基于ISI Web of Science数据库,利用文献计量学方法分析了自1957年以来各国在农田氮流失领域的研究发展态势,综述了农田氮流失特征及氮流失防控措施。结果表明,目前各国对农田氮流失的研究主要集中在施氮对水体水质的污染和监测方法上,...基于ISI Web of Science数据库,利用文献计量学方法分析了自1957年以来各国在农田氮流失领域的研究发展态势,综述了农田氮流失特征及氮流失防控措施。结果表明,目前各国对农田氮流失的研究主要集中在施氮对水体水质的污染和监测方法上,涉及的关键词主要有Groundwater、Water quality、Surface water、Nitrate pollution、Eutrophication、Contamination、Nonpoint source pollution、Lysimeter、Runoff、Subsurface drainage等。中国、美国和加拿大等农业大国的研究机构在这一领域的研究成果最多发文量最多的期刊主要分布在荷兰、美国和中国。文献分析表明,受降水、地形、土壤、施肥等诸多管理措施的影响,不同区域的农田氮流失量差别很大,中国各类农田的氮流失量(13.7—347 kg/hm^2)明显高于欧美国家(4—107 kg/hm^2)。我国单位面积化肥(357.3 kg/hm^2)和氮肥(165.1 kg/hm^2)施用量均远高于世界平均用量(87.5 kg/hm^2和52.9 kg/hm^2),当季氮肥利用率(17%)却明显低于世界平均水平(58%),表明氮肥施用过量且利用率过低是造成氮流失的关键因素。综合分析农田氮流失防控措施发现,从源头控制氮流失是最有效的措施,优化农艺管理措施和氮迁移过程拦截等分别可减少15%—92%、46%—77%的氮素流失,其中针对农田适宜施氮量的研究最多。然而,面对粮食生产需求与资源短缺、水体水质持续恶化的现状,未来的研究重点应从简单的表观平衡向整个农田生态系统的氮素循环过程转变,更为迫切的是加快探索以水质保护为目标的化肥氮施用阈值(造成环境污染的临界施氮量),并推广示范这些有效的氮流失防控措施。展开更多
The variations of muscle tissus, peroxidase and Ca2+-ATPase activities and Vp （pasteurization value） were investigated with the fresh samples of grass carp （Ctenopharyngodon idellus） heated at different temperatur...The variations of muscle tissus, peroxidase and Ca2+-ATPase activities and Vp （pasteurization value） were investigated with the fresh samples of grass carp （Ctenopharyngodon idellus） heated at different temperatures, thus revealing the mechanism of denaturation of muscle protein by heating. The experimental results showed the great difference in muscle tissue structure before and after the samples heated. In the initial stage of heating the water loss accounted for 50% of the total. When the samples were separately heated at 75, 85, and 95℃, fh values （dehydration curve slope） were 5, 4.1 and 1.7, respectively, while Vp reaching 40 minutes were 22, 8 and 5 min, respectively. As to the time for deactivation of peroxidase in the heated samples at the various temperatures, it varied from 38, 10 and to less than 6 min, respectively and the deactivation rates of Ca2+-ATPase in 1 min were 50%, 88% and 100%, respectively. Consequently, the higher temperature the sample was heated, the faster rate of thermocon ductivity, the lower rate of water loss, and the faster rate of peroxidase deactivation it was. Also, the Vp for the least requirement of pasteurization （Vp≥40min） was obtained.展开更多
文摘The variations of muscle tissus, peroxidase and Ca2+-ATPase activities and Vp （pasteurization value） were investigated with the fresh samples of grass carp （Ctenopharyngodon idellus） heated at different temperatures, thus revealing the mechanism of denaturation of muscle protein by heating. The experimental results showed the great difference in muscle tissue structure before and after the samples heated. In the initial stage of heating the water loss accounted for 50% of the total. When the samples were separately heated at 75, 85, and 95℃, fh values （dehydration curve slope） were 5, 4.1 and 1.7, respectively, while Vp reaching 40 minutes were 22, 8 and 5 min, respectively. As to the time for deactivation of peroxidase in the heated samples at the various temperatures, it varied from 38, 10 and to less than 6 min, respectively and the deactivation rates of Ca2+-ATPase in 1 min were 50%, 88% and 100%, respectively. Consequently, the higher temperature the sample was heated, the faster rate of thermocon ductivity, the lower rate of water loss, and the faster rate of peroxidase deactivation it was. Also, the Vp for the least requirement of pasteurization （Vp≥40min） was obtained.
