Dual roles of dissolved organic nitrogen in groundwater nitrogen cycling: Nitrate precursor and denitrification promoter

Dissolved organic nitrogen (DON) has been reported to be prevalent in groundwater worldwide. Owing to the diversity of physicochemical properties, DON plays complex roles in nitrogen cycling processes, which has further implications for nitrate (NO3--N) pollution control in groundwater. To characterize these crucial roles, we investigated the effects of three types of DON (amino acid, urea, and protein) on NO3--N accumulation in groundwater with a 60-day incubation experiment and established quantitative correlations between microbial indicators (bacterial communities and nitrogen functional genes) and nitrogen content. The results showed that NO3--N content increased by 30.3% and 38.8% and was strongly correlated with the presence of amino acid and urea; however, the addition of protein did not lead to an additional increase in NO3--N, possibly due to different extents of mineralization and denitrification caused by different types of DON. Molecular biological experiments demonstrated that Nitrospira (1.8-3.2%) contributed to nitrification in the urea treatment, whereas Arthrobacter (2.0-6.9%) and Thermomonas (11.9-13.1%) were key communities controlling denitrification in amino acid and protein treatments. amoA and nxrA were continuously enriched in the presence of urea; however, amino acid and protein were strongly correlated with napA-dominated and narG-dominated denitrification processes, with the path coefficient - 2.912 and - 2.450 respectively. Combined analyses showed that DON with distinct physicochemical properties played dual roles (NO3--N) precursor and denitrification promoter) to varying degrees, which could have significant impacts on NO3--N accumulation in groundwater. This study may provide guidance for environmental risk evaluation and control strategies for NO3--N pollution in groundwater. (C) 2021 Elsevier B.V. All rights reserved.