Microbial-mediated soil dissolved organic nitrogen transformation pathways: Implications for groundwater nitrogen contamination risk mitigation

The exploration of the molecular characteristics and transformation processes of soil dissolved organic nitrogen (DON) compounds is paramount, as they affect the leaching of DON components into groundwater. Nevertheless, the molecular transformation pathways remain largely uncharted territories. Hence, a comprehensive study of microbial-medicated DON transformation pathways across various land use soils was conducted. The determination of DON components and microbial communities was accomplished utilizing advanced techniques, namely fourier transform ion cyclotron resonance mass spectrometry and 16S rRNA sequencing. These methodologies were synergistically integrated with the FAPROTAX database and network analysis. The results showed that DON emerged as the predominant nitrogen fraction, displaying significant variations in composition across these soils. The dominant constitutes comprised lignin-like and condensed aromatic compounds, varying in their aromaticity and condensation degrees. Key microbial genera, including Solirubrobacter, 67–14, and Bacillus, were identified as crucial for DON decomposition and mineralization. Functional annotation revealed abundant bacteria associated with the breakdown of aromatic compounds, the degradation of aliphatic non-methane hydrocarbons, and the hydrolysis of cellulose. Network analysis emphasized the interconnected and cooperative aspects of DON transformation processes, with notable relationships between N respiration and various degradation activities. Correlation analysis between microbial functions and DON components emphasized distinct mechanisms involved in the transformation of DON components. The orchestrated process of converting high-molecular-weight DON into low-molecular-weight counterparts facilitates potential leaching into groundwater, emphasizing the need for understanding and managing microbial-mediated DON transformation pathways to mitigate groundwater contamination risk.