Significance: Agricultural systems are already major forces of ammonia pollution and environmental degradation. How agricultural ammonia emissions affect the spatio-temporal patterns of nitrogen deposition and where to target future mitigation efforts, remains poorly understood. We develop a substantially complete and coherent agricultural ammonia emissions dataset in nearly recent four decades, and evaluate the relative role of reduced nitrogen in total nitrogen deposition in a spatially explicit way. Global reduced nitrogen deposition has grown rapidly, and will occupy a greater dominant position in total nitrogen deposition without future ammonia regulations. Recognition of agricultural ammonia emissions on nitrogen deposition is critical to formulate effective policies to address ammonia related environmental challenges and protect ecosystems from excessive nitrogen inputs.

Significance: Forest soil carbon (C) storage plays a central role in sequestrating atmospheric CO ₂ on timescales from centuries to millennia. However, our current understanding of soil C sequestration in response to N deposition mainly focuses on mid-to-high latitudes in the Northern Hemisphere, where N supply typically constrains forest growth. We lack data about changes in soil C stocks in tropical forests, where most ecosystems are N-rich or N-saturated. Using more than a decade of continuous N addition experiment and a meta-analysis, we found that excess N deposition can significantly increase soil C in N-rich tropical forests. However, enhanced C sequestration in tropical soils is not a good reason to justify excess N emissions to the atmosphere.

Significance: Elevated atmospheric N deposition threatens ecosystem health through eutrophication in terrestrial ecosystems, but little is known about consequences of N deposition in N-rich tropical ecosystems. We added several levels of N to an N-rich tropical forest and monitored plant growth dynamics, forest nutrient status, plant water use, and water losses from the ecosystem for a decade. We found that plants can acclimate and maintain nutrient balance by altering hydrological cycling. These results demonstrate that while elevated N deposition to already N-rich tropical forests may have minor effects on forest growth, it can exert a detectable influence on hydrological dynamics. Reduced runoff may threaten water supply in rapidly developing tropical regions.