Dissolved organic nitrogen, DON

Generally, N03 dominates over NH4+ at sites close to emission sources (8,12). Dissolved organic nitrogen (DON) concentrations are highly variable in precipitation but often amount to 25-50% of inorganic N deposition (8, 12-14). In some areas, DON can occur in greater concentrations than the inorganic species (15). 

The major objective of this chapter is to examine and synthesize the published literature with respect to sources and production of terrestrially derived DOC, its relationship with dissolved organic nitrogen (DON), and the mechanistic controls on their export from terrestrial ecosystems to surface waters. With the exception of wet precipitation (which is ranked by continental landmass), we have classified data for throughfall and soil solution under biome type. Where possible, we have shown mean and standard deviations of some biomes to illustrate the amount of variance within and between biomes. Relationships between DOC and DON are illustrated using only those studies that report both DOC and DON concentration. Because most research on DOC and DON has been accomplished in relatively undisturbed areas, particularly forests, this chapter concentrates on the aspect of diffuse-source allochthonous inputs to surface waters and not point-source inputs from urban and agricultural areas. Recent work by Westerhoff and Anning (2000), however, indicates that more research on effluent or point-source DOC as a contributor to riverine allochthonous inputs may be... 

Figure 8.5 Experimental fluxes of NELt-1-, NO3U dissolved organic nitrogen (DON), urea, dissolved free amino acids (DFAA), and dissolved combined amino acids (DCAA) across the sediment-water interface in cores collected in Hog Island Bay (USA). (Modified from Tyler et al., 2003.)...

Figure 10.5 Major processes involved in the biogeochemical cycling of N in estuaries and the coastal ocean (1) biological N2 fixation (2) ammonia assimilation (3) nitrification (4) assimilatory NC>3 reduction (5) ammonification or N remineralization (6) ammonium oxidation (speculative at this time) (7) denitrification and dissimilatory NO3 reduction to NH4+ and (8) assimilation of dissolved organic nitrogen (DON). (Modified from Libes, 1992.)...

Dissolved organic nitrogen DON Ethylenediaminetetraacetic acid (EDTA)... 

Table 8.2 Summary of published rates of ammonium (NH +) regeneration and dissolved organic nitrogen (DON) release from Nj fixers in the field in the light. The standard deviations were calculated using propogation of error. 

Table 8.9 Rates of dissolved organic nitrogen (DON) release published and ratios of DON release to gross nitrogen (N) uptake, expressed as a percent, in the field. Rates were determined using N tracer techniques ( N) or changes in concentrations (CC)... 

Open ocean regions receive significant amounts of land-based N from atmospheric deposition ( 25 Tg N yr Dentener pers. comm.). This estimate of atmospheric deposition does not include dissolved organic nitrogen (DON), which may account for from 20% to as much as 80% of the total dissolved N in rainwater in remote marine locations (Cornell et al., 1995, 2001, 2003 Mace et al. 2003). Keeping in mind that this estimate of DON is uncertain because of the... 

The data that have accumulated so far are summarized and discussed in several excellent reviews. Of those, information on the impact of sunlight on aquatic nitrogen compounds can be found in Bronk (2002), Kieber (2000), Mopper and Kieber (2002), Moran and Covert (2003), and Zepp (2003). A reader interested in mechanisms behind the reported phenomena is advised to turn to more chemistry-oriented works by Boule et ah (1999) and Mopper and Kieber (2000). The present chapter is a supplement and update of the above-mentioned studies, specifically focusing on phototransformations of dissolved organic nitrogen (DON). 

Table i8.i Concentrations of Total Dissolved Nitrogen (TDN) and Dissolved Organic Nitrogen (DON) from Surface Waters of a Variety of Estuarine Systems... 

Figure 33.12 A schematic box diagram of the Hood et al. (2001,2004) ecosystem model. The state variables are dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), phytoplankton (P), Trichodesmium (T), heterotrophs (H), and detritus (D). Redrawn and modified with permission from Hood et al. (2004).

---