Treatment of horizontal silage bunker runoff using biochar amended vegetative filter strips

Horizontal silage bunkers produce leachate that contains contaminants that can be detrimental to the environment if released untreated. Vegetated filter strips are used to treat silage bunker runoff to prevent contamination of surface waters via infiltration, however increased infiltration poses risks to groundwater, particularly for nitrate (NO₃^-). Vegetated filter strip plots with a sandy loam soil, half of which are amended with biochar, were investigated to assess the treatment of silage bunker runoff over 20 application events. The subsurface effluent biological oxygen demand (BOD₅), chemical oxygen demand (COD), and total phosphorus (TP) were reduced on average by 40%, 46%, and 75%, respectively, and there was no statistical difference between treatments. The total nitrogen (TN) was reduced by 49 and 64% for control and biochar plots, respectively, which was significantly different between treatments. Biochar significantly reduced nitrate nitrogen (NO₃^--N) leaching by 40% compared to the control, however, the NO₃^--N concentration in leachate was still high ranging from 0.19 to 191.04 mg NO₃^--N L^-1 and 0.18-108.89 mg NO₃^--N L^-1 for control and biochar plots, respectively. A mass balance suggests the primary mechanism for a decrease in TN and NO₃^--N leaching from biochar amended plots was greater retention of NO₃^--N and organic N (ORG-N) within the soil/biochar matrix. The development of oxygenated functional groups and/or formation of organomineral layer on the biochar surface likely enhanced N retention.