Plant uptake and stream chemistry set global bounds on nitrogen gas emissions from humid tropical forests

Denitrification and hydrologic leaching are the two major pathways by which nitrogen is lost from the terrestrial biosphere. Humid tropical forests are thought to dominate denitrification from unmanaged lands globally, but there is large uncertainty about the range and key drivers of total N gas emissions across the biome. We combined pantropical measures of small watershed stream chemistry with ecosystem modeling to determine total nitrogen gas losses and associated uncertainty across humid tropical forests. Our calculations reveal that denitrification in soils and along hydrologic flowpaths contributes on average >45% of total watershed N losses. However, when denitrification occurs exclusively in shallow soils, simulations indicate that gas emissions would exceed N inputs and render plants severely N-limited, which contradicts observations of widespread N-sufficiency in tropical forests. Our analyses suggest an upper bound on soil denitrification of ~80% of total external N losses beyond which tropical plant growth would be compromised.