Soil nitrate reductase (S-NR) catalyzes the reduction of nitrate to nitrite and is a key enzyme in soil nitrate nitrogen reduction. Measurement of S-NR activity is important for rational fertilization strategies and reducing nitrogen loss. S-NR catalyzes the reaction: NO3- + NADH + H+ → NO2- + NAD+ + H2O. The produced nitrite reacts with p-aminobenzenesulfonic acid and α-naphthylamine under acidic conditions to form a red azo compound with maximum absorbance at 540 nm, which is quantified spectrophotometrically.