To account for isotopic fractionation during the NH₃ to pNH4+ transition, the initial δ15N-NH₃ (i.e., δ15N-NH30) was estimated from the concentration-weighted mean δ15N-NH4+ following Pan et al. (2016): 3δ15N-NH30=δ15N-NH4+-εNH4+-NH31-f where εNH4+-NH3 is the isotope fractionation constant (+33 ‰ for equilibrium isotope effect (EIE) between NH_3(g) and pNH4+_(aq∕s)) (Heaton et al., 1997), and f is the fraction of pNH4+ in the NH₃-pNH4+ system. Sensitivity tests regarding the temperature dependence, gas-particle equilibrium, and the approximation of equilibrium on εNH4+-NH3 were performed, as detailed in Supplement Description S3 and Table S1 (Chang et al., 2025). The f values were derived from the Community Multiscale Air Quality (CMAQ) model (version 4.7.1), incorporating meteorological data from the Weather Research and Forecasting (WRF) model (version 3.7.1). The anthropogenic emission database was from the Taiwan Emission Data System (version 12) (Tsai et al., 2024) while biogenic VOC emissions were calculated with the Model of Emission of Gases and Aerosols from Nature algorithm (MEGAN, version 2.04) (Tsai et al., 2015). Concentrations of NH₃ and NH4+ were simulated using the SAPRC99-AERO5 chemical mechanism, in which inorganic gas-particle partitioning of NH3/NH4+ was computed by the ISORROPIA thermodynamic equilibrium module embedded within AERO5. The model-simulated pNH4+ mass concentrations agreed with laboratory measurements within a 20 % uncertainty range (Fig. S2).

For NH₃ source apportionment, four major emission sources were considered: fertilizer (-28.3 ± 5.8 ‰), waste (-17.6 ± 5.6 ‰), NH₃ slip (-8.2 ± 5.5 ‰), and fossil fuel emissions (1.8 ± 3.2 ‰) (Kawashima et al., 2023). These sources can be categorized into two groups based on their δ15N-NH₃ signatures: (1) volatilization-related sources (fertilizer and waste) with lower δ15N-NH₃ values, and (2) combustion-related sources (NH₃ slip and fossil fuel emissions) with higher values (Z.-L. Chen et al., 2022). δ15N-NH₃ obtained using passive techniques was adjusted by 15 ‰ to account for systematic differences between collection methods (Kawashima et al., 2023; Walters et al., 2020). Statistical values for MixSIAR analysis are provided in Table S2.

δ18O-NO3- is applied to evaluate the relative contributions of various HNO₃ formation. Six potential pathways were considered by combining the primary oxidants (PO3 and PRO2) with three terminal mechanisms (OH1, OH2, and heterogeneous processes (het)). These pathways are characterized by their specific δ18O-NO3- signatures (Fig. S3), calculated using a mass-balance approach assuming no kinetic isotope fractionation (Walters and Michalski, 2016). RO₂ was assigned a δ18O of 23.5 ‰, reflecting that O in RO₂ originates from O₂ (Kroopnick and Craig, 1972). O₃ exhibits elevated δ18O values ranging from 90 ‰ to 122 ‰ (Hastings et al., 2003). OH radicals were split into OH1 (-15 ‰ to 0 ‰) (Dubey et al., 1997), and OH2 (38 ‰ to 61 ‰). Daytime mechanisms included PRO2-OH1 (33 ‰–49 ‰), PRO2-OH2 (50 ‰–69 ‰), PO3-OH1 (55 ‰–81 ‰), and PO3-OH2 (73 ‰–102 ‰). Nighttime analysis focused on PRO2-het (50 ‰–69 ‰) and PO3-het (73 ‰–102 ‰), while retaining PRO2-OH1 to account for residual daytime nitrate that persists into the night. The reaction sequences and resulting δ18O-NO3- ranges for each pathway are summarized in Table 1. Sensitivity analyses regarding pathway exclusion are detailed in Supplement Description S4, Tables S3 and S4.

