Bioavailable fractions drive variations in riverine dissolved organic nitrogen along land-use gradients

Abstract

Land-use gradients altered dissolved organic nitrogen (DON) sources. However, the response of riverine DON remains unclear due to interactions between bioavailable fractions and plankton communities. This study examined DON dynamics, bioavailable components, optical properties, and plankton communities (phytoplankton and bacteria) across two hydrological seasons along a river continuum with forest-affected upstream, agricultural midstream, and urban downstream reaches. DON increased downstream from 0.14 to 0.22 mg/L during the dry season but decreased from 0.17 to 0.12 mg/L during the wet season. The proportion of bioavailable DON (urea and dissolved total amino acids) exhibited negative correlations with DON (p < 0.0001), suggesting that bioavailable fractions drive DON variation along land-use gradients. Spectral characterization showed that downstream sites contained more autochthonous DON, associated with highly bioavailable phytoplankton inputs in the wet season and less bioavailable sediment release in the dry season (p < 0.05). Bacterial community composition exhibited pronounced spatial shifts along the land-use gradient, with higher DON bioavailability characterized by an increased dominance of Pseudomonadota. Ammonification driven by Pseudomonadota primarily mediated bioavailable DON consumption (p < 0.001), and nitrate production was the dominant pathway of DON mineralization. This study revealed that land-use gradients drove DON dynamics through changes in bioavailable fractions, providing critical insights for sustainable water management.

Graphical abstract

Keywords

Land-use gradients / Bioavailable DON / Plankton community / Pseudomonadota / Microbial ammonification / Nitrate production

Highlight

	● Land-use gradients significantly altered riverine DON dynamics across seasons.
	● Bioavailable DON ratios were negatively correlated with DON concentrations.
	● Agricultural and urban reaches were enriched in autochthonous DON.
	● Higher DON bioavailability promoted the dominance of Pseudomonadota.
	● Pseudomonadota-dominated deamination mediated bioavailable DON consumption.

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