首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6633篇
  免费   242篇
  国内免费   331篇
林业   142篇
农学   274篇
基础科学   614篇
  800篇
综合类   4923篇
农作物   178篇
水产渔业   14篇
畜牧兽医   128篇
园艺   71篇
植物保护   62篇
  2024年   31篇
  2023年   54篇
  2022年   104篇
  2021年   120篇
  2020年   174篇
  2019年   141篇
  2018年   70篇
  2017年   193篇
  2016年   224篇
  2015年   263篇
  2014年   421篇
  2013年   418篇
  2012年   589篇
  2011年   711篇
  2010年   558篇
  2009年   565篇
  2008年   536篇
  2007年   546篇
  2006年   369篇
  2005年   267篇
  2004年   149篇
  2003年   159篇
  2002年   53篇
  2001年   55篇
  2000年   49篇
  1999年   39篇
  1998年   45篇
  1997年   65篇
  1996年   32篇
  1995年   36篇
  1994年   29篇
  1993年   37篇
  1992年   27篇
  1991年   25篇
  1990年   14篇
  1989年   10篇
  1988年   12篇
  1987年   5篇
  1986年   4篇
  1985年   1篇
  1984年   1篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1978年   1篇
排序方式: 共有7206条查询结果,搜索用时 15 毫秒
141.
The present study was undertaken with the objective of studying use and contribution of information sources in the purchase of agri-inputs by Indian farmers. The importance of an information source was measured in terms of its relevance as perceived by the farmers during the buying process. The study revealed that farmers preferred noncommercial, personal sources of information and the number of information sources used by them depended on the perceived importance of the product, the various options (brands) available to them, and credibility of the information sources. The personal characteristics of farmers did not have a significant influence on the number of information sources used by the farmers.  相似文献   
142.
ABSTRACT

In 1992 the Journal of Extension's Board of Directors authorized a pilot project to test the feasibility of producing and distributing the Journal electronically as a means of reducing the direct and indirect costs of publication. The University of Wisconsin-Extension headed the pilot test and the University of Missouri-Extension was asked to conduct a comprehensive evaluation of the electronic Journal of Extension. This article describes the purposes, methods, content, and preliminary findings of the evaluation of this pilot project.  相似文献   
143.
The agricultural sector is probably the one that will suffer most directly from the climatic variations expected at the global level. In particular, the analysis of the changes expected in water availability and demand is fundamental in order to correctly establish both the present water resource management and the definition of new strategies. In this paper the time series of some climatic and agro-climatic indices in the Region of Umbria (Central Italy) have been analyzed with the aim of finding signs of climate changes and identifying the potential impacts on the agricultural water balance. The aforesaid indices include the precipitation, the mean maximum and minimum temperatures (Tmin, Tmax), the mean temperature range (ΔT), the reference evapotranspiration (ET0) and two drought indices, Standardized Precipitation Index (SPI) and Standardized Deficit Index (SDI, based on the difference between ET0 and precipitation). These indices were analyzed with reference to different periods (in particular the average growing and irrigation seasons). Furthermore, more specific information was obtained by analyzing the simulated water requirement (CWR) and evapotranspiration deficit (ED) of two typical annual crops (corn and sunflower). The time series of the indices were quantified for 38 stations in the region and they were then analyzed with non-parametric tests both at single sites and at the regional level. The tendencies in cumulated precipitation are generally decreasing (particularly during the wet period) and they are also characterized by a defined spatial pattern. The rainfall reduction during the irrigation season, although less widespread, could have the most important practical consequences. The significant trends detected for both the Tmax and Tmin are mainly positive, and they are more evident for Tmin, often resulting in a reduction of ΔT. ET0 shows a prevailing stationary condition due to the counteracting effects of the prevalent reduction observed for the ΔT and the increment of the mean temperature. At any rate, with reference to the irrigation season, ET0 trends are mainly positive. The results obtained for SPI and SDI are in accordance with the tendencies of non-standardized indices with an expectation of extreme drought event occurrence doubled or even tripled over a 30-year time span. Finally, the analysis of the CWR for corn and the ED for sunflower shows a relevant presence of significant positive trends whose impacts can be estimated in respective mean increments of about 23% and 44% over a 50-year time span.  相似文献   
144.
Long-term soil carbon (C) dynamics in agro-ecosystems is controlled by interactions of climate, soil and agronomic management. A modeling approach is a useful tool to understand the interactions, especially over long climatic sequences. In this paper, we examine the performance of the Agricultural Production Systems sIMulator (APSIM) to predict the long-term soil C dynamics under various agricultural practices at four semi-arid sites across the wheat-belt of eastern Australia. We further assessed the underlying factors that regulate soil C dynamics in the top 30 cm of soil through scenario analysis using the validated model. The results show that APSIM is able to predict aboveground biomass production and soil C dynamics at the study sites. Scenario analyses indicate that nitrogen (N) fertilization combined with residue retention (SR) has the potential to significantly slow or reverse the loss of C from agricultural soils. Optimal N fertilization (Nopt) and 100% SR, increased soil C by 13%, 46% and 45% at Warra, Wagga Wagga and Tarelee, respectively. Continuous lucerne pasture was the most efficient strategy to accumulate soil C, resulting in increases of 49%, 57% and 50% at Warra, Wagga Wagga and Tarlee, respectively. In contrast, soil C decreases regardless of agricultural practices as a result of cultivation of natural soils at the Brigalow site. Soil C input, proportional to the amount of retained residue, is a significant predictor of soil C change. At each site, water and nitrogen availability and their interaction, explain more than 59% of the variation in soil C. Across the four sites, mean air temperature has significant (P < 0.05) effects on soil C change. There was greater soil C loss at sites with higher temperature. Our simulations suggest that detailed information on agricultural practices, land use history and local environmental conditions must be explicitly specified to be able to make plausible predictions of the soil C balance in agro-ecosystems at different agro-ecological scales.  相似文献   
145.
