Bulk Chemical Composition of High Molecular Weight Dissolved Organic Nitrogen

Bulk Chemical Composition of High Molecular Weight Dissolved Organic Nitrogen 

Only occasionally has the N content of solid phase extracts been reported. At a site in the Atlantic Ocean the carbon to nitrogen ratio (C N) of XAD 8 and XAD 2 extracts fell in the range of 40-57 (57 0.9 and 41.1 3.3, respectively DrufFel et ai, 1992). In contrast, at the same site XAD 4, when used as the second resin in series with XAD 8 or XAD 2, extracted compounds with lower C N ratios - 19—24 (21.0 2.4). These values are only slighdy higher than ratios reported for total DOM (see below). McKnight and Aiken (1998) reported a C N value of 37 for DOM extracted by XAD 8 at one site in the Pacific Ocean at other sites in the N. Pacific Ocean XAD 2 was found to extract DOM with a C N ratio between 32 and 46.5 (Druffel et al, 1992 Meyers-Schulte and Hedges, 1986). Bronk (2002, Table III) compiled various literature values and arrived at an average C N ratio of 32.8 19.5 for total humic substances isolated from a variety of aqueous environments (see McCarthy and Bronk, this volume). 

The total DON extracted from the Sargasso Sea when using XAD 4 in series with either XAD 8 or XAD 2 is reported in Fig. 3.1A (Druffel et al., 1992). If we assume a total DON concentration of 5.0 pM N in surface waters and 3.0 pM N in deep waters (estimated from Hansell and Carlson, 2001), the organic N extracted by XAD resins represents approximately 10% of the total DON present throughout the water column. The C/N ratio for DOM isolated by Cis resins has not been reported but is probably similar to that for other soHd phase extracts (i.e., 30). 

Values discussed in the previous section represent an oceanic average and individual regions and parts of the water column may behave differendy. For example, Abell et al. (2000, 2005) found that the DOM reminerahzed along isopycnals outcropping in the subtropical North Pacific Ocean is C-rich (C N 30) based on this C N ratio these authors concluded that DOM contributes between 70 and 90% of the total organic matter remineralized in the upper thermocHne. As more data are collected, other sites in the ocean may show similar variabiHty. 

Available stable C and N isotope values (6 C and 5 N) for HMWDOM are listed in Table 3.1. Average values for HMWDOM isolated from surface, mid-depth 

---