Abstract: 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 NO_x released from soil. The main nitrogen species in wet deposition of karst depression was NH₄⁺-N during the rainy season. There was vertical variation of nitrogen species, in that the NH₄⁺-N and DTN in depression were generally higher than 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 δ¹⁵N-NO₃^-and δ¹⁸O-NO₃^- values were -4.8‰±2.3‰ and 58.1‰±7.6‰ in the mountain top, respectively, and were -7.3‰±3.3‰ and 57.3‰±6.6‰ in depression. Nitrate isotopic composition in mountain top was relatively higher than that in depression, which was related to both the contribution of NO_x released from cropland and the possible fractionation in the diffusion process. Monthly variation of δ¹⁵N-NO₃^-value indicated that the contribution of each NO_X source to the nitrate in rainwater was changed monthly.

Key words: niche overlap, nekton species, niche breadth, interspecific association