Both newly fixed N and recycled N (e.g., manure) are processed by terrestrial ecosystems. A substantial portion of this recycled N is subsequently transferred to groundwater, wetlands, lakes, rivers, estuaries, continental shelves and oceanic waters, which has been described as the N Cascade. At each step along this terrestrial to aquatic continuum there is the potential for denitrification to return a portion of land-based N sources to N2. The oceans receive land-based N inputs (through river discharge and direct precipitation to the water surface), as well as N from marine biological N2-fixation occurring primarily in the subtropical gyres. In a hypothetical steady-state world, all the land-based and marine fixed-N sources are eventually denitrified within the terrestrial to marine continuum, and returned to elemental N2. In the real world prior to major human perturbation, the stocks of fixed N varied over time as climate cycles and other controls affected the relative rates of global N2-fixation and denitrification. Currently, human acceleration of N2-fixation raises the urgent question of whether denitrification will keep pace with the extra N2-fixation.
Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here we compare rates of denitrification and controlling factors across a range of ecosystems.
- In aquatic ecosystems, N inputs influence denitrification rates 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 continental shelf sediments account for the largest proportion (44%) of total global denitrification, followed by terrestrial soils (22%), and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification.
- Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per area denitrification rates in soils and groundwater (kg N km-2 y-1) are on-average approximately 10 times lower than per-area denitrification rates in lakes, rivers, estuaries, continental shelves or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.