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 (2 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 -2 d−1, and those rates estimated from the filtrate fraction ranged from -2 d−1. Thus, N2 fixation in the filtrate fraction accounts for on average 50% (ranging from 2 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.