Hydrolysis of cyclotri- and cyclotetraphosphate in soil
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Cyclotriphosphate (Na3P3O9, C3P) and cyclotetraphosphate (Na4P4O12, C4P) are not appreciably sorbed by soil constituents. This property combined with a reduced rate of hydrolysis of C4P and its hydrolysis products may maintain P availability for plant uptake. The primary objective of this study was to determine the residence times of C3P and C4P and their hydrolysis products in four soils. Hydrolysis of C3P and C4P was dependent on soil moisture, temperature, biological activity, and time. Cyclic P hydrolysis followed apparent first-order reaction. Recovery of hydrolysis products of C3P and C4P was higher in soils, such as Falba sandy loam (sl), which demonstrated limited sorption for mono-, di-, and triphosphate. The presence of hydrolysis products may explain the decreasing rate constants observed in soils incubated for longer than 48 h. Cyclotetraphosphate was more stable than C3P in all comparable treatments, often by three to six times. The energy of activation (EA) for C3P and C4P was 45.8 and 61.1 kJ mol-1, respectively, in a Falba sl, and 38.6 and 51.2 kJ mol-1, respectively, in a Branyon clay (c). Cyclotriphosphate was more readily hydrolyzed in soils compared with C4P. These results indicate a potential advantage of C4P over C3P because of its slower hydrolysis in soils, and over all linear phosphates, which are strongly sorbed by soil constituents.
