Diazotroph‐Driven ²¹⁰ Po Scavenging Reveals a Potential Radiotracer for Marine Nitrogen Fixation

Abstract The role of phytoplankton, particularly diazotrophs, in affecting in situ 210 Po scavenging has been little investigated. Here, we first report an enhanced 210 Po deficit in a cyclonic eddy core relative to its edge in the subtropical Pacific, coinciding with an elevated integrated nitrogen fixation rate (NFR). In contrast, particulate organic carbon, chlorophyll a , total particulate matter, and 210 Pb show no core‐edge difference, revealing a selective fractionation between 210 Po and 210 Pb during scavenging. The partition coefficient of 210 Po, but not 210 Pb, correlates with NFR. The 210 Po/ 210 Pb fractionation factor also scales with NFR. These relationships indicate diazotrophs' uptake of 210 Po and provide a field data‐based algorithm linking 210 Po disequilibria to NFR in (sub)tropical oceans. Our findings suggest that 210 Po offers a potential radiotracer for quantifying marine NFR, highlighting a new insight to constrain nitrogen cycling in oligotrophic oceans.