Apportionment of Derivative Uranium from Phosphate Mineral Fertilizer in Agricultural Soil Components after Extended-Period uses

Abstract

Long-term phosphorus (P) fertilizer application can lead to an accumulation of uranium (U) in the surface soil of agricultural fields. This study investigated the soil constituents that participate to the accumulation of U by using chemical extraction methods. Surface soil samples were obtained from paddy, wheat, barley and corn fields cultivated without any phosphate fertilizer (control site), with both NPK fertilizer and TSP for more than 18 years. In extension to the uranium total content in soil, the content of pyrophosphate and acid oxalate-extractable U were evaluation as a measurement of U related with soil total carbon and crystalline Fe/Al minerals in soil, respectively. The total, pyrophosphate-extractable, and acid oxalate-extractable U concentrations were higher in the soil obtained from fertilized sites than in that obtained from the control site. The difference in the U concentrations between phosphate fertilizerd sites and the control sites corresponded with the increased U concentration observed after the application of phosphate fertilizer. In the paddy, wheat and barley field soil with different long application of phosphate fertilizers 57, 42 and 18 year, the increase in pyrophosphate-extractable U was 76.1, 72.9 and 85% of that in Ut respectively, but it was equivalent to the increase than in Ut at the corn fields soil with 34 years period added phosphate fertilizers. Moreover, the range of increase in acid oxalate-extractable U was 21.1–56.3% of that in Ut in the fields soil, In conclusion, most of the phosphate fertilizer-derived U was either incorporated into the soil organic matter or poorly crystalline Fe/Al minerals in the surface soil of agricultural fields. Therefore, soil total organic carbon was an aggregation of U in barley field and corn soil, whereas poorly crystalline Fe/Al minerals were important pools of U in wheat field and paddy soil experiencing alternating changes in redox conditions.