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Title: DENITRIFICATION ACROSS LANDSCAPES AND WATERSCAPES: A SYNTHESIS

Author
item SEITZINGER, SYBIL - RUTGERS UNIV.
item HARRISON, JOHN - RUTGERS UNIV.
item BOHLKE, JOHN - U.S.GEOLOGICAL SURVEY
item BOUWMAN, A. - NETHERLANDS ENV.ASS.AGENC
item Lowrance, Robert
item PETERSON, BRUCE - ECOSYSTEMS CENTER
item TOBIAS, C - U.N.CAROLINA-WILMINGTON
item VAN DRECHT, G - NETHERLANDS ENV. ASS.AGEN

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2005
Publication Date: 12/12/2006
Citation: Seitzinger, S., Harrison, J., Bohlke, J., Bouwman, A.F., Lowrance, R.R., Peterson, B., Tobias, C., Van Drecht, G. 2006. Denitrification across landscapes and waterscapes: a synthesis. Ecological Applications. 16(6):2064-2090.

Interpretive Summary: Denitrification (the microbially mediated reduction of nitrate to di-nitrogen gas) is important because it removes bioavailable nitrogen from natural and human-altered systems and returns di-nitrogen gas to the atmosphere. Much of the excess bioavailable nitrogen (nitrate or ammonium) has been added due to agricultural inputs of N. If the bioavailable N reaches sensitive aquatic system or groundwater it can cause environmental and human health problems. While denitrification has been studied in terrestrial, freshwater, and marine systems, there have been few attempts to compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater and marine systems in which denitrification occurs can be organized along a continuum ranging from: 1) those in which nitrate supplied through nitrification and the subsequent denitrification are tightly coupled in space and time, to 2) those in which nitrate production and denitrification are relatively decoupled. Global, spatially distributed models of denitrification suggest that continental shelf sediments account for the largest proportion (44%) of total global denitrification, followed by terrestrial soils (22%), and oxygen minimum zones (OMZs) of the open oceans (14%). Freshwater systems (groundwater, lakes and rivers) account for about 20% and estuaries 1% of the total global denitrification. In aquatic ecosystems, N inputs appear to 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. The distribution of denitrification of land-based N sources is somewhat different. 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. However, there are a number of regional exceptions to this general trend of decreasing denitrification in a downstream direction.. There are a number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems. However, these have not generally been widely tested or evaluated for their effectiveness at the scales that would be required to significantly reduce N export at the whole watershed scale.

Technical Abstract: 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 ecosystem types. We suggest that terrestrial, freshwater and marine systems in which denitrification occurs can be organized along a continuum ranging from: 1) those in which nitrification and denitrification are tightly coupled in space and time, to 2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs appear to 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 OMZs (14%). Freshwater systems (groundwater, lakes and rivers) account for about 20% and estuaries 1% of the total global denitrification. The distribution of denitrification of land-based N sources is somewhat different. 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. However, there are a number of regional exceptions to this general trend of decreasing denitrification in a downstream direction. 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. There are a number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems. However, these have not generally been widely tested or evaluated for their effectiveness at the scales that would be required to significantly reduce N export at the whole watershed scale.