Longitudinal variability of size‐fractionated N 2 fixation and DON release rates along 24.5°N in the subtropical North Atlantic

Dinitrogen (N 2 ) fixation and dissolved organic nitrogen (DON) release rates were measured on fractionated samples (>10 µm and <10 µm) along 24.5∘N in the subtropical North Atlantic. Net N 2 fixation rates (N 2 assimilation into biomass) ranged from 0.01 to 0.4 nmol N L −1 h −1 , and DON release rates ranged from 0.001 to 0.09 nmol N L −1 h −1 . DON release represented ∼14% and ∼23% of >10 µm and <10 µm gross N 2 fixation (assimilation into biomass plus DON release), respectively. This implies that by overlooking DON release, N 2 fixation rates are underestimated. Net N 2 fixation rates were higher in the east and decreased significantly toward the west ( r s  = −0.487, p  = 0.002, and r s  = −0.496, p  = 0.001, for the >10 µm and <10 µm fractions, respectively). The sum of both fractions correlated with aerosol optical depth at 550 nm (AOD 550 nm) ( r s  = 0.382, p  = 0.017) and phosphate (PO 4 3− ) concentrations ( r s  = 0.453, p  = 0.018), suggesting an enhancement of diazotrophy as a response to aerosol inputs and phosphorus availability. In contrast, DON release was constant among size fractions and did not correlate with any of these variables. We also compared N 2 fixation rates obtained using the 15 N 2 dissolved and bubble methods. The first gave average rates 50% (49% ± 39) higher than the latter, which supports the finding that previously published N 2 fixation rates are likely underestimated. We suggest that by combining N 2 fixation and DON release measurements using dissolved 15 N 2 , global N 2 fixation rates could increase enough to balance oceanic fixed nitrogen budget disequilibria.