全文获取类型
收费全文 | 4081篇 |
免费 | 277篇 |
国内免费 | 2篇 |
专业分类
林业 | 333篇 |
农学 | 78篇 |
基础科学 | 21篇 |
689篇 | |
综合类 | 689篇 |
农作物 | 172篇 |
水产渔业 | 373篇 |
畜牧兽医 | 1615篇 |
园艺 | 126篇 |
植物保护 | 264篇 |
出版年
2023年 | 42篇 |
2022年 | 52篇 |
2021年 | 101篇 |
2020年 | 132篇 |
2019年 | 163篇 |
2018年 | 156篇 |
2017年 | 122篇 |
2016年 | 134篇 |
2015年 | 100篇 |
2014年 | 148篇 |
2013年 | 206篇 |
2012年 | 270篇 |
2011年 | 334篇 |
2010年 | 177篇 |
2009年 | 165篇 |
2008年 | 274篇 |
2007年 | 279篇 |
2006年 | 227篇 |
2005年 | 212篇 |
2004年 | 196篇 |
2003年 | 197篇 |
2002年 | 155篇 |
2001年 | 33篇 |
2000年 | 22篇 |
1999年 | 36篇 |
1998年 | 27篇 |
1997年 | 19篇 |
1996年 | 18篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 24篇 |
1991年 | 21篇 |
1990年 | 21篇 |
1989年 | 17篇 |
1988年 | 19篇 |
1987年 | 8篇 |
1986年 | 16篇 |
1985年 | 13篇 |
1984年 | 17篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1974年 | 8篇 |
1972年 | 7篇 |
1971年 | 9篇 |
1924年 | 6篇 |
排序方式: 共有4360条查询结果,搜索用时 31 毫秒
1.
Tania Khandakar Chris Guppy Sheikh M. F. Rabbi Heiko Daniel 《Soil Use and Management》2021,37(1):120-125
Soil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (<53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (>15 g kg?1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil. 相似文献
2.
3.
Andr Z. Boaratti Thiago M. T. Nascimento Kifayat U. Khan Cleber F. M. Mansano Thaís S. Oliveira Daniel M. A. Queiroz Rafael S. Romaneli Andressa T. Rodrigues Nilva K. Sakomura Joo B. K. Fernandes 《Journal of the World Aquaculture Society》2021,52(1):88-104
The present study aimed to determine the ideal ratios of digestible essential amino acids (EAAs) for pacu (Piaractus mesopotamicus) juveniles by the amino acid (AA) deletion method. A completely randomized design which consisted of 11 treatments and three replicates each was used. The treatments included a control diet (CD) containing 55% of nonpurified natural ingredients and 45% of purified synthetic amino acids and ingredients, and other ten isonitrogenous and isoenergetic EAA limiting diets (LDs), each being deficient in 44.4 ± 0.02% of the respective EAA. Pacu juveniles with initial average body weight of 6.22 ± 0.09 g were distributed among 33 fiber glass tanks. Fish were fed with semipurified and extruded diets for 113 days two times a day until apparent satiation. The ideal ratio of each dietary EAA was calculated on the basis of the relationship between body N retention and amount of EAA deleted from the respective EAA LD. Based on the AA deletion method, the ideal ratios of digestible EAAs for pacu juveniles, relative to lysine requirement of 100% were estimated as: methionine 14.6%, threonine 35.0%, tryptophan 6.6%, arginine 62.8%, histidine 13.6%, isoleucine 26.3%, leucine 43.7%, phenylalanine 27.2%, and valine 35.8%. 相似文献
4.
