Annually, humid tropical forests transform more N2 via biological N fixation, lose more dissolved inorganic N (DIN) to rivers and return more N gas to the atmosphere via denitrification than any other forest biome. These large N fluxes influence global climate by fueling plant drawdown of CO2 and microbial production of nitrous oxide (N2O). Understanding of these processes is essential for climate forecasting and hinges on how DIN in tropical soils is partitioned among plant uptake and hydrologic versus gaseous losses. However, there exists little information on the proportion of N fluxes that plants actually retain, the fate of that which they do not, and how these fluxes vary across the global tropics. Here, we determine DIN uptake by plants and losses via leaching and denitrification in the humid tropics using small watershed budgeting. To determine the relative strength of these fluxes we combined geographically extensive measures of stream chemistry from small watershed forests with a model of soil N processing and solute transport in soils. The model uses satellite estimates of net primary production (NPP) coupled with data on tissue allocation and N concentrations to estimate DIN concentrations in soil water that are available for both leaching and denitrification.
Results/Conclusions
When comparing annual N inputs with hydrologic losses, we find that our tropical watersheds lose <2% of N inputs via streams, equating to >98% of N retained or lost via gas emissions. To place global bounds on potential denitrification fluxes, we consider two alternative mass balance formulations. First, we calculate gaseous loss as a sequential process occurring along hydrologic flowpaths from the bottom of the rooting zone to streams. Second, we calculate denitrification as occurring in parallel with root uptake in soils with both sinks having access to DIN. We calculate average sequential gas fluxes to be 45% of total watershed N loss. In contrast, when we consider denitrification as a parallel process occurring in the rooting zone, our model yields average denitrification fluxes at 89% of total N losses. These results support denitrification as roughly equivalent to dissolved losses at a minimum and >30 times higher than DIN losses at a maximum. Our framework independently constrains global modeling estimates of N gas emissions from the tropics and identifies terrestrial soils as hotspots of watershed-level denitrification. Our approach represents a new way to constrain terrestrial denitrification at the watershed scale that compliments existing modeling and isotopic tools.