全文获取类型
收费全文 | 2150篇 |
免费 | 103篇 |
国内免费 | 1篇 |
专业分类
林业 | 101篇 |
农学 | 69篇 |
基础科学 | 9篇 |
385篇 | |
综合类 | 295篇 |
农作物 | 167篇 |
水产渔业 | 157篇 |
畜牧兽医 | 819篇 |
园艺 | 57篇 |
植物保护 | 195篇 |
出版年
2023年 | 10篇 |
2021年 | 28篇 |
2020年 | 42篇 |
2019年 | 38篇 |
2018年 | 35篇 |
2017年 | 35篇 |
2016年 | 40篇 |
2015年 | 34篇 |
2014年 | 56篇 |
2013年 | 98篇 |
2012年 | 99篇 |
2011年 | 142篇 |
2010年 | 56篇 |
2009年 | 74篇 |
2008年 | 102篇 |
2007年 | 123篇 |
2006年 | 128篇 |
2005年 | 95篇 |
2004年 | 110篇 |
2003年 | 92篇 |
2002年 | 87篇 |
2001年 | 37篇 |
2000年 | 36篇 |
1999年 | 35篇 |
1998年 | 19篇 |
1997年 | 23篇 |
1996年 | 17篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 16篇 |
1992年 | 39篇 |
1991年 | 13篇 |
1990年 | 29篇 |
1989年 | 27篇 |
1988年 | 16篇 |
1987年 | 24篇 |
1986年 | 20篇 |
1985年 | 29篇 |
1984年 | 26篇 |
1982年 | 12篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 13篇 |
1976年 | 18篇 |
1975年 | 11篇 |
1974年 | 19篇 |
1973年 | 14篇 |
1968年 | 9篇 |
1967年 | 14篇 |
排序方式: 共有2254条查询结果,搜索用时 15 毫秒
61.
The question of what determines plant community composition is fundamental to the study of plant community ecology. We examined
the relative roles of historical land use, landscape context, and the biophysical environment as determinants of plant community
composition in regenerating citrus groves in north-central Florida. Results were interpreted in light of plant functional
traits. Herbaceous and woody plants responded differently to broad-scale variables; herbs correlated most strongly with surrounding
land cover at a scale of 8 km, while the only significant determinant of woody species distributions was local land use history.
There were significant correlations between herbaceous species and spatial context, habitat isolation, environmental variables,
and historical variables. Partial Mantel tests indicated that each variable provided a unique contribution in explaining some
of the variation in the herbaceous dataset. The correlation between woody plants and local historical variables remained significant
even with other effects corrected for. In the herbaceous community, species composition was linked to functional traits much
as expected from classical theory. While spatial influences in our study system are important for both woody and herbaceous
plants, the primary determinant of plant community composition in regenerating citrus groves is historical land use. Our results
suggest that the fine-scale mechanisms of local competition, tolerance and facilitation invoked by many classical studies
may ultimately be less important than land use history in understanding current plant community composition in regenerating
agricultural areas. 相似文献
62.
The present study examines the pattern of accumulation of cadmium (Cd) and its biochemical effects on selected tissues of
a variety of African catfish (Clarias gariepinus), after exposure to various doses of Cd. The results obtained indicate that at the end of 21 days of exposure, the total
tissue organ cadmium concentration followed the pattern kidney > gill > liver > muscle for each of the exposure concentrations.
The levels of Cd in these organs were higher than those in ambient water. Moreover, while the rate of uptake of Cd increased
with time in the kidney, liver and muscle, it decreased in the gill. Superoxide dismutase (SOD) was significantly elevated
only in the kidney of catfish treated with 0.2 and 0.4 ppm of Cd for 7 days compared with the control. Conversely, gill SOD
was significantly decreased in the same concentrations of Cd-treated catfish relative to the control. Statistically similar
levels of SOD were observed in the liver, brain and muscle with all the treatments after the same duration of treatment. In
the fish exposed for 21 days, SOD activity was significantly decreased in the kidney with a corresponding increase in lipid
peroxidation (LPO), but it manifested only with the 0.2- and 0.4-ppm Cd treatment relative to the control. In the liver, however,
Cd exposure significantly increased SOD in the 0.2- and 0.4-ppm treatments of the same duration. In conclusion, the present
study indicates that the accumulation of Cd and its effect on SOD and LPO in C. gariepinus is dependent on concentration, tissue and time 相似文献
63.
64.
Water, Air, & Soil Pollution - The basis for the present energy dilemma is described. The current solution to the inadequate petroleum and natural gas supplies and possible long-term solutions... 相似文献
65.
66.
