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Hydrolysis of Trimetaphosphate in Soils¹

ABSTRACT

Trimetaphosphate (TMP), a cyclic polyphosphate that is not sorbed by soils, is hydrolyzed by a series of biochemical reactions to yield triphosphate, pyrophosphate, and orthophosphate; phosphates that are sorbed by soils. In addition to its hydrolysis to triphosphate in soils by the enzyme trimetaphosphatase (trimetaphosphate hydrolyase, EC 3.6.1.2), TMP is hydrolyzed chemically by metal ions such as Ca²⁺ and Mg²⁺. A method was developed for assaying the activity of trimetaphosphatase and for determining the rate of nonenzymic (chemical) hydrolysis of TMP in soils. The method involves incubating 1 g of soil with 3 mL buffered (100 mM Tris) 25 mM TMP at 37°C for 5 h followed by determination of the unaltered substrate. Steam-sterilized soil is used as a control to account for the amount of TMP hydrolyzed nonenzymically. Trimetaphosphatase activity in soils was maximum at a buffer pH of 8 and a temperature of 40°C. Nonenzymic hydrolysis of TMP increased up to the highest incubation temperature (80°C) tested. Trimetaphosphatase in three Iowa surface soils showed apparent K_m values ranging from 6.8 to 7.2 mM TMP, V_max values from 590 to 1200 mg TMP-P hydrolyzed kg^–1 soil 5 h^–1, and energy of activation values from 17 to 26 kJ mol^–1. The activation energy values of the nonenzymic hydrolysis ranged from 29 to 39 kJ mol^–1. Enzymic and nonenzymic hydrolysis rates in the six soils used ranged from 231 to 928 and from 101 to 888 mg TMP-P kg^–1 soil 5 h^–1, respectively. Formaldehyde and EDTA inhibited, and toluene had no effect on, trimetaphosphatase activity in soils. The addition of Ca²⁺ and Mg²⁺ increased nonenzymic hydrolysis rates in two soils tested with Ca²⁺ being twice as effective as Mg²⁺.

¹ Journal Paper no. J-11495 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, Iowa. Projects 2082, 2112, and 2431.

² Graduate Research Assistant and Professor of Soil Chemistry, respectively. Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication June 22, 1984. Accepted for publication November 26, 1984.

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