for every one part of nitrate. Therefore, a solution of 0.015 M VCl3 is capable of reducing 0.005 M NO3 -, which corresponds to a sample concentration of 140 mg L -1 NO3 -. This concentration can be injected into the system and completely reduced if we consider that the mobile phase and reducing agents are mixed in a 1:1 ratio before they enter the reduction oven. The reduction of nitrate with vanadium(III) requires strongly acidic conditions at or below pH 1 , which was provided by the 0. (Elementar Analysensysteme GmbH, Langenselbold, Germany) for referencing purposes. The standard solutions typically contained 50 mg L -1 N-NO3 -(nitrate-N). One standard dilution series was prepared with concentrations of 10, 25, 50, 75, and 100 mg L -1 N-NO3 -to assess the concentration dependence of the system. For the determination of standard bulk isotope signatures of the by EA-IRMS see Supporting Information 3.