2-Nitroacetanilide as substrate for determination of aryl acylamidase activity in soils

A direct colorimetric assay for measuring aryl acylamidase activity in soils is described using 2-nitroacetanilide (2-NAA) as substrate. The assay is based upon an increase in absorption (410 nm) due to formation of the chromogenic hydrolysis product 2-nitroaniline, after incubation of soil with 2-NAA (2.0 mm) in phosphate buffer (pH 8.0). Due to low specific activities (5–53 nmol g−1 soil h−1) incubations of 20–24 h are required. 2-NAA hydrolytic activity in soil was dependent on substrate concentration, with apparent Km values of 180 to 920 μm for the soils examined. 2-NAA aryl acylamidase activity was linear for a 35 h incubation. The activity was reduced by 71 to 85% in the presence of carbaryl (100 μm), an aryl acylamidase inhibitor. Aryl acylamidase activity was increased from 1.31 to 2.78-fold in no-till (NT) soils compared to conventional-till (CT) soils, corresponding with increased organic matter accumulation and microbial activity in the four (NT) soils studied.