Seasonal asynchrony of nitrification and nitrogen uptake in marginal seas: Implications for reassessing new production.

The classical "new production" paradigm, designed for stable open-ocean ecosystems, distorts biological carbon pump assessments when applied to marginal seas-that receive substantial exogenous reduced nitrogen (N). Here, using 15N isotope tracing in the spring and autumn Bohai and Yellow Seas, we simultaneously quantified nitrification and N uptake rates and coupled these with microbial community dynamics. Our results reveal a pronounced seasonal asynchrony between nitrification and N uptake. In spring, high irradiance inhibited ammonia-oxidizing archaea, resulting in negligible nitrification (1.3 nmol L-1d-1) and consequently allowing for substantial ammonium uptake (154.1 nmol L-1d-1). Fueled by substantial exogenous N inputs, this robust ammonium uptake effectively constitutes new production. In autumn, weakened irradiance and intensified vertical mixing promoted enhanced in-situ nitrification (120.5 nmol L-1d-1) co-mediated by expanded AOB and AOA communities, which in turn fueled an exceptionally high nitrate uptake rate (605.8 nmol L-1d-1). This robust autumn nitrate assimilation was sustained primarily by internally recycled N, representing regeneration rather than classical new production. Consequently, we propose a revised new production framework tailored to marginal seas. Upon application of this framework, the classically defined f-ratio of 0.23 in spring increased to a corrected range of 0.44 - 0.96, whereas the autumn estimate decreased from 0.93 to 0.77 - 0.81. Forward, the implementation of this diagnostic framework is essential to prevent the systematic miscalculation of carbon export efficiency in marginal seas.