The daily concentration-weighted δ15N-NH4+ values at the Xitou site ranged from 6.31 ‰ to 14.69 ‰, with an average of 10.95 ± 2.76 ‰ (Fig. 2f, calculation details in Supplement Description S5). These results are consistent with previous winter observations at this site (-3.7 ‰ to 21.39 ‰, with an average of 11.95 ± 2.65 ‰) (T.-Y. Chen et al., 2022). As shown in Fig. S5, these values fall between those typical urban-influenced environments (∼ 19.6 ‰) and remote/forest sites (∼ 5.6 ‰), reflecting the mixed-source characteristics of Xitou (T.-Y. Chen et al., 2022; Kawashima et al., 2023; Hall et al., 2016; Kundu et al., 2010; Moore, 1977; Proemse et al., 2012; Savard et al., 2017; Ti et al., 2018; Walters et al., 2022). This isotopic signature is particularly comparable to other semi-rural receptor sites in East Asia that receive diluted urban plumes mixed with a regional agricultural background.

Temporal patterns showed higher δ15N-NH4+ during the clear period, with daytime and nighttime averages of 11.22 ± 2.13 ‰ and 12.12 ± 2.53 ‰, respectively (Table 2). In contrast, lower values were observed during fog, dropping to 7.35 ‰ (18D), 6.31 ‰ (18N), and 9.60 ‰ (19D). This decline of δ15N during the fog event likely results from several concurrent processes under stagnant conditions: first, stagnant conditions suppressed the upslope transport of δ15N-enriched urban plumes from nearby lowlands; second, continuous exchange between gas-phase NH₃ and pNH4+ shifts the particle phase toward the signature of the local NH₃ pool, which is depleted in δ15N due to agricultural volatilization; third, efficient wet deposition during the fog event may have removed early-formed ¹⁵N-enriched particles, forcing the remaining pNH4+ to re-equilibrate with an increasingly δ15N-depleted gas phase; finally, the hygroscopic growth and aqueous-phase processing under fog conditions likely reflect a shift toward equilibrium isotope exchange driven by the NH₃ gaseous-aqueous phase interactions for acidic particles, leading to a smaller gas-liquid isotope fractionation of ∼ 4 ‰ (Walters et al., 2019). This observed decrease in δ15N-NH4+ during the foggy period contrasts with our previous study, which showed slight increases during shorter fog events (<6 h) at the same site (T.-Y. Chen et al., 2022). The exceptionally prolonged duration of the current fog episode (∼ 26 h) likely allowed for a more complete chemical and physical equilibration between gas-phase NH₃ and the aqueous fog droplets, thereby sustaining isotopic depletion over an extended period. Furthermore, the mass concentrations of both pNH4+ and pNO3- in this study were nearly double those reported by T.-Y. Chen et al. (2022), indicating a stronger partition of NH₃ into the particle phase, leading to much lower δ15N-NH4+ in this study.

Under clear conditions (excluding 19N and 21D), the size-resolved δ15N-NH4+ distribution revealed a bell-shaped distribution (Fig. 4). Values peaked at ∼ 15 ‰ in the accumulation mode (∼ 0.5–2 µm) and decreased to ∼ 7 ‰ in both the ultrafine and coarse particles. This pattern reflects a transition between local emissions and aged, transported aerosols. For ultrafine mode (<0.5 µm), particles maintain isotopic equilibrium with the local NH₃ pool, dominated by δ15N-depleted residual NH₃ or volatilization sources (typically -28.3 ‰ to -17.6 ‰). For accumulation-mode (∼ 0.5–2 µm), particles preserve the signatures of lower-elevation anthropogenic emission regions. Here, combustion-derived NH₃ (typically -8.2 ‰ to 1.8 ‰) reacts with H₂SO₄ to form non-volatile ammonium sulfate and ammonium bisulfate. Unlike ammonium nitrate, which is more volatile and easily re-equilibrates with gas-phase NH₃ during transport, sulfate-bound NH4+ is more resistant to isotopic re-equilibration, allowing it to act as a conservative tracer of urban source signatures during upslope transport (Wu et al., 2022). For the coarse mode (>2 µm), particles are likely dominated by mineral dust and sea salt and present a substantially weaker thermodynamic sink for NH₃ absorption compared to acidic fine-mode sulfate aerosols (Fig. S6) (Pye et al., 2020). Therefore, NH4+ in this mode likely maintains a dynamic equilibrium with local gas-phase NH₃, which becomes progressively depleted in ¹⁵N as the air mass ages.

