N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic time-series study site - art. no. GB1018

摘要

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 N-15/(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 delta(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 delta(15)N, which suggests that most of the DON pool is recalcitrant on this timescale. The TN data require a delta(15)N for the sinking flux that is similar to previous measurements, suggesting that N2 fixation is a minor component of new nitrogen at BATS. Small but measurable differences in the concentration and N-15/N-14 of total organic nitrogen ( TON) between the surface and subsurface ( similar to 250 m) suggest that subsurface remineralization of similar to 0.25 mu M of the surface TON acts to lower the N-15/(14) N of nitrate in the thermocline at BATS.