Ammonia should be considered in field experiments mimicking nitrogen deposition

Excess nitrogen deposition has significant impacts on water eutrophication, soil acidification, eleveted greenhouse gas emissions, and biodiversity loss. These impacts mostly derive from conventional manipulative experiments in the field by adding nitrogen solution directly onto grassland or forest floors. For forest ecosystems, previous field experiments have usually ignored the nitrogen cycles in the canopy, which are important in responses to airborne nitrogen input. Although whole-forest canopy nitrogen fertilization has recently been conducted to promote our understanding of nitrogen deposition processes, spraying nitrogen solution onto plants still largely ignores the dry deposition of ammonia (as well as other gaseous reactive nitrogen species). To date, there have been a limited number of field studies that have investigated the bi-directional exchange of ammonia between the atmosphere and plants, not to mention the impacts of ammonia on natural ecosystems. Due to the increasing trend of atmospheric ammonia concentrations worldwide and its dominant role in nitrogen deposition and haze pollution, the next generation of experiments should mimick nitrogen deposition on natural ecosystems by further considering the dry deposition of ammonia.