资源环境科技发展态势分析平台

Increasing nitrogen (N) deposition in subtropical forests in south China causes N saturation, associated with significant nitrate (NO3-) leaching. Strong N attenuation may occur in groundwater discharge zones hydrologically connected to well-drained hillslopes, as has been shown for the subtropical headwater catchment "TieShanPing", where dual NO3- isotopes indicated that groundwater discharge zones act as an important N sink and hotspot for denitrification. Here, we present a regional study reporting inorganic N fluxes over two years together with dual NO3- isotope signatures obtained in two summer campaigns from seven forested catchments in China, representing a gradient in climate and atmospheric N input. In all catchments, fluxes of dissolved inorganic N indicated efficient conversion of NH4+ to NO3- on well-drained hillslopes, and subsequent interflow of NO3- over the argic B-horizons to groundwater discharge zones. Depletion of N-15- and O-18-NO3- on hillslopes suggested nitrification as the main source of NO3-. In all catchments, except one of the northern sites, which had low N deposition rates, NO3- attenuation by denitrification occurred in groundwater discharge zones, as indicated by simultaneous N-15 and O-18 enrichment in residual NO3-. By contrast to the southern sites, the northern catchments lack continuous and well-developed groundwater discharge zones, explaining less efficient N removal. Using a model based on (NO3-)-N-15 signatures, we estimated denitrification fluxes from 2.4 to 21.7 kg N ha(-1) year(-1) for the southern sites, accounting for more than half of the observed N removal. Across the southern catchments, estimated denitrification scaled proportionally with N deposition. Together, this indicates that N removal by denitrification is an important component of the N budget of southern Chinese forests and that natural NO3- attenuation may increase with increasing N input, thus partly counteracting further aggravation of N contamination of surface waters in the region.