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61.
Plants often exhibit proportionately larger photosynthetic responses to the transition from glacial to modern [CO(2)] than from modern to future [CO(2)]. Although this pattern may reflect increased nutrient demand with increasing [CO(2)], few studies have examined the role of nutrient supply in regulating responses to the range of [CO(2)] from glacial to future [CO(2)]. In this study, we examined the effects of P supply (0.004-0.5 mM) on photosynthetic responses of Populus deltoides (cottonwood) seedlings to glacial (200 micromol mol(-1)), modern (350 μmol mol(-1)) and future (700 micromol mol(-1)) [CO(2)]. The A(sat) (light-saturated net photosynthetic rates at the growth [CO(2)]) response to future [CO(2)] decreased with decreasing P supply such that there was no response at the lowest P supply. However, P supply did not affect A(sat) responses to an increase from glacial to modern [CO(2)]. Photosynthetic capacity [e.g., final rubisco activity, apparent, maximal Rubisco-limited rate of photosynthesis (V(cmax)), apparent, maximal electron transport-limited rate of photosynthesis (J(max))], stomatal conductance (g(s)) and leaf P generally increased with increasing P supply but decreased with increasing [CO(2)]. Measures of carbohydrate sink capacity (e.g., leaf mass per unit leaf area, leaf starch) increased with both increasing P supply and increasing [CO(2)]. Changes in V(cmax) and g(s) together accounted for 78% of the variation in A(sat) among [CO(2)] and P treatments, suggesting significant biochemical and stomatal controls on photosynthesis. However, A(sat) responses to increasing [CO(2)] did not reflect the changes in the carbohydrate sink capacity. These results have important implications because low P already constrains responses to increasing [CO(2)] in many ecosystems, and our results suggest that the P demand will increasingly affect A(sat) in cottonwood as [CO(2)] continues to increase. 相似文献
62.
63.
The effects of thermo-mechanical refining conditions on the properties of medium density fiberboard (MDF) made from black
spruce (Picea mariana) bark were evaluated. The bark chips were refined in the MDF pilot plant of Forintek Canada Corporation under nine different
refining conditions in which preheating retention time was adjusted from 3 to 5 to 7 min and steam pressure was set at either
0.6, 0.9 or 1.2 MPa. The resulting bark fibers were blended with 12% UF resin (based on oven-dry fiber weight) using a mechanical
blender. The resinated fibers were manually formed into fiber mats and hot-pressed into MDF panels using consistent parameters.
Two panels for each refining condition were produced, resulting in a total of 18 panels. Analysis of variance (ANOVA) was
used to analyze the significance of factors. Regression coefficients and 3D contour plots were used to quantify the relationship
between panel properties and the two test factors. The results from this study indicated that the preheating retention time
was a significant factor for both modulus of rupture (MOR) and modulus of elasticity (MOE), the steam pressure was a significant
factor for internal bond strength (IB), MOR and MOE, whereas both factors were insignificant for thickness swelling, water
absorption and linear expansion. The properties of MDF panels were quadratic functions of retention time and steam pressure.
Compared to the ANSI standard for 120-grade MDF, most panels with a nominal density of 950 kg/m3 had very high IB (>1 MPa) and acceptable MOR, MOE and dimension stabilities. These results suggest that black spruce bark
residues can be considered as a potentially suitable raw material for manufacturing MDF products. 相似文献
64.
Weston F. Mwase Åsmund Bjørnstad James M. Bokosi Moses B. Kwapata Brita Stedje 《New Forests》2007,33(3):297-307
Biodiversity is declining throughout southern African miombo woodlands due to poor land use practices that are detrimental
to soil, vegetation and habitat. We aimed at examining tree and shrub species diversity and distribution in Uapaca kirkiana-dominated woodlands under three land tenure types; forest reserves, customary and leasehold land. The study was conducted
at different forest sites within similar agroecological zone in southern Malawi. Study sites were located at a range of altitudes
between 900 and 1,200 m a.s.l. Three circular plots each with 32.60 m radius were established at each study site to sample
tree and shrub species composition, structure and distribution under three land tenure types. We found that forest reserves
had higher species diversity with an average of 16 tree families, 27 genera and 34 species as compared to only 10 tree families,
6 genera and 6 species on customary forest lands. Comparisons of diameter at breast height size class distributions showed
that customary land had significantly low numbers of small (5.0–10.0 cm) and very large (≥30 cm) diameter classes suggesting
lower levels of regeneration and high rate of anthropogenic activities. The high species diversity and richness in forest
reserves and leasehold land indicate high potential for protected lands to restore tree species diversity. It is concluded
that levels of human activities as influenced by land tenure type reduce tree species diversity, composition and distribution
at the different sites, and this confirms the hypothesis that open access lands are not compatible with conservation of tree
and shrub species diversity because of high anthropogenic activities. 相似文献
65.
Clement C.R. Weber J.C. van Leeuwen J. Astorga Domian C. Cole D.M. Arévalo Lopez L.A. Argüello H. 《Agroforestry Systems》2004,61(1-3):195-206
Agroforestry Systems - Peach palm (Bactris gasipaes) was domesticated as a fruit crop by the first Amazonians in traditional Neotropical agroforestry systems, but research and development... 相似文献
66.
