全文获取类型
收费全文 | 3668篇 |
免费 | 189篇 |
国内免费 | 389篇 |
专业分类
林业 | 802篇 |
农学 | 243篇 |
基础科学 | 265篇 |
813篇 | |
综合类 | 1233篇 |
农作物 | 152篇 |
水产渔业 | 185篇 |
畜牧兽医 | 262篇 |
园艺 | 173篇 |
植物保护 | 118篇 |
出版年
2024年 | 14篇 |
2023年 | 51篇 |
2022年 | 63篇 |
2021年 | 63篇 |
2020年 | 93篇 |
2019年 | 116篇 |
2018年 | 94篇 |
2017年 | 135篇 |
2016年 | 141篇 |
2015年 | 135篇 |
2014年 | 214篇 |
2013年 | 286篇 |
2012年 | 306篇 |
2011年 | 285篇 |
2010年 | 282篇 |
2009年 | 260篇 |
2008年 | 188篇 |
2007年 | 248篇 |
2006年 | 209篇 |
2005年 | 134篇 |
2004年 | 121篇 |
2003年 | 119篇 |
2002年 | 93篇 |
2001年 | 84篇 |
2000年 | 70篇 |
1999年 | 51篇 |
1998年 | 49篇 |
1997年 | 42篇 |
1996年 | 47篇 |
1995年 | 36篇 |
1994年 | 35篇 |
1993年 | 35篇 |
1992年 | 41篇 |
1991年 | 24篇 |
1990年 | 31篇 |
1989年 | 9篇 |
1988年 | 20篇 |
1987年 | 12篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有4246条查询结果,搜索用时 15 毫秒
991.
关于园林绿化质量管理问题的探讨 总被引:4,自引:0,他引:4
本文结合园林绿化行业特点,对园林绿化质量管理的涵义进行了阐述,并对如何认识园林绿化质量管理特殊性的问题进行了探讨。 相似文献
992.
993.
Pedro Le o de Sousa Luí s L. Silva Ricardo P. Serralheiro 《Agricultural Water Management》1999,40(2-3):341-351
Traditional irrigation systems still cover a large area of irrigated lands in Portugal. The needs for competitiveness leads to the gradual abandon of traditional systems, resulting in social and economic impacts. Pressurised on-farm irrigation systems provide labour savings but imply important investments. Surface irrigation does not require expensive equipment and is low energy and low labour consuming, if modern systems are implemented. Flexible-pipe, gated-pipe, surge-flow valve and cablegation are the main equipment issues available to Portuguese farmers for modern surface irrigation practice. Conditions for application of these systems have been studied relative to different soil and topographic conditions. Plane land, either horizontal or sloped and undulated fields can be irrigated by surface systems, if a convenient design is accomplished. The adequacy of the irrigation systems has been evaluated and a cost-benefit study was carried out. A case study of a situation with undulated land, representative of South Portugal, is presented in this paper, comparing surface irrigation issues and a sprinkler system. Conclusions show a particular economic interest on adopting surge-valve, gated-pipe and cablegation. 相似文献
994.
Quantitative analysis of earthworm burrow systems with respect to biological soil-structure regeneration after soil compaction 总被引:6,自引:0,他引:6
M. Langmaack S. Schrader U. Rapp-Bernhardt K. Kotzke 《Biology and Fertility of Soils》1999,28(3):219-229
On arable land, tilled with conventional tillage (CT) and conservation tillage (CS) respectively, plots were compacted by
wheeling them 6 times with a 5 Mg wheel load in spring 1995. Immediately after compaction, undisturbed soil monoliths were
excavated from the compacted and uncompacted plots. The monoliths were defaunated and inoculated with either Lumbricus terrestris or Aporrectodea caliginosa. One monolith from each plot remained uninoculated as a control. After 6 months the monoliths were defaunated again and then
scanned with X-ray helical computed tomography. The data were transformed, the void systems inside the monoliths were reconstructed
and visualised, and the parameters total void length, total void volume, tortuosity and continuity were quantified. The parameters'
values were generally lower in the controls than in the inoculated monoliths. Differences in burrow construction could be
explained by the different life strategies of the two earthworm species. Changes in burrow morphology due to tillage system
and soil compaction were minor. Only the continuity of the burrow systems clearly changed: decreasing for L. terrestris and increasing for A. caliginosa. This can be explained by a change in the earthworms' burrowing activity to minimise energy expenditure in compacted soil.
By extrapolating field data, we concluded that earthworms have great potential for biologically regenerating the soil structure
after a single compaction event. Due to higher earthworm abundances in soil managed by CS the regeneration of the soil structure
is assumed to be better in these plots than those tilled by CT.
Received: 17 December 1997 相似文献
995.
硒的生物有效性是决定硒在土壤-植物系统中转运、积累的关键因素,然而目前关于该方面研究进展还缺乏系统的总结和论述。本文结合赣南富硒脐橙调查研究结果,以硒在土壤-植物系统中的运移为切入点,系统总结和论述了硒在土壤-植物系统中的氧化-还原、吸收、转运过程,探讨了土壤理化性质、微生物、施肥、农艺管理和植物生长阶段等对硒生物有效性和迁移的影响,并提出未来硒的研究热点:土壤微生物活性与硒生物地球化学循环的耦合关系;植物生长、果实品质与硒代谢相关的分子机制;结合同位素示踪技术和薄膜扩散梯度(DGT)技术原位探究硒在生物地球化学中的归趋。本综述可为土壤硒生物有效性调控、植物体系中硒的有效积累和农艺生物强化富硒措施方面的研究提供理论参考。 相似文献
996.
