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N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time‐series Study site
Author(s) -
Knapp Angela N.,
Sigman Daniel M.,
Lipschultz Fredric
Publication year - 2005
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2004gb002320
Subject(s) - nitrate , nitrogen , thermocline , water column , oceanography , environmental chemistry , nitrogen cycle , flux (metallurgy) , isotopes of nitrogen , nitrogen fixation , environmental science , chemistry , geology , organic chemistry
To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N 2 fixation in the nitrogen cycle at the Bermuda Atlantic Time‐series Study (BATS) site, we measured the 15 N/ 14 N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June 2000 through May 2001. The annually averaged TN δ 15 N in the upper 100 m is 3.9‰, which is greater than thermocline nitrate (2–3‰ at 250 m) and similar to literature values for shallow sinking nitrogen at BATS (3.7‰). We discern no seasonal variation in TN δ 15 N, which suggests that most of the DON pool is recalcitrant on this timescale. The TN data require a δ 15 N for the sinking flux that is similar to previous measurements, suggesting that N 2 fixation is a minor component of new nitrogen at BATS. Small but measurable differences in the concentration and 15 N/ 14 N of total organic nitrogen (TON) between the surface and subsurface (∼250 m) suggest that subsurface remineralization of ∼0.25 μM of the surface TON acts to lower the 15 N/ 14 N of nitrate in the thermocline at BATS.