Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi‐site dual nitrate isotopes

Abstract

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 15 N- and 18 O-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 15 N and 18 O 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 15 NO3 - signatures, we estimated denitrification fluxes from 2.4 to 21.7\xa0kg\xa0N\xa0ha-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.