During the fog event, the bell-shaped distribution flattened significantly. Weakened wind suppressed the upslope transport of enriched anthropogenic pollutants, while enhanced aqueous-phase processing promoted uniform isotopic re-equilibrium across all particle sizes. Hygroscopic growth and aqueous processing during fog shifted the dominant partitioning pathway from gas–solid equilibrium (∼ 33 ‰) toward a gas–liquid exchange mechanism driven by the NH₃ gaseous-aqueous phase interactions with a much smaller isotope fractionation factor (∼ 4 ‰) (Walters et al., 2019). This reduced isotopic separation between the gaseous and condensed phases, while promoting uniform chemical processing across all size bins, effectively dampens the size-dependent isotopic gradient. Concurrently, the high pNO3- concentration from aqueous chemistry can thermodynamically favor the partition of locally-generated isotopically depleted NH₃ into the particle phase, further suppressing accumulation-mode enrichment.

The flat δ15N-NH4+ size distribution was also observed on 19N and 21D. For 19N, it is likely a legacy effect of the preceding fog period (18D, 18N, and 19D). The NH₃ pool remained isotopically depleted from sustained fog-driven scavenging and re-equilibration, and regional transport had not yet replenished the accumulation-mode signal. However, high sub-micrometer NO3- concentration (Fig. S4) suggests that fog-like aqueous chemistry persisted, maintaining isotopic signatures similar to 18D. Conversely, the flat distribution on 21D reflects a distinct source influence. HYSPLIT back-trajectories (Figs. S7–S8) indicate that the air parcel traveled through agricultural regions characterized by heavily depleted NH₃ (e.g., livestock waste, -28.3 ‰ to -17.6 ‰). The partitioning of this isotopically light NH₃ during transit effectively erased the characteristic accumulation-mode enrichment.

The δ15N-NH30 values were derived using f values estimated from CMAQ-simulated NH₃ and measured pNH4+ concentrations (Fig. S9, as described in Eq. 3). The resulting δ15N-NH30 ranged from -12.55 ‰ to -6.71 ‰, consistently lower than the concentration-weighted δ15N-NH4+, as shown in Fig. 5a. Based on the derived δ15N-NH30 values and assuming a single-source contribution, NH₃ can be mainly attributed to NH₃ slip. However, because real-world air parcels are typically influenced by a complex mixture of emission sources, the MixSIAR Bayesian framework was employed to quantitatively assess multiple source contributions (Fig. 5b). Overall, combustion-related sources (fossil fuel and NH₃ slip) were the dominant contributors to NH30 (i.e., NH4+ + NH₃) in the Xitou area, accounting for 50 %–83 % of total NH₃ emissions. During periods with higher δ15N-NH30, such as 17D, 17N, and 19D, fossil fuel combustion dominated (54 ± 3 %), followed by NH₃ slip (26 ± 1 %), waste (13 ± 1 %), and fertilizer (8 ± 1 %). In comparison, during periods with lower δ15N-NH30, the source contributions were more evenly distributed. Fossil fuel combustion and NH₃ slip remain significant, accounting for 30 ± 5 % and 29 ± 1 %, respectively, while the relative contributions from waste and fertilizer sources increased to 24 ± 2 % and 17 ± 3 % (Fig. S10).

It is noted that the temperature effect yields εNH4+-NH3 values ranging between 33 ‰ and 37 ‰ (Kawashima et al., 2023), while consideration of gas–particle equilibrium pathways under normal (neutral) and foggy (acidic) conditions would lead to an isotope fractionation with εNH4+-NH3 values of 33 ‰ and 4 ‰, respectively, as described in Sect. 3.3.1. (Chang et al., 2025; Walters et al., 2019). In real-world environments, these two isotope fraction effects likely operate concurrently, and the δ15N-NH30 values are expected to lie between the estimations derived from εNH4+-NH3=4 ‰ and 33 ‰. Nevertheless, despite this sensitivity of δ15N-NH30 calculation to those shifting εNH4+-NH3 values, the MixSIAR Bayesian model outputs remained consistent with combustion-related sources accounting for 88 %–95 % of total NH₃ emissions (see Supplement Description S3 and Table S1). The persistent dominance of combustion-related sources at this remote mountain site, even during the stagnant fog condition, strongly demonstrates the profound, overarching influence of transport from urban and industrial areas.

These findings underscore the need for regional emission control strategies targeting anthropogenic NH₃ sources, which may represent a more cost-effective mitigation strategy for Taiwan, potentially yielding greater air quality benefits than NO_x or SO₂ reductions alone (P.-C. Huang et al., 2024). It should be noted, however, that the differences in source contributions between daytime and nighttime periods, or between fog and clear conditions, remain within the overlapping ranges of the posterior distributions; consequently, the current sample number precludes statistically robust discrimination between these conditions. Expanded measurements encompassing multiple seasons and contrasting meteorological regimes would be necessary to establish whether systematic diurnal or fog-driven shifts in NH₃ source apportionment exist at this site.

