## 2 Materials and Methods  
Nitrate extraction efficiency was measured using biochar produced by Carbon Terra from wood chips (80% confierous, 20% deciduous) at a pyrolysis temperature of 700 \({}^{\circ}\)C for 36 h. While biochar can be produced from a variety of biomasses at varying temperatures, Carbon Terra biochar was chosen for analysis as it has been used in a previous study where nitrate extraction methods were identified as having low nitrate recovery after composting [11]. Prior to oxidation, nitrate sorption, and extraction, biochar composition of C, H, and N was determined to be 86.0%, 4.6%, and 0.2%, respectively, using a PerkinElmer Elemental Analyzer (PerkinElmer, Waltham, MA, USA), following method ASTM D537. Oxygen content was determined to be 7.6%, based on the remaining composition after subtracting C, H, N, ash (via ASTM D1102 using a muffle furnace at 600 \({}^{\circ}\)C for 2 h). The pH was measured using a HACH HQ440d Benchtop Multi Meter pH probe (Hach Company, Loveland, CO, USA), after diluting the biochar 1:20 by mass with DI water, and was determined to be 9.4 for the initial biochar. In this study, biochar with a particle size of 2.00 to 4.75 mm was used. As biochar nitrate sorption is low without modification or aging, biochar was oxidized using a sodium hypochlorite (NaClO) oxidation method [10]. All oxidized biochar was dried at 105 \({}^{\circ}\)C for at least 24 h before beginning the nitrate sorption process. A 50 mg NO\({}_{3}\)-N L\({}^{-1}\) stock solution made from sodium nitrate (NaNO\({}_{3}\)) was mixed with 2.5 g oxidized biochar at a ratio of 1:10 (w/v). Control samples containing only stock solution were included. Samples were covered and shaken at 100 rpm on a G10 Gyrotory 141 Shaker (New Brunswick Scientific Co., Edison, NJ, USA) for 24 h at room temperature. Samples were then vacuum filtered through 1.5 \(\upmu\)m filters, and the filtrate was acidified with H\({}_{2}\)SO\({}_{4}\) and stored at 4 \({}^{\circ}\)C until analysis. Biochar was transferred to clean bottles and stored at 4 \({}^{\circ}\)C until extraction. All nitrate solutions were analyzed using a SEAL AQ2 automated discrete analyzer (SEAL Analytical Inc., Werkstrasse, Germany) following the EPA-126-A Rev 9 [17]. Nitrate absorbed was determined by the nitrate concentration difference in the initial and final solutions. Following nitrate sorption, the biochar was extracted using methods that varied in extraction duration and temperature, Table 1. All methods reported in this paper used a KCl extraction solution. Different extraction solutions were initially evaluated (0.5 M K\({}_{2}\)SO\({}_{4}\) and 0.1 M HCl) but were excluded from this paper due to recovery efficiencies below 20%. There are several nitrate extractants that could be used (i.e., CaSO\({}_{4}\), NH\({}_{4}\)F, H\({}_{2}\)SO\({}_{4}\), CaCl\({}_{2}\), NaHCO\({}_{3}\), CuSO\({}_{4}\), Ag\({}_{2}\)SO\({}_{4}\)), nonetheless, 2 M KCl was chosen because it is the most commonly used and accepted method for determining inorganic nitrogen [14; 15]. In this study, biochar was mixed with 25 mL of a 2 M KCl solution to an approximate ratio of 1:10 w/v and shaken at room temperature for 24 h (RT24), longer than the one-hour in the above methods, as suggested by [PERSON] et al. [11]. After an initial vacuum filtration, three rinses were performed by pouring 25 mL of deionized (DI) water over biochar and vacuum filtering. Filtrate from each rinse was acidified with H\({}_{2}\)SO\({}_{4}\) and stored at 4 \({}^{\circ}\)C until analysis. Biochar was dried in an oven at 105 \({}^{\circ}\)C for 24 h and then weighed to account for any biochar losses during the extraction process. Extraction duration and temperature were evaluated. Past studies have found that standard extraction durations are not sufficient to remove nitrate form biochar, thus, using a 24 h extraction has been suggested [11].