Ics 2100 ion chromatograph

Na⁺ concentrations were determined in porewater aliquots acidified with 2% suprapure ${\mathrm{HNO}}_3^{-}$ . The analyses were carried out at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) using an Arcos ICP-OES (internal standard deviation < 1%) after 1000-fold dilution. Cl⁻ concentrations were measured with an ICS-2100 ion chromatograph (DIONEX) after 100-fold dilution. Alkalinity was determined by titration at the Analysis and Education Lab at Eawag.

Incubation and baseline chemistry samples were collected, frozen, and sent for analysis at the Environmental Analytical Laboratory in the Department of Land Resources and Environmental Sciences at Montana State University (MSU). Inorganic nutrients (NH₄⁺, NO₃⁻, NO₂⁻, and PO₄³⁻) in unamended (background) stream waters were determined at μg L⁻¹ levels on a QuAAtro39 continuous segmented flow analyzer (Seal Analytical, Inc.). We calculated dissolved inorganic nitrogen (DIN) as the sum of NH₄⁺, NO₃⁻, and NO₂⁻. Acetate and other dissolved solutes in the treated incubation samples (NO₃⁻, NO₂⁻, and Cl⁻) were measured at mg L⁻¹ levels on an ICS 2100 Ion Chromatograph (Dionex, Thermo Scientific) equipped with an anion column (ASX‐18 column). DOC and total nitrogen (TN) in all samples were determined using a V‐TOC CSH Total Carbon Auto‐Analyzer with a TNM‐1 Total Nitrogen Module (Shimadzu Corporation). Analytical uncertainties were determined for each instrument based on replication of samples and results for several representative working standards (supporting information). Samples were kept frozen at MSU until analysis, and values were only accepted if uncertainty was less than 10%. When concentrations were below the limit of detection, we set their values to half the detection limit.