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

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 ¹⁵N‐ and ¹⁸O–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 ¹⁵N and ¹⁸O 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 ¹⁵NO₃^? 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.