Seasonal pattern of ammonium 15 N natural abundance in precipitation at a rural forested site and implications for NH 3 source partitioning.

Excess ammonia (NH ₃ ) emissions and deposition can have negative effects on air quality and terrestrial ecosystems. Identifying NH ₃ sources is a critical step for effectively reducing NH ₃ emissions, which are generally unregulated around the world. Stable nitrogen isotopes (δ ¹⁵ N) of ammonium (NH ₄ ⁺ ) in precipitation have been directly used to partition NH ₃ sources. However, nitrogen isotope fractionation during atmospheric processes from NH ₃ sources to sinks has been previously overlooked. Here we measured δ ¹⁵ NNH ₄ ⁺ in precipitation on a daily basis at a rural forested site in Northeast China over three years to examine its seasonal pattern and attempt to constrain the NH ₃ sources. We found that the NH ₄ ⁺ concentrations in precipitation ranged from 5 to 1265 μM, and NH ₄ ⁺ accounted for 65% of the inorganic nitrogen deposition (20.0 kg N ha ^-1 yr ^-1 ) over the study period. The δ ¹⁵ N values of NH ₄ ⁺ fluctuated from -24.6 to +16.2‰ (average -6.5‰) and showed a repeatable seasonal pattern with higher values in summer (average -2.3‰) than in winter (average -16.4‰), which could not be explained by only the seasonal changes in the NH ₃ sources. Our results suggest that in addition to the NH ₃ sources, isotope equilibrium fractionation contributed to the seasonal pattern of δ ¹⁵ NNH ₄ ⁺ in precipitation, and thus, nitrogen isotope fractionation should be considered when partitioning NH ₃ sources based on δ ¹⁵ NNH ₄ ⁺ in precipitation.
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