Denitrification Across Landscapes and Waterscapes: A Synthesis

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 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 portion (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 x km(-2) x yr(-1)) are, on average, approximately one-tenth the per-area rates of denitrification 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.

Modeling increased riverine nitrogen export: Source tracking and integrated watershed-coast management

“…2b). Water residence time and N loading are two important factors influencing the rates of denitrification in a river system (Seitzinger et al, 2006). Long water residence time may facilitate N removal capacity through denitrification while denitrification effectiveness may decline in hyper-N rich rivers (Chen et al, 2014;Mulholland et al, 2008;Seitzinger et al, 2006;Seitzinger et al, 2002).…”

“…Water residence time and N loading are two important factors influencing the rates of denitrification in a river system (Seitzinger et al, 2006). Long water residence time may facilitate N removal capacity through denitrification while denitrification effectiveness may decline in hyper-N rich rivers (Chen et al, 2014;Mulholland et al, 2008;Seitzinger et al, 2006;Seitzinger et al, 2002). Direct measurement of excess dissolved N 2 (ΔN 2 , denitrification product) and estimation of area-weighted N 2 emission flux in the Jiulong River network during 2010-2011 suggest that gaseous N removal accounts for 25% (North River) and 21% (West River) of riverine N export (Chen et al, 2014).…”

“…Dam reservoirs retain more sediment by increasing water residence time, which provides greater opportunity for N removal processes such as denitrification and ANAMOX (Fan and Shibata, 2015;Mayorga et al, 2010;Salvia-Castellvi et al, 2005). Deep water, lower oxygen and sediment together cause more anaerobic sediment and hence denitrification in dam reservoirs (Seitzinger et al, 2006;Trimmer et al, 2012). The North River has more artificial reservoirs with a larger storage capacity compared with the West River, contributing to a higher gaseous N removal through denitrification (Chen et al, 2014).…”

Denitrification Potential and Denitrifier Abundance in Downstream of Dams in Temperate Streams

“…The availabilities of nitrate and organic carbon are thus important in determining denitrifying activity in soils (Starr and Gillham, 1993;Philippot et al, 2007; Enwall et al, 2010). Denitrification usually occurs at oxic/suboxic interfaces, where higher nitrate concentrations and abundant denitrifiers (mostly facultative anaerobes) can exist (Seitzinger et al, 2006). However, the occurrence of denitrification in a wide range of ecosystems revealed in previous studies implied the diverse environmental conditions for denitrifying activity (McClain et al, 2003;Seitzinger et al, 2006).…”

“…In many cases, researchers have observed the increasing amounts of bioavailable nitrogen to enter rivers together with uncoupling low in-stream N levels (Boyer et al, 2002;Mulholland et al, 2008). As such, numerous empirical measurements of denitrification rates in the sediments of a range of natural aquatic ecosystems (rivers, streams, lakes, and coastal marines) have been conducted to evaluate such a fundamental process (Seitzinger et al, 2006;Wallenstein et al, 2006). In natural streams, denitrification often accounts for a great portion of the total nitrate removal (Mulholland et al, 2008).…”