Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning

Ammonia (NH₃) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NH_x). Accurately quantifying its contribution to regional atmospheric NH_x deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ¹⁵N) is a promising indicator to trace atmospheric NH_x sources, presupposing a reliable nitrogen isotopic signature of NH₃ emission sources. To obtain more specific seasonal δ¹⁵N values of soil NH₃ volatilization for reliable regional seasonal NH₃ source partitioning, we utilized an active dynamic sampling technique to measure the δ¹⁵N-NH₃ values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ¹⁵N-NH₃ variations were related to the main NH₃ production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ¹⁵N variation in soil-volatilized NH₃ could result in an overestimate by up to absolute 38% for agricultural volatile NH₃ to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ¹⁵N profile of agricultural volatile NH₃ in regional source apportionment.