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 δ¹⁵N 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 δ¹⁵N of total DON in surface waters correlated well with the δ¹⁵N of nitrate supplied to the euphotic zone from the subsurface. SPE-DON δ¹⁵N was also correlated with nitrate δ¹⁵N, but SPE-DON δ¹⁵N values were confined to a narrower range compared to those of total DON. The combined concentration and δ¹⁵N 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. The longer average turnover time of SPE-DON suggests that greater molecular polarity and/or charge directly increase or are otherwise correlated with the biogeochemical lability of different DON pools.