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 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.

Drivers of Hot Spots and Hot Moments of Denitrification in Agricultural Systems

“…Although the NFARM intact core direct measurements are an improvement of previous methods involving inhibitors or disturbed soils (Groffman et al., 2006), there are concerns about the disturbance of plant:soil interactions, and holding times with this method (Wang, Pan, et al., 2020). There will be a strong need for independent evaluation of denitrification estimates produced from sensors and models, either with detailed mass balances or future development of alternative time and area‐integrated methods, perhaps with flux towers and/or isotopes (Hattori et al., 2019; Yu et al., 2019).…”

What can we learn from N2O isotope data? – Analytics, processes and modelling

“…In addition, the bulk isotopic composition of the N precursor might not be representative of the actually utilized N substrate pool. Particularly in soils where the soil matrix can be markedly heterogeneous and “hotspots” of denitrification can occur in isolated anoxic soil microsites, 82 NO 3 − near and in the reactive zones may be strongly enriched in 15 N compared with the bulk soil 41,91 and may also be derived from various soil N pools including organic and mineral N 92 . Similarly, in the case of nitrate consumption at strong redox gradients in the ocean and in lakes, most denitrifying activity is localized where the δ 15 N value of the nitrate pool has already been elevated.…”

Humid Subtropical Forests Constitute a Net Methane Source: A Catchment‐Scale Study

“…Hence, the accuracy of CH 4 flux estimates for the whole catchment depends primarily on sampling frequency and replicate measurements of CH 4 fluxes in the groundwater discharge zone, in addition to a precise estimate of its areal contributions (Sakabe et al, ). The catchment‐scale assessment and upscaling as done for TSP in this study is of relevance for the subtropical region of South China at large, where topography and landscape elements are similar to that found at TSP (Yu, Mulder, et al, ). Hillslopes, which account for 96% of the total catchment area, played a secondary role in determining the interannual variability of whole‐catchment CH 4 budgets (Table ).…”

“…Previously, Muzamil () reported TOC levels in top soils of the groundwater discharge zone comparable to those on the hillslope (Figure e). Since the groundwater discharge zone at TSP receives both seepage water from the valley bottom (upstream) and from interflow from the Acrisols on the surrounding hillslopes (Yu, Mulder, et al, ) and because of the large area ratio between the hillslope and groundwater discharge zone in the TSP catchment (160:1), substantial amounts of DOC are expected to be laterally transported from the surrounding hillslopes to the groundwater discharge zone (Gundersen, ), especially in the rainy seasons. The low DOC concentration in the groundwater discharge zone seems to contradict with our assumption (Figure f; Gundersen, ), but it may indicate a rapid decomposition of DOC in the valley bottom and the groundwater discharge zone.…”