全文获取类型
收费全文 | 1428篇 |
免费 | 54篇 |
国内免费 | 5篇 |
专业分类
林业 | 77篇 |
农学 | 37篇 |
基础科学 | 8篇 |
195篇 | |
综合类 | 535篇 |
农作物 | 51篇 |
水产渔业 | 101篇 |
畜牧兽医 | 345篇 |
园艺 | 67篇 |
植物保护 | 71篇 |
出版年
2023年 | 18篇 |
2022年 | 23篇 |
2021年 | 36篇 |
2020年 | 28篇 |
2019年 | 45篇 |
2018年 | 35篇 |
2017年 | 37篇 |
2016年 | 44篇 |
2015年 | 22篇 |
2014年 | 40篇 |
2013年 | 53篇 |
2012年 | 81篇 |
2011年 | 100篇 |
2010年 | 51篇 |
2009年 | 38篇 |
2008年 | 86篇 |
2007年 | 73篇 |
2006年 | 69篇 |
2005年 | 90篇 |
2004年 | 86篇 |
2003年 | 77篇 |
2002年 | 71篇 |
2001年 | 48篇 |
2000年 | 42篇 |
1999年 | 21篇 |
1998年 | 21篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1968年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有1487条查询结果,搜索用时 847 毫秒
61.
Intensive grow‐out of goldfish, Carassius auratus (L.), larvae and juveniles in closed systems requires the control of environmental conditions and feeding. This study investigates the use of different types of live food and combinations of live food and dry food in a series of four rearing experiments. Juvenile goldfish can be weaned from Artemia onto live food at about 24 days after the onset of feeding without causing a reduction in growth and survival. The replacement of Artemia by Daphnia at day 10 appears feasible, as growth and survival were not significantly affected. Fish fed decapsulated Artemia cysts grew better than fish fed live Artemia. Within the first 14 days, goldfish juveniles should be fed at least 155 cysts per fish per day to achieve fast growth and to minimize size variation. 相似文献
62.
63.
A study of the environmental effects associated with the trestle cultivation of Pacific oysters, Crassostrea gigas Thunberg, was conducted at a commercial cultivation site in the River Exe estuary, Devon, England. Small, but significant, changes were detected in the macrofaunal community sampled beneath oyster trestles compared with that found in adjacent uncultivated areas. These changes were associated with an increase in organic and silt composition and a reduction in the depth of the oxygenated layer of the sediment beneath the trestles. Water velocity was decreased by the presence of the trestles which probably led to the increase in sedimentation rate observed beneath them. Although biological and physical changes were observed, they were relatively minor compared with the extreme environmental changes associated with the suspended culture techniques used for other bivalve species and fishes. However, other studies suggest that the environmental effects associated with oyster cultivation become more severe in areas of large-scale (hectares) cultivation. 相似文献
64.
Nimesha Fernando Joe Panozzo Michael Tausz Robert M. Norton Glenn J. Fitzgerald Samuel Myers Marc E. Nicolas Saman Seneweera 《Journal of Cereal Science》2014
In order to investigate the intra-specific variation of wheat grain quality response to elevated atmospheric CO2 concentration (e[CO2]), eight wheat (Triticum aestivum L.)cultivars were grown at two CO2 concentrations ([CO2]) (current atmospheric, 389 CO2 μmol mol−1vs. e[CO2], FACE (Free-Air CO2 Enrichment), 550 ± 10% CO2 μmol mol−1), at two water levels (rain-fed vs. irrigated) and at two times of sowing (TOS1, vs. TOS2). The TOS treatment was mainly imposed to understand whether e[CO2] could modify the effects of timing of higher grain filling temperatures on grain quality. When plants were grown at TOS1, TKW (thousand kernel weight), grain test weight, hardness index, P, Ca, Na and phytate were not significantly changed under e[CO2]. On the other hand, e[CO2] increased TKW (16%), hardness index (9%), kernel diameter (6%), test weight (2%) but decreased grain protein (10%) and grain phytate (11%) at TOS2. In regard to grain Zn, Mn and Cu concentrations and some flour rheological properties, cultivar specific responses to e[CO2] were observed at both sowing times. Observed genetic variability in response to e[CO2] in terms of grain minerals and flour rheological properties could be easily incorporated into future wheat breeding programs to enable adaptation to climate change. 相似文献
65.
