Export flux of unprocessed atmospheric nitrate from temperate forested catchments: a possible new index for nitrogen saturation

Abstract

Abstract. To clarify the biological processing of nitrate within temperate forested catchments using unprocessed atmospheric nitrate exported from each catchment as a tracer, we continuously monitored stream nitrate concentrations and stable isotopic compositions, including 17O excess (Δ17O), in three forested catchments in Japan (KJ, IJ1, and IJ2) for more than 2 years. The catchments showed varying flux-weighted average nitrate concentrations of 58.4, 24.4, and 17.1 µmol L−1 in KJ, IJ1, and IJ2, respectively, which correspond to varying export fluxes of nitrate: 76.4, 50.1, and 35.1 mmol m−2 in KJ, IJ1, and IJ2, respectively. In addition to stream nitrate, nitrate concentrations and stable isotopic compositions in soil water were determined for comparison in the most nitrate-enriched catchment (site KJ). While the 17O excess of nitrate in soil water showed significant seasonal variation, ranging from +0.1 ‰ to +5.7 ‰ in KJ, stream nitrate showed small variation, from +0.8 ‰ to +2.0 ‰ in KJ, +0.7 ‰ to +2.8 ‰ in IJ1, and +0.4 ‰ to +2.2 ‰ in IJ2. We conclude that the major source of stream nitrate in each forested catchment is groundwater nitrate. Additionally, the significant seasonal variation found in soil nitrate is buffered by the groundwater nitrate. The estimated annual export flux of unprocessed atmospheric nitrate accounted for 9.4 %±2.6 %, 6.5 %±1.8 %, and 2.6 %±0.6 % of the annual deposition flux of atmospheric nitrate in KJ, IJ1, and IJ2, respectively. The export flux of unprocessed atmospheric nitrate relative to the deposition flux showed a clear normal correlation with the flux-weighted average concentration of stream nitrate, indicating that reductions in the biological assimilation rates of nitrate in forested soils, rather than increased nitrification rates, are likely responsible for the elevated stream nitrate concentration, probably as a result of nitrogen saturation. The export flux of unprocessed atmospheric nitrate relative to the deposition flux in each forest ecosystem is applicable as an index for nitrogen saturation.

Vertical Changes in the Flux of Atmospheric Nitrate From a Forest Canopy to the Surface Soil Based on Δ17O Values

“…These values are comparable with reported values of extracts from the forest floor (+3.6 ± 2.4‰; n = 4) and surface soil (0-30 cm; +2.0 ± 1.1‰; n = 16) in a temperate forest in northern lower Michigan, USA (Costa et al, 2011) and those of the soil water at a depth of 20 cm (from +0.1‰ to +5.7‰) in a coniferous forest in Niigata prefecture, Japan (Nakagawa et al, 2018). The average % NO 3 − atm decreased from 76.3%-100% for the throughfall to 1.1%-11.8% for the soil water (Table 1 (Hattori et al, 2019;Nakagawa et al, 2018;Osaka et al, 2010;Shi et al, 2014). The large decreases in δ 15 N from throughfall to forest floor leachate (Figure 3c and Table S1) also suggest the occurrence of nitrification in the forest floor as nitrification produces δ 15 N-depleted NO 3 − (Högberg, 1997).…”

“…The average % direct leaching for oak and Sasa in the canopy gap was larger than the values calculated from stream water NO 3 − atm in previous studies. Nakagawa et al (2018), for instance, determined the Δ 17 O value of NO 3 − in stream water in forested watersheds in central Japan and estimated that 2.6%-9.4% of the NO 3 − input to the watershed was directly leached. If we assume that a similar magnitude of direct leaching of NO 3 − atm to a stream as that observed by Nakagawa et al (2018) occurred in our study watershed, then the decline in the direct leaching of NO 3 − atm from soil water to stream water suggests significant NO 3 − consumption in deeper soil and groundwater through plant and microbial assimilation or denitrification.…”

“…(Durka et al, 1994 (Kendall et al, 2007). In addition, the δ 18 O value of NO 3 − in a sample is known to increase during biological processes such as plant and microbial assimilation of NO 3 − and denitrification (Granger et al, 2008(Granger et al, , 2010 (Costa et al, 2011;Michalski et al, 2004;Nakagawa et al, 2013Nakagawa et al, , 2018Tsunogai et al, 2010Tsunogai et al, , 2016 (Michalski et al, 2003(Michalski et al, , 2004. Moreover, Δ 17 O is stable during mass-dependent isotope fractionation processes such as assimilation and denitrification (Michalski et al, 2004 (Rose et al, 2015;Sebestyen et al, 2019;Tsunogai et al, 2014).…”

Nitrogen saturation of forested catchments in central Japan - Progress or recovery?

“…Interestingly, the NO 3 concentration in stream water in KJK is still increasing (Sase et al 2021). A recent analysis of 17 O excess in NO 3 clarified that unprocessed atmospheric NO 3 was leached from both ecosystems in KJK and IJR (Nakagawa et al 2018). This study's objectives are to assess the current N saturation situation in forest areas in central Japan, and to identify factors influencing the current situation.…”

Determination of the triple oxygen isotopic composition of tropospheric ozone in terminal positions using a multistep nitrite‐coated filter‐pack system

“…where the constant β = 0.5279. [36][37][38][39] The Δ 17 O value defined by the power-law definition (Equation 1)…”