本文回顾了农业旱灾系统恢复性研究进展, 探讨了灌溉农区农业旱灾系统恢复性基本问题, 提出了其恢复性评价思路与框架, 建立了综合指数评价模型, 并以河北省邢台市为例进行了评估。结果表明, 在生长季或年际尺度上, 自然降水偏少对全市旱情有显著影响, 但并不决定最终灾情。旱灾严重程度和系统恢复能力还受到农业资源环境基础、旱灾应对能力及社会经济发展水平等多方面因素影响。持续超采浅层地下水进行灌溉的中部平原地带的县市, 农业旱灾系统恢复指数较高; 缺水、人均耕地资源不足且经济相对落后的西部山地丘陵地区各县市农业旱灾系统恢复指数较低; 在深层地下水超采、地下水漏斗扩展的东部地区, 取水能力强,经济承受力较高的县市, 农业旱灾系统恢复指数较高, 经济落后的县, 恢复指数最低。在生长季或年际尺度上,虽然以水资源持续超采进行灌溉, 全市旱灾总体恢复能力在提高, 但因其忽视了区域水平衡和生态稳定性,所以不具有可持续性。  相似文献   
146.
乡土野花组合是筛选以乡土野花为主体的,并通过混合播种建立群落的一种景观植被建植模式,是欧美国家近年来较为流行的一种农业景观恢复与建设的技术措施。已有的研究发现乡土野花组合能够改善景观结构与生境质量,吸引传粉生物和自然天敌,从而起到增强农业景观的传粉及害虫控制功能,并改善农作物品质、提高农作物产量;与此同时,乡土野花组合也具有较好的文化功能。乡土野花组合的设计对建植效果具有重要作用,其物种筛选、配比、播种方式及布局是设计中需要主要解决的问题。本文在总结欧美国家乡土野花组合设计模式与生态监测结果的基础上,提出乡土野花组合物种筛选的3原则:乡土性、功能多样性与包含特定关键物种;并对其生长周期、物种数、播量、播种面积与空间布局等技术细节进行了讨论,以促进乡土野花组合的本土化、区域化发展。欧美国家提出了一系列推动产业发展及农户补贴的有关政策,其中美国主要以推动基础研究与产业发展政策为主,注重限制植物材料本土化;欧洲国家通过多项立法及农业环境管护政策为农户提供多样化的生态补贴,以促进农户充分参与乡土野花组合栽植过程。在我国,尚缺乏成熟的乡土野花组合应用案例。中国的乡土野花组合需充分发掘种质资源并进行生态、栽培与景观特性评估,以实现本土化的乡土野花组合。本文最后提出,通过多学科合作实现乡土野花组合的设计与推广,政府、产业、科研机构与农民共同参与乡土野花组合的设计过程,并提出市场化的生态补贴政策,以推动乡土野花组合在我国农业景观的应用,为改善我国农业生态环境做出应有贡献。  相似文献   
147.
基于2010-2018年河北省农资投入、农作物产量、耕地面积和农作物播种面积等统计数据,计算河北省农作物生产投入品碳排放,分析河北省农作物生产投入品碳排放的动态变化和不同投入品碳排放所占比例,为农业生产节能减排,实现低碳农业提供理论依据。结果表明,2010-2018年河北省农作物生产投入品碳排放以2015年为转折点,呈现先上升后下降的趋势。研究期内河北省农作物生产碳排放总量、单位产量碳排放量和单位耕地面积碳排放量分别为2219万~2674万tCO2eq、0.21~0.30tCO2eq·t-1和3.40~4.10tCO2eq·hm-2。2018年河北省农作物生产碳排放总量、单位产量碳排放量和单位耕地面积碳排放量最低,分别比最高值降低17.0%、30.0%和17.0%。化肥、农药、农膜、柴油和灌溉用电9a平均碳排放量占比分别为38.6%、2.1%、11.5%、34.7%和13.1%。化肥和柴油是最主要的农业碳排放源,占比均在30%以上。农作物生产投入品碳排放的动态变化受国家政策影响...  相似文献   
148.
农业建筑与环境工程专业现存的主要问题是:专业的发展方向、地位、业务归口单位应该进一步明确;专业需要得到全社会的承认和支持;对专业的边缘性和综合性的特点应该有充分的认识,要在工程技术和生物技术的结合方面作深入的研究工作;专业跨多门传统学科的特点与课程安排之间的矛盾要很好地解决;专业本身要主动适应大农业经济建设的需要,进一步加快和深化改革。  相似文献   
149.
我国加入WTO后农业对策与生态农业发展趋势   总被引:3,自引:2,他引:3  
阐述了我国加入世界贸易组织 (WTO)后农业面临的主要问题和生态农业发展趋势 ,指出我国农业发展的对策。  相似文献   
150.
系统地论述了世界银行贷款“中国农业支持服务项目”可行性研究的方法论。这类项目就其类型和复杂性,在我国农业建设项目中属首次,与农业生产性项目比较,其可行性研究有许多特点,如项目准备期长、项目设计涉及面广、项目评价和风险分析相对复杂等,作者在这方面的研究总结,为开拓中国农业项目可行性研究提供了新的方法论内容。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号