Abstract
Large rivers are important terrestrial dissolved organic matter (DOM) sources to marginal seas, and dissolved organic nitrogen (DON) plays an essential role in DOM cycling. The Yellow River ranks as the fifth largest river (in length) in the world and is well-known for its high dissolved inorganic nitrogen (DIN) concentration and relatively low DON concentration, leading to extreme measuring uncertainties in DON and nitrogen isotopic composition (δ15N), consequently leaving its DON cycling as an unresolved puzzle. To fill such a knowledge gap, we analyzed 17 samples from the middle to downstream with a combination of spectroscopy, tangential flow filtration, nitrogen isotope, and DNA sequencing. DON<1kDa dominated the DON pool and significantly correlated inversely with DIN, indicating the DON<1kDa mineralized into nitrate. This finding was further supported by the observed Rayleigh fractionation in δ15NDON<1kDa and the spatial distribution pattern of ammonia-oxidizing bacteria/archaea abundance. The redundancy analysis revealed that the geographical features and the microbial community were closely related, which joined together to drive the DON cycling. In addition, we propose a rational method to quantify the flux of mineralized DON in large rivers. This study discovered the active DON cycling hidden in high DIN large river and highlighted the importance of DON mineralization as well as its role in marginal seas carbon cycling.
Published Version
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