Hydrolysis of polyphosphates in soils

We studied hydrolysis in soils of seven linear oligomers ranging from P2 to P65 and of a cyclic polyphosphate (trimetaphosphate). Each of four Iowa surface soils was treated with 500 μg of polyphosphate-P/g soil and incubated under aerobic conditions at 10, 20, or 30°C for 7 d. We performed a similar experiment under aerobic or waterlogged conditions at 25°C for times ranging from 1 to 14 d. The amount of orthophosphate (Pi) released was determined. The amounts of Pi produced from the polyphosphates in air-dried soils ranged from 66 to 96% of those produced in field-moist soils. Increasing the temperature of incubation from 10 to 30°C increased the hydrolysis rates. In general, the rate of hydrolysis decreased with increasing polyphosphate chain length. This was particularly evident during the first few days of incubation. In two acid soils (Webster and Muscatine), the rates of hydrolysis of the polyphosphates under waterlogged conditions were higher (5 to 16%) than those obtained for aerobic conditions. Calculation of the Q10 values suggested that, with the exception of the oligomers (P15 to P65) in the three acid soils incubated at 10 to 20°C, the hydrolysis was dominated by enzyme reactions. Energy of activation (Ea) values for acid soils increased with increasing chain length, with a range from 10.7 to 71.2 kJ/mol. The relationship between the nonhydrolyzed polyphosphate and time of incubation showed that polyphosphate hydrolysis was controlled by two first-order reactions. The initial faster rate (k1) changed to a slower rate (k2) at incubation times ranging from 2 to 7 d, depending on the oligomer, soil type, and soil moisture status. The k1 and k2 values obtained for soils incubated under aerobic conditions ranged from 1.7 X 10 to 3.3 X 10 min and from 4.5 X 10 to 7.4 X 10 min, respectively. The corresponding values for soils incubated under waterlogged conditions ranged from 2.4 X 10 to 2.6 X 10 min and from 5.2 X 10 to 1.0 X 10 min.