Vanadium iii chloride

Hydrochloric acid (37%), sulfanilamide (≥99.0%), N-(1-naphthyl) ethylenediamine dihydrochloride (NEDD, >98%),
glucose (≥99.0%), and sodium nitrate (≥99.0%) were purchased
from Sigma-Aldrich, UK. Vanadium(III) chloride (≥99.0%) was
obtained from Alfa Aesar, UK. Ultrapure water was obtained from a
Milli-Q Direct Water Purification System, with a resistance of 18.2
MΩ (Millipore, Merck). Fluorinert FC40 oil was obtained from
3M, UK. The assay used here is a modified Griess reagent method, whereby
nitrate is first reduced by vanadium(III) to nitrite, which then reacts
with a mixture of sulfanilamide and N-naphthyl-ethylenediamine
(NEDD) to produce a purple/pink-colored diazonium product,^{23 (link)} giving a summation of nitrate and nitrite. For
the forest soil sample, the assay mainly quantifies nitrate since
nitrite does not usually accumulate in soil.^{24 (link)} Preparation of standard solutions and Griess reagent is described
in the Supporting Information (Text S1 and S2). All soil used in the lab-based
tests was collected from Writtle forest, Chelmsford, UK (51°41′37.9″N,
0°22′20.4″E). The soil detailed information, preparation
of nitrate-free soil, standard dried soil, spiked soil columns, and
soil characterization (i.e., maximum water holding capacity (WHC%)
and conventional analysis) are also stated in Text S3–S5.

Unless otherwise stated, water was ultrapure grade (18.2 MΩ, Barnstead EASYpure RODI) and all chemicals were obtained from Sigma Aldrich, UK. A modified Griess assay for nitrite and nitrate measurement was formulated as previously described.^{10,25 (link)} Briefly, 1.25 g of vanadium(iii) chloride (99.0%, Alfa Aesar, UK) was added to a 250 ml volumetric flask along with 50 ml of ultrapure water to form a dark brown solution. 15 ml of concentrated (37%) hydrochloric acid was added, causing the solution to turn dark turquoise, then 1.25 g of sulfanilamide (≥99.0%) and 0.125 g of N-(1-naphthyl)ethylenediamine dihydrochloride (>98%) were added, dissolved, and the solution finally made up to the volumetric mark using ultrapure water. A 100 mM stock solution of sodium nitrite was made by dissolving 1.725 g of sodium nitrite (≥99.0%) in ultrapure water in a 250 ml volumetric flask and then diluted as required to make up lower concentrations. Fluorinert FC40 oil was obtained from Acota Ltd, UK.