Humans have doubled the natural rate of N inputs to terrestrial systems globally (270 Tg N/yr). As this N cascades down the terrestrial-freshwater-marine continuum, N is removed by denitrification. Where along this continuum denitrification occurs, how much N is denitrified at each step, the global spatial distribution of denitrification, and unifying principals across systems will be addressed in this talk.
 
In aquatic ecosystems, N inputs appear to influence the total amount of denitrification whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves.
 
Spatially distributed global models of denitrification suggest that on a watershed basis, the amount of N denitrified is generally highest in terrestrial soils (124 Tg N/y), with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries (total 110 Tg N/y). However, there are a number of regional exceptions to this general trend of decreasing denitrification in a downstream direction. For example, in Europe groundwater appears to be the major site of denitrification in watersheds, with lesser amounts in soils and considerably less in rivers and lakes. In eastern Asia, considerably less N is denitrification in lakes/reservoirs and groundwater than in rivers. Continental shelves and open ocean oxygen minimum zones (OMZs) also are estimated to be important in removing N from land-based sources (69 Tg N/y). Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per area denitrification rates in soils and groundwater (kg N/km2/y) are on-average approximately 10 times lower than per-area denitrification rates in lakes, rivers, estuaries, continental shelves or OMZs.
 
Some of the largest uncertainties in denitrification rates are in terrestrial systems and in stream/river networks, where a large proportion of land-based N sources are denitrified. Advanced methods to measure denitrification at ecosystem scales, integration of information across systems, as well as dynamic biogeochemical models to further understand the effects of denitrification on ecosystem dynamics are needed.

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