Significance of N2 fixation in dissolved fractions of organic nitrogen

Abstract

Using the 15N2 tracer method and high-sensitivity 15N analytical systems, we determined N2 fixation rates by dividing fractions into particulate organic nitrogen (PON: >0.7 μm) and dissolved organic nitrogen (DON: <0.7 μm). While N2 fixation in the 5 DON fraction had been ignored in previous studies, we found significant N2 fixation signal in the DON fraction in our study. The areal N2 fixation rates estimated from the PON fractions varied from <1–160 μmolNm−2 d−1, and those estimated from the DON fractions ranged from <0.5–54 μmolNm−2 d−1. Thus, N2 fixation in the DON fractions accounted for 50% (ranging from <10% to 84%) of the total N2 fixation rates on an 10 average. The new total N2 fixation flux, which includes fixation in DON fractions, has possibility to double the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.