Enhanced Vadose Zone Nitrogen Removal by Poplar During Dormancy

A pilot-scale, engineered poplar tree vadose zone system was utilized to determine effluent nitrate (NO₃^?) and ammonium concentrations resulting from intermittent dosing of a synthetic wastewater onto sandy soils at 4.5°C. The synthetic wastewater replicated that of an industrial food processor that irrigates onto sandy soils even during dormancy which can leave groundwater vulnerable to NO₃^? contamination. Data from a 21-day experiment was used to assess various Hydrus model parameterizations that simulated the impact of dormant roots. Bromide tracer data indicated that roots impacted the hydraulic properties of the packed sand by increasing effective dispersion, water content and residence time. The simulated effluent NO₃^? concentration on day 21 was 1.2 mg-N L^?1 in the rooted treatments compared to a measured value of 1.0 ± 0.72 mg-N L^?1. For the non-rooted treatment, the simulated NO₃^? concentration was 4.7 mg-N L^?1 compared to 5.1 ± 3.5 mg-N L^?1 measured on day 21. The model predicted a substantial “root benefit” toward protecting groundwater through increased denitrification in rooted treatments during a 21-day simulation with 8% of dosed nitrogen converted to N₂ compared to 3.3% converted in the non-rooted test cells. Simulations at the 90-day timescale provided similar results, indicating increased denitrification in rooted treatments.