Since the industrial revolution, the atmospheric nitrogen (N) deposition has rapidly increased as a result of the ever-accelerated increasing of anthropogenic emissions of reactive forms of N. Excessive N deposition can have a series of negative effects on terrestrial ecosystems, such as soil acidification, water body eutrophication and biodiversity loss. Deposition of N compounds mainly originates from ammonia (NH₃) and nitrogen oxide (NOx) sources that emitted into the atmosphere (which also play important roles in the formation of atmospheric haze). To reduce the adverse effects of N deposition, China has implemented measures to control NOx emissions since the 12th Five-Year Plan period.

But so far no national policy concerning NH₃ emission control has been set forth. Identifying NH₃ sources is a critical step for formulating policies controlling NH₃ emissions. The rapid development of stable isotope techniques has made it possible to partition NH₃ sources by measuring the natural abundance (expressed by δ¹⁵N) of ammonium (NH₄+). However, most such studies were performed at urban and agricultural areas, and data from remote forested areas is lacking. In addition, previous studies neglected the nitrogen isotope fractionation during atmospheric processes forming ammonium (NH₄+).

To determine the seasonal pattern of δ15N-NH₄+ in rainfall and its sources, Prof. FANG Yunting and his PhD student HUANG Shaonan, two researchers of the Institute of Applied Ecology (IAE), the Chinese Academy of Sciences (CAS), collected precipitation samples from 2014 to 2016 and quantified the N deposition at the Qingyuan Forest Ecosystem Research Station of the CAS (referred to as Qingyuan Station hereinafter). The researchers, in their paper, reported that the amount of inorganic N deposition at the study site was 20 kg N ha-1 yr-1, of which 2/3 was in the form of NH₄+. The concentrations of δ¹⁵N-NH₄+ fluctuated widely from -24.6 to +16.2‰ (average -6.5‰), showed a repeatable seasonal pattern with higher values in summer than in winter. In summer, NH₃ emissions from agricultural activities increase with rising air temperature, which give rise to large amounts of NH₃ gas and result in isotope equilibrium fractionation during the conversion of NH₃ gas to aerosol NH₄+ in the atmosphere.

The researchers thus suggested that N isotope fractionation must be considered when partitioning NH₃ sources based on δ¹⁵N-NH₄+ in precipitation. When taking N isotope fractionation into account, the researchers reported that, of the total amount of NH₄+ in precipitation, the NH₃ emitted from agricultural activities accounted for 60–94% in summer, while fossil fuel combustion contributed to 7–40% of that in summer and 44-62% in winter.

The research was financially supported by the National Key R&D Project of China, the Key Research Program of Frontier Sciences of the CAS, the National Natural Science Foundation of China and the Premier’s Fund - Special Project for Air Pollution Control.

The study entitled "Seasonal pattern of ammonium ¹⁵N natural abundance in precipitation at a rural forested site and implications for NH₃ source partitioning”, has been published in Environmental Pollution.