| Abstract: | Abstract In this study, the rate of pyrophosphate hydrolysis was measured in four organic soil materials similar in pyrophosphate retention values and in all other respects except pyrophosphatase activity. These soil materials had a bulk density of 0.20 kg/L and a pH (0.01 M CaCl2) of 5.1, and contained 16% ash and 103 mg P/kg as Mehlich No. 1 extractable P. The difference in pyrophosphatase activity between the soil materials was attributed to response to Cu of previous crops at low values of 1 N HCl‐extractable Cu (10 vs 76 mg Cu/kg) and to pyrophosphatase inhibition by Cu at high Cu values (201 and 479 mg Cu/kg). Pyrophosphatase activity increased from 35.6 to 38.3 mmol/kg/(2h) with Cu contents increasing from 10 to 76 mg/kg, and decreased to 31.9 and 28.3 mmol/kg/ (2h) with Cu contents increasing respectively to 201 and to 479 mg/kg. However, no significant difference in course of pyrophos‐phate hydrolysis was observed among the soil materials under both laboratory and field conditions. The half‐life of pyrophosphate was nearly 1 day under laboratory conditions. The decrease in substrate concentration over time and the corresponding increase in products competing with substrate on active sites of enzymes tended presumably to mask the difference shown in pyrophosphatase activity among organic soil materials. Thus, a difference in pyrophosphatase activities larger than the one considered in this study would be required to influence the rate of pyrophosphate hydrolysis in organic soils of similar pyrophosphate retention characteristics. Such a difference in activity levels was not obtained despite a large difference in Cu contents among the soil materials. It was thus apparent that Cu contents, exceeding that generally found in Quebec organic soils, would not reduce the rate of orthophosphate production from polyphosphate fertilizers containing large amounts of pyrophosphate. Pyrophosphate hydrolysis would be rapid and not affected by a small reduction in pyrophosphatase activity. |
