Adsorption-desorption characteristics of phosphorus in purple soils applied with ammonium polyphosphate
[Objective]The study aimed to investigate the adsorption-desorption characteristics of phosphorus(P)in soils applied with ammo-nium polyphosphates(APPs)of different polymerization distribution,and to compare these characteristics with those of soils applied with tra-ditional orthophosphates(Monoammonium phosphate,MAP;Diammonium phosphate,DAP).It can provide a theoretical basis for the efficient use of APP.[Method]Three kinds of APP(18-58-APP,16-60-APP and 11-37-APP,whose N-P2O5-K2O were 18-58-0,16-60-0 and 11-37-0,and the distribution of polymerization degree were 1-2,1-4 and 1-7,respectively),MAP and DAP were used as test fertilizers.The fertiliz-ers were applied to calcareous and acid purple soils and then were incubated for 56 days.The adsorption-desorption characteristics of P and the physicochemical properties of the soils were determined.[Result]Both Langmuir and Freundlich equations fit the adsorption characteris-tics of P in the two purple soils well(R2=0.825-0.997).In the calcareous purple soil,compared with MAP-and DAP-treated soils,the APP-treated soil showed a 15.1％-20.7％decrease in P adsorption rate and a 4.5％-164.9％increase in desorption rate.The Langmuir KL,adsorption free energy(|ΔG|),and Freundlich KF values of the APP-treated soil decreased by 20.4％-54.0％,3.3％-10.6％,and 23.8％-43.8％,respectively,indicating a reduction in the binding force of soil for P.The 11-37-APP treated soil showed the largest chan-ges in these values(KL,|ΔG|and KF values were reduced by 40.8％-54.0％,7.2％-10.6％and 43.6％-43.8％,respectively).In acid purple soil,the impact of different P fertilizers on the adsorption-desoprtion characteristics of P was not significant,but APP-treated soil tended to reduce P adsorption and promote P desorption.Correlation analysis showed that pH,iron and aluminum oxides,and Olsen-P were significantly correlated with P adsorption parameters in calcareous purple soil and soil Olsen-P was significantly correlated with P adsorption parameters in acid purple soil.APP application in calcareous purple soil decreased complexed iron and aluminum oxides while increased free iron and aluminum oxides,resulting in decreased P adsorption and increased desorption.Compared to calcareous soil,acid purple soil had a lower available P content and a stronger adsorption capacity,making the adsorption-desorption characteristics less susceptible to the influence of different P fertilizers in the short term.[Conclusion]The adsorption-desorption characteristics of P in APP-treated purple soils are influ-enced by the soil properties and the APP's polymerization distribution.The significant changes are observed in calcareous purple soil,espe-cially for 11-37-APP with a broader polymerization distribution.
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