Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH 3 Source Partitioning.

Ammonia (NH 3 ) 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 δ 15 N) is a promising indicator to trace atmospheric NH x sources, presupposing a reliable nitrogen isotopic signature of NH 3 emission sources. To obtain more specific seasonal δ 15 N values of soil NH 3 volatilization for reliable regional seasonal NH 3 source partitioning, we utilized an active dynamic sampling technique to measure the δ 15 N-NH 3 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 δ 15 N-NH 3 variations were related to the main NH 3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ 15 N variation in soil-volatilized NH 3 could result in an overestimate by up to absolute 38% for agricultural volatile NH 3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ 15 N profile of agricultural volatile NH 3 in regional source apportionment.