Colorimetric determination of reducing sugars in soils

Reducing sugars are the end products of many biological processes and enzymatic reactions in soils. They are determined in assay of several soil enzymes, including cellulase activity. Five colorimetric methods [phenol-sulfuric acid, anthrone-sulfuric acid, dinitrosalicylic acid (DNS), reaction with potassium ferric hexa-cyanide reagent (Prussian blue), and the Somogyi-Nelson (molybdenum blue) methods] were evaluated for determination of reducing sugars and total saccharides extracted from soils. Results showed that the Prussian blue and the molybdenum blue methods were the most sensitive and accurate for determination of reducing sugars in soils. Metals extracted from soils interfered with molybdenum blue color development. These metals, however, could be removed by K-saturated resin before analysis. The trace amount of metals extracted from soils did not interfere with the Prussian blue color development, but this method is too sensitive to be useful for determination of reducing sugars in soil extracts. Unlike the Prussian blue method, which is very sensitive and has 1 h color stability, the molybdenum blue method has color stability of at least 24 h. Reducing sugar values in soils increased significantly upon air-drying of field-moist soils or incubation of soils with acetate buffer (50 mM, pH 5.5) at 30°C for 24 h, suggesting enzymatic hydrolysis of the native substrates. Calibration graphs showed that the phenol-sulfuric acid, DNS, and anthrone-sulfuric acid methods are not as sensitive as the other two methods.