Direct Canopy Uptake of Atmospheric Reactive Nitrogen: A Significant Pathway for Airborne Nitrogen Input into Rice Paddy Ecosystems

Direct canopy uptake of atmospheric reactive nitrogen (ARN) is an important process, but the magnitude of ARN assimilation in agricultural ecosystems is unclear. We used a combination of a water-culture rice-growing system with a 15N tracer to investigate canopy uptake of ARN under field conditions. Gross uptake was quantified using the plant N partitioning approach, and the net uptake of ARN was estimated based on the system N balance. Gross ARN uptakes were 23.1 and 38.2 kg N·ha-1 during vegetative and reproductive growth, respectively. Although a certain amount of N was lost mainly from the rice canopy during the reproductive phase, the net gain of ARN was 34.3 kg N·ha-1 over the entire growing season, much higher than the present estimates of dry N deposition using an indirect inferential approach. Our results highlight the magnitude of direct canopy uptake of ARN in rice paddy ecosystems, an important N process that is rarely considered in present N budgets.