中文摘要：

以位于北亚热带丘陵地区不同利用条件下的2个相邻小流域（F：森林、FA：森林/农田）为研究区域,2007年3月～2009年2月通过定期监测雨水和径流水以及对其NH 4^＋-N和NO 3^--N的测定,分析了流域氮素的平衡与源汇特征.结果表明,雨水中无机氮的输入量为16.72 kg·（hm^2·a）^-1,NH 4＋-N占56%;2个小流域（F、FA）径流水中无机氮的输出量分别为5.31kg·（hm^2·a）^-1和8.21 kg·（hm^2·a）^-1,NO 3--N占75%～82%,流域内农业活动加剧了径流水中氮素的输出.大气干湿沉降输入的无机氮总量为20.06～23.41 kg·（hm^2·a）^-1,约占当地氮肥施用量13%～15%.2个小流域（F、FA）由于氮沉降及其转化产生的H＋量分别为355 mol·（hm^2·a）^-1和461 mol·（hm^2·a）^-1,流域内农业活动在一定程度上加速了土壤酸化进程.流域氮素收支平衡表明2个小流域（F、FA）氮素净滞留量分别为13.35～16.70 kg·（hm^2·a）^-1和17.89～23.38 kg·（hm^2·a）^-1,受到农业活动影响的流域（FA）氮素的净滞留率（33%～40%）远比森林流域（F）（65%～70%）低,表明我国亚热带地区以森林为主的流域目前仍具有氮汇作用,但流域内农业活动降低了生态系统的氮汇潜力.

英文摘要：

The present study takes two small watersheds（F：forest,FA：forest /farmland） with different land uses as the study areas,which are located in the hilly area of subtropical China.The rain water and stream water samples were collected from March 2007 to February 2009 and were determined for NH 4＋-N and NO3--N,to estimate nitrogen（N） budgets and source-sink characteristics of the two studied watersheds.The results show that inorganic N input in rain water is 16.72 kg·（hm^2·a）^-1,in which NH 4＋-N accounts for 56%;inorganic N output in stream water in the two small watersheds（F,FA） is 5.31 kg·（hm^2·a）^-1 and 8.21 kg·（hm^2·a）^-1 respectively,in which NO 3--N accounts for 75%-82%,indicating that agricultural activities in the watershed have increased N output in runoff.Total inorganic N input by atmospheric dry and wet deposition is 20.06-23.41 kg·（hm^2·a）^-1,which equals to approximately 13%-15% of the local N fertilizer application.The net production of H ＋ caused by N deposition and transformations in the two small watersheds（F,FA） is 355 mol·（hm^2·a）^-1 and 461 mol·（hm^2·a）^-1 respectively,indicating that agricultural activities lead to accelerated soil acidification.Based on N budgets,the net retention of N in the two small watersheds（F,FA） is 13.35-16.70 kg·（hm^2·a）^-1 and 17.89-23.38 kg·（hm^2·a）^-1 respectively.N retention efficiency in the FA watershed（33%-40%） as impacted by agricultural activities is much lower than that in the forested watershed（F）（65%-70%）,indicating that the forest ecosystem in subtropical China is still a sink for N,but agricultural activities have decreased the nitrogen-sink potential of the ecosystem.