The daily concentration-weighted δ15N-NO3- values ranged from -6.54 ‰ to -0.24 ‰, with an average of -2.98 ± 1.97 ‰ (Fig. 2g). These values are consistent with those reported in mountain sites such as Mt. Lulin (-3.6 ± 3.8 ‰) (Guha et al., 2017) and the Himalayan-Tibetan Plateau (0.44 ± 4.89 ‰) (Lin et al., 2021), as illustrated in Fig. S11. Compared to wintertime measurements at the same site (2.98 ± 1.20 ‰) (T.-Y. Chen et al., 2022), these lower springtime values likely reflect seasonal shifts in NO_x sources and transport-related fractionation. The depletion relative to urban values, such as ∼ 11.5 ‰ in Beijing and Gosan (Fan et al., 2020; Kundu et al., 2010), indicates that NO_x-to-HNO₃ conversion during transport to the mountain receptors is accompanied by progressive isotopic fractionation (Freyer et al., 1993; Gobel et al., 2013).

Under clear conditions, δ15N-NO3- showed minor diurnal variation, with slightly higher values during daytime (-1.50 ± 1.30 ‰) than nighttime (-3.46 ± 1.67 ‰; Table 2), likely reflecting shifts between transported urban and local biogenic NO_x sources or the depletion through deposition. During the fog, δ15N-NO3- decreased significantly from -2.19 ‰ (18D) to -4.80 ‰ (18N), and further to -6.54 ‰ (19D) (Fig. 2g). This decline, contrasting with previous shorter fog events (T.-Y. Chen et al., 2022), reflects the combined effects of prolonged stagnation and a reduced influx of urban air. These conditions promote enhanced aqueous-phase pNO3- formation, predominantly driven by δ15N-depleted NO_x from locally derived biogenic precursors or precursors that have undergone extensive fractionation during transport (Vicars et al., 2013; Gobel et al., 2013).

Relatively low concentration-weighted δ15N-NO3- values were observed outside the fog periods on 20N and 22N, likely attributable to distinct mechanisms. For 20N, the depletion is linked to enhanced local production, supported by elevated sub-micrometer NO3- concentration shown in Fig. S4. For 22N, the low δ15N coincides with significant deposition loss, as evidenced by low total NO3- levels. Because HNO₃ readily reacts with coarse-mode mineral dust and sea salt, the deposition efficiency during transport is high; this preferential removal of δ15N-enriched species often leaves the residual nitrate pool isotopically lower. Furthermore, the thermodynamic partitioning of HNO₃, governed by ambient temperature and particle acidity regulated by sulfate and ammonium, critically influences the observed δ15N. Ultimately, the observed δ15N at this site serves as an integrated signal of physical and chemical dynamics processes within the air parcel during its transit.

Size-segregated δ15N-NO3- values ranged from -10.05 ‰ to 0.78 ‰, showing a weak positive correlation with particle diameter (Fig. 6a). Fine particles (PM₁) had slightly lower δ15N-NO3- values (-3.59 ± 2.38 ‰) compared to larger, coarse-mode particles (PM_1–10, -2.07 ± 2.10 ‰), a trend consistent with previous findings (T.-Y. Chen et al., 2022). This size-dependent distribution likely reflects isotopic fractionation occurring during HNO₃ formation and its subsequent partitioning across different size modes. In environments (nearby coast) proximal to major NO_x sources, HNO₃ may react preferentially with coarse-mode particles (e.g., NaCl or mineral dust), forming δ15N-enriched pNO3-. The residual NO_x pool, consequently depleted in δ15N, subsequently forms HNO₃ that condenses onto fine-mode particles as they undergo transport toward the sampling site (Gobel et al., 2013). However, it is important to note that the complex physical and chemical dynamic processes within the air parcel during its transit, as detailed above, may diminish these distinct isotopic signatures under varying daily meteorological conditions.

