Publication Date:
2019
Description:
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 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 yr−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.
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Print ISSN:
1354-1013
Electronic ISSN:
1365-2486
Topics:
Biology
,
Energy, Environment Protection, Nuclear Power Engineering
,
Geography
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