Nhi H. Y. Nguyen Lan T. Trinh Da T. Chau Kartik Baruah Torbjrn Lundh Anders Kiessling 《Aquaculture Nutrition》2019,25(4):970-979
A 90‐day feeding experiment was conducted in order to test spent brewer's yeast as a replacement for fishmeal in diets for giant freshwater prawn (Macrobrachium rosenbergii) reared in either a recirculating aquaculture system (RAS) or a biofloc system. Triplicate groups, each of 20 prawn (6.7 ± 0.03 g), were randomly assigned to 24 identical tanks. Twelve tanks were connected to the RAS, and the other 12 tanks were connected to the biofloc system which were initially supplemented with Bacillus subtilis, molasses and blood meal to achieve a C:N ratio of 20:1. Four iso‐nitrogenous (350 g/kg) and iso‐energetic (19 MJ/kg) diets where 0%, 20%, 40% and 60% of fishmeal protein was replaced by spent brewer's yeast. Neither growth nor survival was affected (p > 0.05) by increasing levels of brewer's yeast. The exception, being fish given 60% replacement with brewer's yeast, reared in biofloc, which displayed a higher (p < 0.05) growth than fish given 40% replacement, reared in clear water. The feed conversion ratio of prawn reared in the biofloc environment was lower than in the clear water. We concluded that spent brewer's yeast represents a possible 60% substitute for fishmeal in giant freshwater prawn diets, especially for prawn reared in a biofloc environment. 相似文献
997.
Leydy Y. Sandoval‐Vargas Martha N. Jimnez‐Amaya Jos Rodríguez‐Pulido Diana N. Guaje‐Ramírez Juan A. Ramírez‐Merlano Víctor M. Medina‐Robles 《Aquaculture Research》2020,51(9):3865-3878
The aim of this study was to assess the zootechnical performance and water quality of a cachama blanca (Piaractus brachypomus) culture using biofloc technology (BFT) versus a system with daily water exchange (DWE). To do this, 180 juveniles (mean initial weight: 5.40 ± 0.19 g) were distributed in 12 circular plastic tanks with stocking densities of 20 or 40 individuals m3; then, they were cultured for 91 days. BFT treatments kept a C:N ratio approximately of 15:1. Temperature, pH and oxygen were monitored daily, while the other variables were measured weekly. Most productive variables were significantly influenced by both culture system and stocking density with significantly higher values of daily weight gain, total weight gain and total length for fish kept in DWE 20. However, only minor differences were observed within the BFT system. With the exception of the toxic nitrogen compounds (NH4+ and NO2?), all the other water quality parameters were within the acceptable ranges for the cultivation of tropical fish. Microorganisms started to settle from the first week. A total of 23 genera were present, the most outstanding of which being seven genera of ciliates and three rotifers, rhizopods and chlorophytes. In conclusion, both systems BFT and DWE are useful for increasing the production of P. brachypomus in captivity. Additionally, the BFT system can potentially be applied for growing juveniles of this specie in regions with scarce water resources. 相似文献
998.
Sharks and their cartilaginous relatives are one of the world's most threatened species groups. The primary cause is overfishing in targeted and bycatch fisheries. Reductions in fishing mortality are needed to halt shark population declines. However, this requires complex fisheries management decisions, which often entail trade‐offs between conservation objectives and fisheries objectives. We propose the mitigation hierarchy (MH)—a step‐wise precautionary approach for minimizing the impacts of human activity on biodiversity—as a novel framework for supporting these management decisions. We outline a holistic conceptual model for risks to sharks in fisheries, which includes biophysical, operational and socioeconomic considerations. We then demonstrate how this model, in conjunction with the MH, can support risk‐based least cost shark conservation. Through providing examples from real‐world fishery management problems, we illustrate how the MH can be applied to a range of species, fisheries and contexts, and explore some of the opportunities and challenges hereto. Finally, we outline next steps for research and implementation. This is important in the context of increasing international regulation of shark fishing and trade, which must lead to reductions in shark mortality, while managing trade‐offs between conservation objectives and the socioeconomic value of fisheries. 相似文献
999.
Matthew R. Baker Wayne Palsson Mark Zimmermann Christopher N. Rooper 《Fisheries Oceanography》2019,28(6):629-657
Bottom trawl surveys provide fishery‐independent data on relative abundance and life history parameters for a wide range of marine taxa. Survey data are used to assess species distribution, biological interactions, and ecosystem structure and to manage marine resources. Not all bottom types or oceanographic conditions accommodate this survey method. We applied National Ocean Service hydrographic smooth sheets to evaluate physical attributes associated with habitat available to surveys. Random forests were used to evaluate the relative influence of benthic terrain and oceanographic predictors in determining accessibility to bottom trawl gear. We examined the marginal importance of each predictor, quantified the response gradient, and applied piecewise regression to determine threshold breakpoint values. Thresholds were used to develop predictive maps and distinguish untrawlable habitat at the scale of discrete towpaths and survey stations. Untrawlable habitat was associated with increased complexity in terrain, roughness, slope, surface curvature, substrate coarseness, current, and aspect. Maps of critical thresholds suggest different variables constrain the probability of a successful trawl in the nearshore, shelf, and continental slope. Overlay analysis of the model projection demonstrates the utility of archived smooth sheet data and identifies areas where higher resolution data might improve results. The model and maps produced in this analysis might be used to identify habitats available to and impacted by commercial trawl fisheries, inform the relative availability of various species and habitat types to bottom trawl surveys, evaluate bias in assessment indices and ecosystem metrics derived from survey data, and advance habitat‐specific biomass estimates. 相似文献
1000.