The daily concentration-weighted δ18O-NO3- ranged from 30.98 ‰ to 73.27 ‰, with an average value of 51.82 ± 16.47 ‰ (Fig. 2h). Similar to δ15N-NO3-, these values align with measurements from other mountain regions such as 10.8 ‰–92.4 ‰ at Mt. Lulin (Guha et al., 2017) and 64.71 ± 11.52 ‰ at the Himalayan-Tibetan Plateau (Lin et al., 2021), but are lower than previous winter observations (72.66 ± 3.42 ‰) (T.-Y. Chen et al., 2022), as illustrated in Fig. S12. In contrast, δ18O-NO3- at urban sites such as Changchun (68.16 ± 9.52 ‰) and Beijing (83.8 ± 13.4 ‰) are substantially higher, consistent with O₃-dominated oxidation under polluted conditions. The relatively lower δ18O-NO3- values at Xitou and other mountain sites suggest that RO₂-initiated pathways contribute more substantially to nitrate formation under less-polluted conditions. A sharp decrease in δ18O-NO3- from 70.44 ‰ (18D) to 33.81 ‰ (18N and 19D) occurred during the fog, coinciding with δ15N-NO3- depletion. This shift from O₃- to RO₂-dominated oxidation is likely driven by stagnant winds, suppressed urban input, and enhanced local aqueous-phase chemistry under high RH.

Notably, no significant systematic size-dependent trend of δ18O-NO3- on particle size was observed (Fig. 6b). The spatial heterogeneity of oxidants (O₃ and RO₂), coupled with the complex HNO₃ partitioning dynamics within the air parcel, likely erases any size-dependent isotopic pattern and daily variation. However, the δ15N-NO3- vs. δ18O-NO3- relationship (Figs. 6c, S13, and S14) resolves two geochemically distinct regimes reflecting contrasting formation environments and aerosol histories. The higher δ18O-NO3- (55 ‰–83 ‰) with enriched δ15N-NO3- (-6 ‰ to 1 ‰) can be assigned as an urban/transported regime. Freshly emitted in metropolitan areas contributed to elevated δ15N, while O₃-driven oxidation leads to high δ18O (Gobel et al., 2013). The observed lower δ18O-NO3- (9 ‰–38 ‰) and depleted δ15N-NO3- (-10 ‰ to -2 ‰) is assigned as a rural/fog regime. These signatures reflect RO₂-initiated oxidation and δ15N-depleted NO_x from locally derived biogenic precursors or after extensive fractionation during transport, particularly under stagnant conditions. A moderate positive correlation (r=0.66) between δ18O-NO3- and δ15N-NO3- further highlights the intrinsic interplay between oxidation processes and nitrogen cycling at the site. These data demonstrate a δ18O-NO3- and δ15N-NO3- relationship that covers a broad isotopic range, spanning from the signatures observed at the Himalayan-Tibetan Plateau to those at regional background sites like Mt. Lulin (Fig. S15).

Formation pathways of pNO3- were quantitatively analyzed using the MixSIAR framework based on δ18O-NO3- values, as shown in Fig. 7. During daytime periods characterized by low δ18O-NO3- values (31 ‰–41 ‰) and low CO concentrations (<0.22 ppm, 17D, 19D, and 23D), PRO2-OH1 accounted for 82 %–92 % of pNO3- formation (Fig. 7a and b). These conditions are consistent with periods with limited influence from urban air masses, suggesting the dominance of local RO₂ oxidation processes. In contrast, during daytime with higher δ18O-NO3- values (63 ‰–70 ‰) and elevated CO concentrations (0.24–0.27 ppm, 18D, 21D, 22D), the dominant pathways shifted toward PRO2-OH2, PO3-OH1, and PO3-OH2, contributing an average of 29 ± 1 %, 27 ± 3 %, and 28 ± 5 %, respectively. This shift probably reflects the enhanced transport of urban air masses enriched in O₃ and NO_x precursors.

Although nighttime nitrate formation is theoretically dominated by heterogeneous reactions, our results reveal a more complex process. On nights with elevated δ18O-NO3- values, heterogeneous pathways (PRO2-het and PO3-het) were the dominant contributors, accounting for 33 ± 6 % and 47 ± 9 % of pNO3- formation on 19N, 21N, 23N. In contrast, on nights with lower δ18O-NO3- values (32 ‰–43 ‰, 17N, 18N, 20N, and 22N), PRO2-OH1 remained a major contributor, accounting for 80 %–94 % of formation. This high PRO2-OH1 contribution might stem from the residual pNO3- from daytime or nighttime ⚫OH generated via reactions between O₃ and alkenes and/or terpenes. The latter pathway is chemically plausible at Xitou given the documented nighttime accumulation of monoterpenes at this site (Salvador et al., 2020) and the known positive correlation between monoterpene concentrations and dark OH/HO2 radical production in forested environments, which likely proceeds via ozonolysis (Kroll et al., 2001; Aschmann et al., 2002; Kanaya et al., 2007; Salvador et al., 2020). However, direct OH/RO2 measurements would be needed in future campaigns to quantify the relative importance of these mechanisms. The weak night-to-night covariation between δ18O-NO3- and CO concentrations further suggests that nighttime nitrate isotope signatures are governed by the interplay among fog processing, reduced advection, and heterogeneous chemistry, rather than transport intensity alone.

