Distribution and molecular chemodiversity of dissolved organic nitrogen in the vadose zone-groundwater system of a fluvial plain, northern China: Implications for understanding its loss pathway to groundwater

Nitrogen (N) pollution in groundwater has become a worldwide environmental geological issue due to the excessive N application into the vadose zone and furthered N leaching. Dissolved organic nitrogen (DON) are proposed as an overlooked pathway of N loss from agricultural systems to groundwater recently. Here, we collected soil (0–320 cm) and groundwater samples in a historic agricultural area to characterize the distribution and chemodiversity of DON in the vadose zone-groundwater system, and identified specific linkages between DON traits and the bacterial community. The results showed that DON and NO₃⁻-N were the main forms of dissolved N in the vadose zone-groundwater system. The deep vadose zone (> 100 cm) was an important storage area for DON (44.9%), having implications for long-term groundwater quality degradation. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that the DON was dominated by condensed aromatics and lignins (57.2%) in the vadose zone, whereas amino sugars, proteins, peptides and lignins (72.5%) were dominant in groundwater. By analyzing shared and ubiquitous DON molecular formulas detected among different layers, it was found that < 2.52% of DONs could be leached from surface soil to groundwater directly, and most DONs went through biological conversion during the whole leaching path. It was identified that bacterial community played an important role in DONs transformation. The most active bacteria in the transformation were Nitrospira, Bacillus, and Sphingomonas and they tended to interact with DON of high N/C and H/C ratios, causing molecules with high unsaturation, high aromaticity and high oxidation to accumulate. The results would be helpful to elucidate DON occurrence in groundwater and track the key processes governing DON transport from the surface soil to groundwater.