Cesar Ricardo Arruda Fernanda Ramiro Vanessa Faria Roberto Barcelos Daniel Pontes Fernanda Passos Fabiano Kaiser Karoline dos Santos Teixeira Aline Maria Serrano Aline Abreu Leticia Siqueira Danielle Teixeira Matheus Vezzone Mariana Polivanov Helena Castilhos Zuleica 《Journal of Soils and Sediments》2022,22(2):547-558
Journal of Soils and Sediments - This study consists of the evaluation of metals acute toxicity levels taking into consideration a hypothetic scenario of dam breaking and land deposition of two... 相似文献
66.
Hans Pretzsch Michael Heym Samuel Pinna Robert Schneider 《Scandinavian Journal of Forest Research》2014,29(3):222-233
Silviculture heading for structural heterogeneity creates many single trees standing at stand margins, inner edges or in remnant tree groups. As they played just a minor role in the age class forest, the growth behaviour of strongly released trees is rather unexplored. Here we show how retention cutting, presently spreading in the boreal of Québec province, affects stem and coarse root growth of remained single black spruce [Picea mariana (Mill.) Britton]. Increment cores from roots and stems of 125 trees show that retention cutting triggers coarse root growth of the remaining trees. Compared with reference trees retention trees accelerate root in relation to stem growth. Mean and variability of the root–stem allometry significantly rise after retention cutting. The found acceleration of root in relation to stem growth means mechanical stabilisation of the retention trees and corroborates the retention cutting method. Evaluation of silvicultural treatments can be incomplete and misleading as long as they are just based on aboveground reactions and neglect root growth. 相似文献
67.
Maxine P. Piggott Samuel C. Banks Ben T. Broadhurst Christopher J. Fulton Mark Lintermans 《水产资源保护:海洋与淡水生态系统》2021,31(1):173-184
- Detecting rare species is often a necessity for conservation and management, yet challenging for many field survey methods. Environmental DNA (eDNA) is a highly promising solution that has been shown to outperform many established survey methods.
- Macquarie perch (Macquaria australasica) is an endangered native species that has declined significantly in range and abundance. Detection of M. australasica was compared with an abundant alien fish species (Oncorhynchus mykiss) using eDNA and three conventional survey methods: gill nets, electrofishing and fyke nets.
- eDNA occupancy estimates for both fish species were compared using four different models to investigate what effect these differences have on false positives and false negatives for the rare and common fish species. These models used unadjusted eDNA detections in water samples, eDNA detections that have been screened using a limit of detection method to remove potential false positives, eDNA data supplemented with a second survey method, or eDNA data augmented with sequencing of positive polymerase chain reaction replicates.
- eDNA surveying as a single detection method was found to be more efficient and sensitive compared with each capture method separately and combined. Occupancy estimates for the common and rare species did not vary significantly between the four site occupancy-detection models, suggesting that supplementary data may not have as much effect on occupancy estimates compared with other approaches such as temporal or spatial sampling.
- We conclude that eDNA outperforms the three established survey methods for both a rare and common freshwater fish species. Although there was no significant effect of augmenting eDNA survey methods with other survey data, additional data may improve confidence in detection, and provide confirmatory evidence for unexpected or new detections of a species.
68.
