Determination of total N 2 fixation rates in the ocean taking into account both the particulate and filtrate fractions
Abstract. Using the 15N2 tracer method and high-sensitivity δ15N analytical systems, we determined N2 fixation rates for ocean samples by dividing them into particulate (>0.7 μm) and filtrate (<0.7 μm) fractions. While N2 fixation in the filtrate fraction had been ignored in previous studies, we found a significant N2 fixation rates in the filtrate fraction in our study. The areal N2 fixation rates in the western North Pacific Ocean estimated from the particulate fraction varied from <1 to 160 μmol N m-2 d−1, and those rates estimated from the filtrate fraction ranged from <0.5 to 54 μmol N m-2 d−1. Thus, N2 fixation in the filtrate fraction accounts for on average 50% (ranging from <10% to 84%) of the total N2 fixation rates. If these results are confirmed generally in the ocean, the new total N2 fixation flux, which includes fixation in the filtrate fraction, possibly doubles the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.