J. H. Heckman H. G. Zuckerman C. H. Metzger B. A. Brown E. R. Bennett W. B. Ward C. L. Fitch O. H. Elmer Jhon S. Gardner Julan C. Miller Frank W. Hussey E. L. Newdick Ralph W. Donaldson Lionel Harris H. O. Werner Wm. H. Martin E. V. Hardenburg T. E. Odland Robert Schmidt Geo E. Prince Brooks D. Drain Harold L. Bailey G. S. Ralston Chase D. Gaines J. G. Milward John Tucker 《American Journal of Potato Research》1935,12(6):158-170
67.
68.
Jeroen H. van Houts Siqun Wang Huiping Shi George W. Kabalka 《Wood Science and Technology》2006,40(6):437-443
The previous paper in this series demonstrates the use of a nuclear magnetic resonance (NMR) micro-imaging system to observe the movement of liquid water through small specimens (11 × 16 mm2 cross-section) of oriented strandboard (OSB) and solid wood. In the present paper, a NMR body scanner is utilized to obtain images of moisture penetration into ASTM D 1037–99 standard sized (152 × 152 mm2) thickness swelling specimens at different intervals during water soaking. As with the micro-imaging study, silicone was applied to the surfaces and edges of some of the specimens in order to observe the directional characteristics of moisture movement through OSB. Specimens with commercially applied edge sealant were also included in the study in order to observe the effectiveness of the sealant of reducing moisture penetration and thickness swelling. The presence of voids and areas of low density were shown to contribute significantly to moisture penetration into the specimens. 相似文献
69.
George I. Mantanis Jozef Martinka Charalampos Lykidis Libor Ševčík 《Wood material science & engineering》2020,15(5):303-311
ABSTRACT The objective of the work was to evaluate the efficacy of two new polyphosphate-based fire retardants (FRs) and one commercial product named Siriono® on the fire performance and physical–mechanical properties of medium density fibreboard (MDF) fabricated in the laboratory from Scots pine (Pinus sylvestris L.) wood. The fibres were treated with aqueous solutions of fire retardants, at 12% loading (dry salt on dry wood), and bonded with a melamine urea formaldehyde (MUF) adhesive. The physical and mechanical properties of panels were assessed using the European standards, whereas their fire performance was evaluated using an in-house method and the Cone calorimeter. In overall, the chemicals added enhanced the fire and smoke properties of the panels to varying degrees. Critical FR parameters such as peak heat release rate (peak HRR), total heat release (THR) and total smoke production (TSP) were significantly improved in the FR-treated panels, as exhibited in cone calorimeter tests. However, the internal bond strength of treated panels largely decreased by the addition of fire retardants, while thickness swell and water absorption negatively affected to a significant extent. In contrast, the formaldehyde release of the panels was considerably decreased at the E1 class level, with the incorporation of the polyphosphate-based additives. 相似文献
70.
We compared radiation-use efficiency of growth (epsilon;), defined as rate of biomass accumulation per unit of absorbed photosynthetically active radiation, of forest plots exposed to ambient (approximately 360 micro l l-1) or elevated (approximately 560 micro l l-1) atmospheric CO2 concentration ([CO2]). Large plots (30-m diameter) in a loblolly pine (Pinus taeda L.) plantation, which contained several hardwood species in the understory, were fumigated with a free-air CO2 enrichment system. Biomass accumulation of the dominant loblolly pines was calculated from monthly measurements of tree growth and site-specific allometric equations. Depending on the species, leaf area index (L*) was estimated by three methods: optical, allometric and litterfall. Based on the relationship between tree height and diameter during the first 3 years of exposure, we conclude that elevated [CO2] did not alter the pattern of aboveground biomass allocation in loblolly pine. There was considerable variation in L* estimates by the different methods; total L* was 18-42% lower when estimated by the optical method compared with estimates from allometric calculations, and this discrepancy was reduced when optical measurements were corrected for the non-random distribution of loblolly pine foliage. The allometric + litterfall approach revealed a seasonal maximum total L* of 6.2-7.1 with about 1/3 of the total from hardwood foliage. Elevated [CO2] had only a slight effect on L* in the first 3 years of this study. Mean epsilon; (+/- SD), calculated for loblolly pine only, was 0.49 +/- 0.05 and 0.62 +/- 0.04 g MJ-1 for trees in the ambient and elevated [CO2] plots, respectively. The 27% increase in epsilon; in response to CO2 enrichment was caused primarily by the stimulation of biomass increment, as there was only a small effect of elevated [CO2] on L* during the initial years of fumigation. Long-term increases in atmospheric [CO2] can increase epsilon; in closed-canopy forests but the absolute magnitude and duration of this increase remain uncertain. 相似文献