Evaluation of Soil Labile Phosphorus Using a Double—Labeling （^32P and ^33P） Technique

Isotopic exchangeability of phosphorus in four Chinese soils with and without P application was studied by ^32P and ^33P double-labeling technique in relation to routine chemical extractions.The results showed that Bray-I and Bray-Ⅱ reagents could extract most of the fast exchangeable P.Not all of the Olsen-P belonged to fast exchangeable P,but it was about the same quantity of fast exchangeable P in a calcareous soil and a neutral soil without P application.Sequential fractionation of the soil phosphorus showed that most of the added radioisotopes in high P fixation red soils were tightly held by iron and aluminium oxides,which could be totally extracted only by 0.1M NaOH solution.In the neutral and calcareous soils most of the radioisotopes added were loosely held on the surface of soil particles and could be extracted by anion exchange resin.Phosphate application increased the resin-P fraction significantly for all the soils studied.