterozygosity estimated from both marker types was positively correlated with stand age. Population subdivision (Fst) estimated among forested sites using microsatellite and AFLP markers was 0.033 and 0.095, respectively. Lower genetic variation and heterozygosity in recent clearcuts suggest that clearcut logging may be associated with local population declines. Our genetic findings are consistent with previous studies that report lower salamander densities in recently clearcut sites. 相似文献
998.
Understanding metapopulation dynamics in large carnivores with naturally fragmented populations is difficult because of the large temporal and spatial context of such dynamics. We coupled a long-term database of visitor sighting records with an intensive 3-year telemetry study to describe population dynamics of recolonization by black bears (Ursus americanus) of Big Bend National Park in Texas during 1988-2002. This population, which occurs within a metapopulation in western Texas and northern Mexico, increased from a single pair of known breeding-age animals in 1988 to 29 bears (including 6 females of breeding age) in March 2000 (λ = 1.25/year). A migration and dispersal event in August-December 2000 reduced the population to 2 adult females and as few as 5-7 individuals. One-way movement distances from the study area during this event averaged 76 km for females (n = 7) and 92 km for males (n = 4), and 3 animals conducted migrations of at least 154, 178, and 214 km, respectively. Our observations exemplify the importance of stochastic events on demographics of small populations and highlight the potential scale of bear movement among montane islands of southwestern North America. They also provide insight into the use of dispersal data in parameterizing metapopulation models for large carnivores. 相似文献
999.
Nicolas S. Bloom Eric M. Prestbo Björn Hall Eric J. Von Der Geest 《Water, air, and soil pollution》1995,80(1-4):1315-1318
An established industrial monitoring technique has been refined to allow the accurate determination of total gaseous Hg in air at background levels (1–3 ng·m?3). Samples are collected under vacuum through a commercially available iodide-impregnated carbon trap. For analysis, the trap is digested in a 7∶3 (v/v) mixture of HNO3+H2SO4, and an aliquot of the digest analyzed by SnCl2 reduction, dual gold amalgamation, and cold vapor atomic fluorescence spectrometric (CVAFS) detection. The traps exhibit little breakthrough (<5%) under flow rates of up to 1.0 l·min?1 for periods up to 10 days. The mean value for the blank on the half traps was found to be 0.40±0.16 ng (n=20), resulting in a 3 α detection limit of 0.07 ng·m (1 week sample at 1.0 L·m?3). Replicate field collections show a between-trap RSD of ± 6.4%, while field intercomparison with gold trapping resulted in 104±20% recovery in the 1–4 ng/m3 concentration range. 相似文献
1000.
Damià Barceló Mira PetroYic Eric de Deckere Joop F. Bakker Mark Scrimshaw Julia Stegemann Grazyna Kowalewska Peter Heininger Carlos Vale Peter Diedrieh Hansen María Jesús Belzunce Segarra Andrew Parker Véronique Poulsen Werner Manz Juliän Blasco Ángel DelValls Casillas 《Journal of Soils and Sediments》2004,4(4):223-224