This study aimed to reveal differences in the relevance of particulate as well as water-soluble organic matter (OM) fractions from topsoils to the easily biodegradable soil organic matter (SOM). We selected eight paired sites with quite different soil types and soil properties. For each of these sites, we took samples from adjacent arable and forest topsoils. Physically uncomplexed, macro-, and micro-aggregate-occluded organic particle, as well as water-soluble OM fractions were sequentially separated by a combination of electrostatic attraction, ultrasonic treatment, density separation, sieving, and water extraction. The easily biodegradable SOM of the topsoil samples was determined by measuring microbial respiration during a short-term incubation experiment (OCR). The organic carbon (OC) contents separated by i) the physically uncomplexed water-soluble OM, ii) the macro-, and iii) the micro-aggregate-occluded organic particle as well as water-soluble OM fractions were significantly correlated with OCR. The correlation coefficients vary between 0.54 and 0.65 suggesting differences in the relevance of these OM fractions to the easily biodegradable SOM. The strongest correlation to OCR was detected for the OC content separated by the physically uncomplexed water-soluble OM indicating the most distinct relation to the easily biodegradable SOM. This was found to be independent from land use or soil properties. 相似文献
69.
Negligible contribution from roots to soil-borne phospholipid fatty acid fungal biomarkers 18:2ω6,9 and 18:1ω9 总被引:1,自引:0,他引:1
The phospholipid fatty acid biomarkers 18:1ω9, 18:2ω6,9 and 18:3ω3,6,9 are commonly used as fungal biomarkers in soils. They have, however, also been found to occur in plant tissues, such as roots. Thus, the use of these PLFAs as fungal biomarkers in sieved soil, which may still contain small remains of roots, has been questioned. We used data from a recent beech tree girdling experiment to calculate the contribution of roots to these biomarkers and were able to demonstrate that not more than 0.61% of 18:1ω9 and 18:2ω6,9 in sieved soil samples originated from roots (but 4% of 18:3ω3,6,9). Additionally, the abundance of the biomarker 18:2ω6,9 in the soil was found to be highly correlated to ectomycorrhizal root colonization, which further corroborates its fungal origin. PLFA biomarkers were substantially reduced in vital roots from girdled trees compared to roots of control trees (by up to 76%), indicating that the major part of PLFAs measured in roots may actually originate from ectomycorrhizal fungi growing inside the roots. We calculated, that even a near to 50% reduction in fine root biomass - as observed in the girdling treatment - accounted for only 0.8% of the measured decrease of 18:2ω6,9. Our results demonstrate that both 18:1ω9 and 18:2ω6,9 are suitable biomarkers for detecting fungal dynamics in soils and that especially 18:2ω6,9 is a reliable biomarker to study mycorrhizal dynamics in beech forests. 相似文献
70.
Samuel C. Pierce Matt T. Moore Dan Larsen S. R. Pezeshki 《Water, air, and soil pollution》2010,207(1-4):73-84
Vegetated drainages are an effective method for removal of pollutants associated with agricultural runoff. Leersia oryzoides, a plant common to agricultural ditches, may be particularly effective in remediation; however, research characterizing responses of L. oryzoides to flooding are limited. Soil reduction resulting from flooding can change availability of nutrients to plants via changes in chemical species (e.g., increasing solubility of Fe). Additionally, plant metabolic stresses resulting from reduced soils can decrease nutrient uptake and translocation. The objective of this study was to characterize belowground and aboveground nutrient allocation of L. oryzoides subjected to various soil moisture regimes. Treatments included: a well-watered and well-drained control; a continuously saturated treatment; a 48-h pulse-flood treatment; and a partially flooded treatment in which water level was maintained at 15 cm below the soil surface and flooded to the soil surface for 48 h once a week. Soil redox potential (Eh, mV) was measured periodically over the course of the 8-week experiment. At experiment termination, concentrations of Kjeldahl nitrogen, phosphorus (P), potassium (K), iron (Fe), and manganese (Mn) were measured in plant tissues. All flooded treatments demonstrated moderately reduced soil conditions (Eh < 350 mV). Plant Kjeldahl nitrogen concentrations demonstrated no treatment effect, whereas P and K concentrations decreased in aboveground portions of the plant. Belowground concentrations of P, Mn, and Fe were significantly higher in flooded plants, likely due to the increased solubility of these nutrients resulting from the reductive decomposition of metal–phosphate complexes in the soil and subsequent precipitation in the rhizosphere. These results indicate that wetland plants may indirectly affect P, Mn, and Fe concentrations in surface waters by altering local trends in soil oxidation–reduction chemistry. 相似文献