Active dissolved organic nitrogen cycling hidden in large river and environmental implications

Area of Science:

• Environmental Chemistry
• Riverine Biogeochemistry
• Microbial Ecology

Background:

• Large rivers are critical sources of terrestrial dissolved organic matter (DOM) to marginal seas.
• Dissolved organic nitrogen (DON) is crucial for DOM cycling, but its dynamics in high dissolved inorganic nitrogen (DIN) rivers like the Yellow River remain poorly understood.
• The Yellow River's high DIN and low DON concentrations present measurement challenges and obscure DON cycling.

Purpose of the Study:

• To investigate the dissolved organic nitrogen (DON) cycling in the Yellow River, a major global river with high dissolved inorganic nitrogen (DIN).
• To elucidate the role of DON in the river's biogeochemical processes and its contribution to marginal seas.
• To develop a method for quantifying mineralized DON flux in large rivers.

Main Methods:

• Analysis of 17 water samples from the Yellow River's middle to downstream sections.
• Utilized spectroscopy, tangential flow filtration, nitrogen isotope analysis (δ¹⁵N), and DNA sequencing.
• Employed redundancy analysis to assess relationships between geographical features, microbial communities, and DON cycling.

Main Results:

• Dissolved organic nitrogen smaller than 1 kDa (DON_<1kDa) dominated the DON pool and inversely correlated with DIN, suggesting mineralization into nitrate.
• Nitrogen isotope data (δ¹⁵N_DON<1kDa) and ammonia-oxidizing bacteria/archaea abundance supported the mineralization hypothesis.
• Geographical features and microbial communities were identified as key drivers of DON cycling.

Conclusions:

• Active DON cycling, particularly DON mineralization, occurs in large rivers with high DIN concentrations.
• DON mineralization plays a significant role in the nitrogen and carbon cycling within these rivers and subsequently in marginal seas.
• A novel method for quantifying mineralized DON flux in large rivers was proposed.