Luis Daniel Jimnez‐Martínez Dariel Tovar‐Ramírez Carlos Alfonso lvarez‐Gonzlez Emyr Pea‐Marín Susana Camarillo‐Coop Rafael Martínez‐García Elena Palacios‐Mechetnov Rosario Martínez‐Yaez Bartolo Concha‐Frías 《Aquaculture Research》2020,51(7):2629-2640
Five experimental diets containing different lipid sources, fish oil (D1), soybean lecithin (D2), corn oil (D3), canola oil (D4) and olive oil (D5), were evaluated in Atractosteus tropicus larvae for the relative gene expression of the enzymes fatty acid synthase (fas), acetyl‐CoA carboxylase 1 (acc1) and carnitine palmitoyltransferase 1C (cpt1c), in addition to their effects on larval growth, survival and cannibalism during a 30‐day feeding trial. Higher growth and survival were obtained in treatments D1 and D2, and lower performance in diets D3, D4 and D5. The highest levels of expression of fas and acc1 occurred in larvae fed with D1, which contained high amounts of n‐3 long‐chain polyunsaturated fatty acids (LC‐PUFA), mainly DHA and EPA FA are regulators of lipogenesis. The higher cpt1c expression in plant‐based diets is attributed to the fact that these diets are rich in α‐linolenic acid (ALA) and low DHA, EPA and ARA levels that favour ß‐oxidation. In conclusion, the diets with fish oil (D1) and soybean lecithin (D2) were the best treatments for larval growth, survival and cannibalism and thus appear to meet both lipid and energy requirements of A. tropicus larvae, meanwhile the use of vegetable oils influences the expression of intermediary lipogenic genes. 相似文献
5.
Freshwater fishes are threatened globally, and often too little is known about threatened species to effectively guide their conservation. Habitat complexity is linked to fish species diversity and persistence, and degraded streams often lack habitat complexity. Beaver Castor spp., in turn, have been used to restore streams and increase habitat complexity. The northern leatherside chub Lepidomeda copei is a rare, small‐bodied, drift‐feeding minnow that has anecdotally been observed to use complex habitats associated with beaver dams in the western United States. To investigate this anecdote, we conducted fish and habitat surveys, the latter focusing on quantifying habitat complexity, in a sub‐basin of the Upper Snake River Basin in the USA. Complementary generalised linear model and path analyses revealed that northern leatherside chub occurred more often at sites with complex streamflows, and streamflows were more complex when beaver dams were present and pools were deeper. Northern leatherside chubs were also more likely to occur when temperatures were warmer, aquatic macrophytes were abundant and stream channels were narrow and deep. The linkage between chubs, complex streamflows and beaver dams needs to be evaluated more broadly to completely understand its role in the rangewide status of the species. However, it does suggests that increased use of beaver reintroductions and dam analogues for stream restoration could be a boon for the northern leatherside chub, but such efforts should be monitored to determine their effectiveness to help adapt beaver‐based restoration approaches to best benefit the species. 相似文献
6.
Krzysztof Kupren Katarzyna Paliska‐arska Sawomir Krejszeff Daniel arski 《Aquaculture Research》2019,50(9):2528-2536
The morphological development and allometric growth patterns of Eurasian perch, Perca fluviatilis L., a highly valued commercial species, were studied under intensive rearing conditions from hatching up to 50 DPH (Days Post Hatch). Based on the external morphology, four different phases during early development of Eurasian perch were identified: pre‐flexion larva 0–20 DPH (5.70–10.16 mm TL); flexion larva 22–30 DPH (11.09–15.14 mm TL) and post‐flexion larva/juvenile 32–50 DPH (18.00–24.75 mm TL). The results indicate that growth period when final replacement of all temporary (larval) structures and most important changes in the shape of P. fluviatilis occurred (between 13.95 and 24.06 mm TL, during flexion and post‐flexion phase) can be considered as a transitional period between the larva and juvenile. All body segments, except trunk length and tail length showed fast growth (positive allometry) throughout the entire studied period or up to the respective inflexion point with a common tendency to isometry. In addition, the specific behaviours (e.g. pelagic way of life) of Eurasian perch larvae resulted in some characteristic allometric growth patterns in the posterior region, different from the majority of other teleosts. The results are discussed with respect to the ontogeny of the functional morphology under both ecological and aquaculture considerations. 相似文献
7.
