Multiyear dual nitrate isotope signatures suggest that N‐saturated subtropical forested catchments can act as robust N sinks

Abstract

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH ), causes significant nitrate (NO ) leaching from well-drained acid forest soils on hill slopes (HS), whereas significant retention of NO occurs in near-stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ N and δ O) of nitrate (NO ) in soil pore water along a hydrological continuum in the N-saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH and production of NO in soils on HS were associated with a significant decrease in δ N-NO , suggesting efficient nitrification despite low soil pH. The concentration of NO declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ N and δ O of residual NO , providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent N enrichment factor (ε) of NO of about -5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ N and δ O-NO in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ N-NO values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N-saturated, subtropical forests with important implications for regional N budgets.

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

“…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. This is expected to drive reductive processes, as recently shown by the strong denitrification activity in the valley bottom and groundwater discharge zone, which is indicated by the sharp increase in natural abundance of 15 N and 18 O of NO 3 − (Yu et al, ).…”

“…An alternative explanation for the lower CH 4 emission in the valley bottom may be the inhibition of methanogenesis by other electron acceptors such as NO 3 − and SO 4 2− (Conrad, ; Le Mer & Roger, ; Veldkamp et al, ). Along the TSP catchment, the concentrations of NO 3 − and SO 4 2− in soil remain high in the valley bottom (NO 3 − ; Figures h and S3) until significant denitrification (Yu et al, ) and sulfate reduction (Yu, Si, et al, ) take place in the groundwater discharge zone.…”

“…In this study, we selected a 4.2‐ha subcatchment that is categorized into four landscapes elements as hillslope, foot of hillslope, valley bottom, and groundwater discharge zone, following a topographic gradient (Yu et al, ). Our field observations covered three full years (September to August) of 2009–2010, 2012–2013, and 2013–2014.…”

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

“…These scenarios differ fundamentally from observations in South China, where large amounts of NO 3 − are produced in well‐drained top soils on hillslopes and removed in groundwater discharge zones (Larssen et al, ). In a previous study in SW China, we observed a robust multi‐year 15 N and 18 O pattern in nitrate (NO 3 − ) along a hydrological flow path in a headwater catchment, suggesting that large amounts of NO 3 − are produced by nitrification on hillslopes and transported by interflow over argic horizons of common Acrisols to wet soils in groundwater discharge zones, where they are denitrified (Yu, Zhu, Mulder, & Dörsch, ). Similar geomorphological and soil‐related patterns of N turnover, transport and removal have been recently reported in temperate and subtropical systems (Anderson, Groffman, & Walter, ; Griffiths et al, ).…”

Changing nitrogen deposition with low δ15N−NH4 + and δ15N−NO3 − values at the Experimental Lakes Area, northwestern Ontario, Canada

“…Most published δ 15 N−NO 3 − data are from heavily impacted systems in Europe, the eastern USA, northern Europe, and China (e.g., Hastings et al 2003;Elliott et al 2007Elliott et al , 2009Zhang et al 2008;Fang et al 2011;Koszelnik and Gruca-Rokosz 2013;Korth et al 2014;Yang et al 2014;Guerrieri et al 2015;Yu et al 2016). In general, fewer studies with NH 4 + isotopes in atmospheric deposition are available (e.g., Garten 1992;Zhang et al 2008).…”