Microcosm evaluation of saturation and labile carbon controls on nitrate persistence in vadose zone soils

Groundwater table recovery increases water saturation in the lower vadose zone, altering nitrate persistence. To clarify how soil properties and substrate availability modulate these dynamics, two microcosm experiments compared static water-filled pore space levels (50%, 80%, and 100%) and various nutrient additions in a Beijing soil profile. Between days 1 and 45, nitrate remained stable at 50% WFPS but declined by 39.15%~53.52% at 100% WFPS, with the largest reduction in the deepest soil due to higher initial organic carbon. Glucose addition enhanced the 45-day decline to 85.30% by inducing reducing conditions (-89 mV) within the first 15 days. In contrast, ammonium inputs persisted as the dominant dissolved form, while glycine triggered a sequential cascade marked by organic nitrogen decrease and transient ammonium, nitrite, and nitrate peaks. Day 45 sequencing confirmed that glucose and glycine shifted bacterial structures and gene profiles. Although gaseous measurements or isotope tracing are required to resolve specific pathways, these findings demonstrate that labile carbon governs the extent of nitrate decline under high saturation, while substrate composition shapes inorganic nitrogen accumulation timing.