A Comparison of the Effects of Two Prey Enrichment Media on Growth and Survival of Pacific Bluefin Tuna,Thunnus orientalis,Larvae 下载免费PDF全文
Maria S. Stein Daniel Margulies Jeanne B. Wexler Vernon P. Scholey Katagiri Ryo Tomoki Honryo Tsukasa Sasaki Angel Guillen Yasuo Agawa Yoshifumi Sawada 《Journal of the World Aquaculture Society》2018,49(1):240-255
Three experiments were conducted to investigate the growth, survival, and standardized cohort biomass of Pacific bluefin tuna, Thunnus orientalis, larvae fed nutritionally enhanced prey during the first week of feeding using two commonly used, commercially available enrichment media, AlgaMac Enrich and Marine Glos. T. orientalis larvae exhibited exponential growth in standard length and dry weight. The daily specific growth rates in length and weight are the first reported for T. orientalis larvae and the averages ranged from 3.8 to 4.1% and 27.5%, respectively, for larvae in the AlgaMac treatment and from 4.1 to 6.1% and 31.5%, respectively, in the Marine Glos treatment. Average daily growth rates in length ranged from 0.16 to 0.23 mm/d for larvae in the AlgaMac treatment and from 0.17 to 0.27 mm/d for those in the Marine Glos treatment. Daily growth rates in length were similar to those reported for other tuna larvae reared in the laboratory but slower than most published estimates for larval tunas in situ at similar water temperatures. Mean prey number per gut was positively associated with mean prey level in the tank. Both enrichment media appear to be good sources of nutritional improvement of planktonic prey for T. orientalis larvae. 相似文献
8.
Elienai Ferreira da Silva Mara Regina Moitinho Daniel De Bortoli Teixeira Elton da Silva Bicalho Guilherme Adalberto Ferreira Castioni Gener Tadeu Pereira Newton La Scala 《Soil Use and Management》2020,36(4):658-670
Soil CO2 emission (FCO2) in agricultural areas results from the interaction of different factors such as climate and soil conditions. Our objective was to investigate the spatiotemporal variation of FCO2, temperature (Tsoil), moisture (Msoil) and air-filled pore space (AFPS), as well as their interactions, during the sugarcane field reform. The study was conducted on a 90 × 90 m sampling grid with 100 points at 10 m spacings. Ten assessments of FCO2, Tsoil and Msoil were carried out at each point over a 28-day period. The greatest mean values of FCO2 (0.74 g m−2 hr−1) and Msoil (31.7%) were obtained on Julian day 276, 2013, being associated with precipitation events at the study site. Also, the smallest values of AFPS (19.17%) and Tsoil (20.90°C) were observed on the same day. The spatial variability of FCO2, Tsoil, Msoil and AFPS was best described by an adjusted spherical model, although an exponential model better fitted some results. The spatial pattern of all soil attributes showed little temporal persistency, indicating a high complexity for FCO2 during precipitation. Correlation maps assisted in identifying regions where Msoil and AFPS better controlled the emission process and where Tsoil was important. A major challenge for world agriculture is to increase the efficiency of conventional soil management practices. We highlight the importance of the spatial pattern of soil properties that directly influence the CO2 emission dynamics. Future mitigation actions should involve less intense tillage and ensure homogeneous applications of soil inputs, thereby reducing production costs and the contribution of these activities to CO2 emissions during the sugarcane field reform. 相似文献
9.
Robert M. Boddey Daniel Rume Casagrande Bruno G. C. Homem Bruno J. R. Alves 《Grass and Forage Science》2020,75(4):357-371
Beef cattle producers seldom use fertilizers for their pastures in tropical regions of Brazil. Slowly, this is changing but because of the need for repeated applications, N fertilizer is rarely applied. The introduction of a forage legume is an appropriate solution for this problem, but until recently adoption has been very low as the legumes generally have not persisted in the sward. We report research on how grazing management can affect the persistence of stoloniferous legumes in pastures of Brachiaria spp. and the problems of establishing and maintaining crown-forming legumes such as Stylosanthes spp. With suitable management, milk or bovine carcass yields can be equal or greater from mixed than from grass-alone pastures fertilized with 120 or 150 kg Nha-1 year−1. In addition to savings in CO2 emissions from fossil fuels for the production and distribution of N fertilizers, nitrous oxide emissions from cattle excreta and legume residues are lower than those from N-fertilized brachiaria grass monocultures. Other studies indicate that enteric methane emissions from cattle may be mitigated when forage legumes are included in their diet. The use of forage legumes in mixed pastures for tropical regions is emerging as a feasible strategy to keep meat and milk production at acceptable levels with reduced greenhouse gas emission rates. 相似文献
10.
Palavecino Pablo Martín Curti María Isabel Bustos Mariela Cecilia Penci María Cecilia Ribotta Pablo Daniel 《Plant foods for human nutrition (Dordrecht, Netherlands)》2020,75(3):326-336
Plant Foods for Human Nutrition - Sorghum is a major cereal crop with various agronomic advantages, contains health-promoting compounds and is gluten-free. There is a growing tendency to use... 相似文献