Monicah Mucheru-Muna Daniel Mugendi James Kung’u Jayne Mugwe Andre Bationo 《Agroforestry Systems》2007,69(3):189-197
Soil nutrient depletion as a result of continuous cultivation of soils without adequate addition of external inputs is a major
challenge in the highlands of Kenya. An experiment was set up in Meru South District, Kenya in 2000 to investigate the effects
of different soil-incorporated organic (manure, Tithonia diversifolia, Calliandra calothyrsus, Leucaena leucocephala) and mineral fertilizer inputs on maize yield, and soil chemical properties over seven seasons. On average, tithonia treatments
(with or without half recommended rate of mineral fertilizer) gave the highest grain yield (5.5 and 5.4 Mg ha−1 respectively) while the control treatment gave the lowest yield (1.5 Mg ha−1). After 2 years of trial implementation, total soil carbon and nitrogen contents were improved with the application of organic
residues, and manure in particular improved soil calcium content. Results of the economic analysis indicated that on average
across the seven seasons, tithonia with half recommended rate of mineral fertilizer treatment recorded the highest net benefit
(USD 787 ha−1) while the control recorded the lowest (USD 272 ha−1). However, returns to labor or benefit-cost ratios were in most cases not significantly improved when organic materials were
used. 相似文献
67.
Bare-root stock of Douglas-fir, lodgepole pine, white fir, and grand fir, inoculated with Pisolithus tinctorius and handled by standard nursery, cold storage and planting practices performed no better than stock which was not intentionally inoculated but which had formed mycorrhizae with indigenous, nursery fungi on a variety of different outplanting sites in southwestern Oregon. Climate, planting sites and nursery practices in the Pacific Northwest differ drastically from those in the southeastern United States, where P. tinctorius has increased plantation survival and growth. Before P. tinctorius can be recommended for nursery inoculation in the Pacific Northwest, its reaction to each phase of the nursery and planting process needs to be carefully analyzed to determine where the problems lie.This article was written and prepared by US Government employees on official time, and is therefore in the public domain. 相似文献
68.
One-day rate measurements for estimating net nitrification potential in humid forest soils 总被引:2,自引:0,他引:2
Donald S. Ross Guinevere Fredriksen Austin E. Jamison Beverley C. Wemple Scott W. Bailey James B. Shanley Gregory B. Lawrence 《Forest Ecology and Management》2006,230(1-3):91-95
Measurements of net nitrification rates in forest soils have usually been performed by extended sample incubation (2–8 weeks), either in the field or in the lab. Because of disturbance effects, these measurements are only estimates of nitrification potential and shorter incubations may suffice. In three separate studies of northeastern USA forest soil surface horizons, we found that laboratory nitrification rates measured over 1 day related well to those measured over 4 weeks. Soil samples of Oa or A horizons were mixed by hand and the initial extraction of subsamples, using 2 mol L−1 KCl, occurred in the field as soon as feasible after sampling. Soils were kept near field temperature and subsampled again the following day in the laboratory. Rates measured by this method were about three times higher than the 4-week rates. Variability in measured rates was similar over either incubation period. Because NO3− concentrations were usually quite low in the field, average rates from 10 research watersheds could be estimated with only a single, 1-day extraction. Methodological studies showed that the concentration of NH4+ increased slowly during contact time with the KCl extractant and, thus, this contact time should be kept similar during the procedure. This method allows a large number of samples to be rapidly assessed. 相似文献
69.
The effects of using 100% black spruce (Picea mariana) bark fibers as core layer material accounting for up to 70% of the board and its resin content (between 6 and 10%) on the
properties of three-layer medium-density fiberboard (MDF) were investigated using a full factorial experimental design with
two independent variables and three levels. Five response variables, namely internal bond strength, modulus of rupture, modulus
of elasticity, thickness swelling and water absorption were statistically analyzed using a response surface methodology and
two-way analysis of variance. The effects of the proportion of core layer (bark fibers) and its resin content on panel properties
were significant. All properties studied were positively affected by increasing core layer resin content. The effects of the
proportion of core layer (bark fibers) on mechanical properties and water absorption were negative, but positive on thickness
swelling. Simultaneous optimization of panel properties indicated that at a density of 850 kg/m3, a three-layer MDF with a core layer resin content of 6.5%, a face resin content of 12 and 60% of core layer proportion (spruce
bark fibers) would satisfy the minimum requirements of ANSI standard for 130-grade MDF. Overall, black spruce bark, a major
residue source in the Eastern Canada, should be considered as a supplemental furnish for the core layer materials of a three-layer
MDF. 相似文献
70.
Butternut (Juglans cinerea L.) is a native, cold-tolerant, hard-mast species formerly valued for its nuts and wood, which is now endangered. The most immediate threat to butternut restoration is the spread of butternut canker disease, caused by the exotic fungus Sirococcus clavigignenti-juglandacearum Nair, Kostichka & Kuntz. Other threats include the hybridization of butternut with the exotic Japanese walnut (Juglans ailantifolia Carr.) and poor regeneration. The hybrids, known as buartnuts, are vegetatively vigorous, highly fecund, more resistant than butternut to butternut canker disease and difficult to identify. We review the vegetative and reproductive morphological traits that distinguish butternut from hybrids and identify those that can be used by field biologists to separate the taxa. No single trait was sufficient to separate butternut from hybrids, but pith color, lenticel size, shape and abundance, and the presence or absence of a notch in the upper margin of leaf scars, can be used in combination with other traits to identify butternuts and exclude most hybrids. In at least one butternut population, reduced symptoms of butternut canker disease were significantly associated with a dark barked phenotype. We also describe two randomly amplified polymorphic DNA (RAPD) markers that differentiate butternuts from hybrids based on DNA polymorphism. Together, these results should assist in the identification and testing of non-hybrid butternut for breeding and reintroduction of the species to its former habitats. 相似文献