The isotopic composition of solid-phase extracted dissolved organic nitrogen in seawater

Dissolved organic nitrogen (DON) is likely comprised of multiple compound classes with varying reactivities and turnover times, resulting in numerous roles in ocean biogeochemistry. Here, we present measurements of total DON and solid-phase extracted DON (SPE-DON) concentrations and δ 15 N values from sampling sites across the global ocean. An optimized SPE protocol was developed to maximize total DON recovery, with a recovery of 41.0 ± 9.4% for surface DON across the global ocean and 56.6 ± 11.2% for deep water DON from the Sargasso Sea. SPE-DON concentrations were 1.8–1.9 μM across most sampling sites, in contrast to greater variation in total DON concentration (4.0–5.9 μM). However, in the equatorial upwelling zones, SPE-DON concentrations were slightly (0.3–0.4 μM) higher than in other regions. The δ 15 N of total DON in surface waters correlated well with the δ 15 N of nitrate supplied to the euphotic zone from the subsurface. SPE-DON δ 15 N was also correlated with nitrate δ 15 N, but SPE-DON δ 15 N values were confined to a narrower range compared to those of total DON. The combined concentration and δ 15 N data indicate that while SPE-DON is biased toward long-lived DON, it still retains some reactive components introduced through regional inputs in the upper ocean, and even some of its longer-lived components may be labile on the timescales of deep ocean circulation. Thus, the evidence for isotopic variation in SPE-DON reported here suggests that SPE-DON can contain DON fractions with varying turnover timescales, highlighting its potential utility for probing dissolved organic matter fluxes in the ocean.