Kinetic and molecular evidence for DON transformation in the deep vadose zone: Important implications for soil nitrogen budgeting and groundwater nitrate management

Abstract

The neglect of dissolved organic nitrogen (DON), especially with regard to its transformation in the deep vadose zone, can result in high uncertainty surrounding estimated nitrogen (N) leaching losses to groundwater. This study provided kinetic and molecular evidence for DON transformation in the deep vadose zone by focusing on a typical agricultural area in Eastern China. DON was as important as inorganic N for N stocks, and 28.04% ∼ 71.94% of DON was sealed in the deep vadose zones (i.e., > 1 m depth). Soil N mineralization rates gradually decreased as incubation proceeded, and aerobic condition was more favorable for soil N mineralization. Both shallow and deep vadose zones could be potential hotspots for N mineralization, and the trend of N mineralization rate varying with soil depth was site-specific. Fourier-transform ion cyclotron resonance mass spectrometry data revealed that DON was dominated by lipids before incubation, whereas lignins were dominant after incubation was complete. Lignin-like compounds were not intrinsically recalcitrant, instead persisting in soil due to concurrent removal, transformation, and formation. DON transformation occurred along the entire soil profile regardless of redox conditions, potentially having different microbe-mediated pathways. These findings emphasize the implications of considering DON transformation in the deep vadose zone for soil N budgeting and groundwater nitrate management.