Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi‐site dual nitrate isotopes. (13th March 2019)

• Record Type: Journal Article
• Authors: Yu, Longfei; Mulder, Jan; Zhu, Jing; Zhang, Xiaoshan; Wang, Zhangwei; Dörsch, Peter
• Is Part Of: Global change biology. Volume 25:Number 5(2019)
• Page Start: 1765
• Page End: 1778
• Publication Date: 2019-03-13
• Subjects: groundwater discharge zone -- hydrological continuum -- isotopic modelling -- nitrification -- nitrogen removal -- δ15N and δ18O of nitrate; Climatic changes -- Environmental aspects -- Periodicals; Troposphere -- Environmental aspects -- Periodicals; Biodiversity conservation -- Periodicals; Eutrophication -- Periodicals; 551.5
• DOI: 10.1111/gcb.14596
• Languages: English
• ISSNs: 1354-1013

Abstract

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 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. Abstract : Forests in South China receive high loads of atmospheric N deposition, which causes significant nitrate leaching from soils. However, at the catchment level, moderate N export by stream runoff indicates strong N removal. We conducted a regional study with N fluxes over 2 years and dual nitrate isotopic signatures in two summers across seven forested catchments. Our findings suggest that denitrification in groundwater discharge zones is an important N sink in subtropical forests. Model estimates indicate that denitrification scales with N deposition, highlighting the role of natural forests in counteracting aggravated N pollution in the region.