Ammonia fluxes over an agricultural field in growing and fallow periods using relaxed eddy accumulation

Asia has a considerable risk of nitrogen deposition effects on ecosystems due to the high deposition amounts. Ammonia (NH3) emissions, primarily from agricultural production, have increased in East Asia, and the increasing contribution of NH3 deposition to the total nitrogen deposition is concerning. However, NH3 deposition in this region is uncertain due to lacking flux measurements and the complex bidirectional nature of NH3 atmosphere−surface exchange. To better understand the NH3 deposition in this region, we investigated NH3 fluxes over an agricultural field in Tokyo, Japan. Fluxes were determined using the relaxed eddy accumulation method incorporating the denuder/filter-pack sampling technique and continuously conducting daytime and nighttime measurements in soybean-growing (July 28–August 5, 2020) and fallow (March 4–March 10, 2021) periods. NH3 fluxes were mostly downward in both periods, indicating that the agricultural field was a sink for NH3 during the periods (mean flux = −0.091 μg m−2 s−1). The deposition velocities in the growing period were lower than in the fallow period possibly due to the smaller turbulence. The deposition velocities correlated positively with friction velocity, indicating that the turbulence is the dominant factor driving NH3 deposition. We compared inferred fluxes using a compensation point model with the measured fluxes. The model did not reproduce the observation results when using default inputs, but showed a mean NH3 emission of 0.054 μg m−2 s−1. Revised inputs considering on-site soil and foliage information improved the model's results; however, the inferred fluxes were still smaller than the measured fluxes (mean flux = −0.017 μg m−2 s−1). These results indicate that there were possibly other processes enhancing NH3 deposition in the agricultural field.