喀斯特洼地湿沉降氮形态及硝酸盐同位素组成的垂向分布特征

Vertical distribution of nitrogen species and nitrate isotopic composition in wet deposition in karst depression

为探明喀斯特洼地湿沉降中氮形态及硝酸盐同位素组成的垂向分布特征,于2015年雨季(5—8月)采集了贵州省普定县的典型喀斯特农业洼地湿沉降样品,对样品中各形态氮的浓度和硝酸盐同位素组成进行了分析。结果表明,湿沉降对洼地排放的铵态氮具有较强的捕获效应,湿沉降中有机氮的来源与农业活动密切相关。湿沉降中硝态氮浓度垂向差异性不大,但其同位组成表明较多土壤释放氮氧化物的贡献。雨季喀斯特洼地湿沉降的氮主要形态为NH_4^+-N,各形态氮浓度的垂向差异表现为洼地的NH|_4^+-N和DTN的浓度普遍高于山顶,5—7月洼地的DON浓度高于山顶,8月洼地的DON浓度低于山顶。山顶降雨中δ^(15)N-NO_3^-值为-4.8‰±2.3‰,δ^(18)O-NO_3^-为58.1‰±7.6‰;洼地降雨中δ^(15)N-NO_3^-值为-7.3‰±3.3‰,δ_(18)O-NO_3^-值为57.3‰±6.6‰。山顶的硝酸盐同位素组成较洼地偏正,这与农田释放的NO_x贡献及其扩散过程中可能发生的分馏作用有关。δ^(15)N-NO_3^-值的月际变化特征表明,在不同月份各NO_X源对雨水硝酸盐的贡献存在差异。

To understand the vertical distribution of nitrogen species and nitrate isotopic composition in wet deposition in karst depression, we collected wet deposition samples within a typical karst agricultural depression in the rainy season （May to August 2015 ） in Puding County, Guizhou Province. The concentration of nitrogen species and nitrate isotopic composition were examined. The results showed that there was a stronger capture effect on the ammonium released from the depression in wet deposition. The source of dissolved organic nitrogen was closely related to agricultural activities. There was no obvious vertical variation of nitrate concentration, but its isotopic composition presented high contribution of NOx released from soil. The main nitrogen species in wet deposition of karst depression was NHa＋-N during the rainy season. There was vertical variation of nitrogen species, in that the NH4＋-N and DTN in depression were generally higherthan that in the mountain top. DON concentration of depression was higher than that from the mountain top from May to July, but lower than that from the mountain top in August. The 815 N- N03- andδ^（18）O-NO3^- values were -4.8%v±2.3%o and 58.1%o＋7.6%o in the mountain top, respectively, and were -7.3%0±3.3%o and 57.3%0±6.6%0 in depression. Nitrate isotopic composition in mountain top was relatively higher than that in depression, which was related to both the contribution of NOx released from cropland and the possible fractionation in the diffusion process. Monthly variation of δ15N-N03- value indicated that the contribution of each NOx source to the nitrate in rainwater was changed monthly.