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

Ammonia (NH) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NH). Accurately quantifying its contribution to regional atmospheric NH deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δN) is a promising indicator to trace atmospheric NH sources, presupposing a reliable nitrogen isotopic signature of NH emission sources. To obtain more specific seasonal δN values of soil NH volatilization for reliable regional seasonal NH source partitioning, we utilized an active dynamic sampling technique to measure the δN-NH 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 δN-NH variations were related to the main NH production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δN variation in soil-volatilized NH could result in an overestimate by up to absolute 38% for agricultural volatile NH to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δN profile of agricultural volatile NH in regional source apportionment.