Characterization of ester sulphate in a gypsum-amended podzol using an immobilized sulphatase reactor

An immobilized sulphatase reactor column was successfully used to determine the biochemical stability of ester sulphate in soil organic matter extracted from a podzol amended with gypsum. The sulphatase from Helix pomatia was covalently attached to controlled-porosity glass beads, and the immobilized enzyme was packed into a small glass column. The optimum pH, the time required to reach equilibrium, and the percentage of substrate consumed for the enzymatic hydrolysis of soil ester sulphate (pH 7.7, 90 min, 23–59%) were substantially different from those of p-nitrophenyl sulphate at similar concentrations of substrate (pH 7.0, 40 min, 99%). The striking difference in the biochemical stability and kinetic behaviour between soil ester sulphate and the simple synthetic substrate reflected their different chemical nature and structural features. The amounts of enzymatically hydrolysable (labile) ester sulphate in soil organic matter extracted from the podzol amended with gypsum at rates of 0, 50, and 200 kg S ha^-1 were significantly different (P=0.0004), being 0.5, 1.1, and 1.4 g S ml^-1 soil extract (or 5, 11, and 14 g S g^-1 soil), respectively. The labile ester sulphate was not correlated with the total hydriodic acid-reducible organic sulphate with the soil organic matter extracts but with the hydriodic acid-reducible organic sulphate: organic C ratio, which increased as a result of gypsum amendment. This study revealed that input of inorganic sulphate as gypsum substantially increased the accumulation of labile ester sulphate in a podzol.