A distinct shift in nitrate formation pathways was observed during the fog event. On 18D, high δ18O-NO3- values (∼ 70 ‰) and elevated CO concentrations (0.24 ppm) were associated with the dominance of PRO2-OH2 (30 ± 20 %) and PO3-OH2 (36 ± 16 %), indicating that O₃ from transported urban precursors remained active (Fig. S16). As fog intensified during 18N and persisted into 19D, δ18O-NO3- values decreased sharply to ∼ 34 ‰, and PRO2-OH1 became dominant (93 ± 5 % on 18N, and 90 ± 5 % on 19D). This transition is driven by two coupled effects: (1) the attenuation of incoming solar radiation suppresses O₃ photolysis and thereby reduces the availability of high-δ18O oxidants, and (2) the scavenging of upslope transported particles by wet deposition diminishes the influx of urban NO_x and O₃. Together, these foggy conditions favor locally derived low δ18O-RO₂ cycling as the predominant oxidant driving nitrate formation. While low values of individual isotopic tracers were occasionally observed during non-fog periods, attributable to source footprint effects or transport-induced fractionation (Sect. 3.3.1 and 3.4.1), the fog episode represents the unique period during this campaign where simultaneous depletion across all three tracers (δ15N-NH4+, δ15N-NO3- and δ18O-NO3-) occurred with coherent temporal progression. This multi-tracer convergence supports the concurrent operation of several fog-induced processes: intense atmospheric stagnation, progressive wet scavenging, aqueous-phase re-equilibration with isotopically depleted gas-phase species, and a mechanistic shift toward RO₂-dominated oxidation, all of which are mechanistically coupled to fog conditions rather than to individual source or regional transport effects.

Distinguishing the relative contributions helps clarify how nitrate formation processes vary with air mass origin, photochemical activity, and fog processing, which is directly relevant to emission control strategies since the efficacy of nitrate reduction depends on the dominant oxidation pathway. At the global scale, model estimated that the NO₂ + ⚫OH (same as our PO3-OH1/PO3-OH2) and N₂O₅ hydrolysis (same as our PRO2-het/PO3-het) pathways each contributed approximately 41 % to tropospheric nitrate formation (Alexander et al., 2020), while observation studies in both urban and mountain environments suggest that heterogeneous pathways become more important for nitrate formation under elevated pollution levels (Fan et al., 2020; Lin et al., 2021). The pathway apportionment presented here provides finer resolution than gas-phase and heterogeneous partitioning by explicitly distinguishing RO₂- from O₃-initiated formation, which can be useful in regions where RO₂ oxidation dominates.

Beyond inorganic HNO₃ formation, the total particulate nitrogen budget includes particulate organic nitrate (pON) species, which can account for 17 %–31 % of total particulate nitrogen (Yu et al., 2024) in mixed urban-biogenic environments, and up to 50 % in urban plumes (Murphy et al., 2025). pON forms via the reaction of NO with RO₂ (NO + RO₂ → RONO₂) during the day, and NO₃ with alkenes and biogenic VOC at night (NO₃ + R → RONO₂) (Murphy et al., 2025; Ward et al., 2025; Guo et al., 2024). These mechanisms operate through the same RO₂- and NO₃-driven pathways identified here (PRO2-OH1/PRO2-OH2 and PRO2-het/PO3-het, respectively), suggesting that the oxidation chemistry inferred from δ18O-NO3- signatures at Xitou also drives concurrent pON production. Because isotope analysis in this study assumes TN comprises only NO3- and NH4+, organic nitrogen contribution could bias the derived δ15N-NH4+ toward the signature of pON. While this was partially mitigated by excluding samples with low [NH4+] (<60 % of ([TN] - [NN])), future research should prioritize direct δ15N-NH4+ measurement, which would enable the estimation of organic nitrate isotopes via residual mass balance (i.e., [ON] = [TN] - [NH4+] - [NO3-]) and provide a more complete characterization of the Nr budget in mountain forest environments.