Kinetics Of Accumulated Phosphorous Releasing And Phosphorous Fractions Change In Black Soil

Due to the influence of long-term fertilization,the accumulation of phosphorus in farmland soil increased rapidly and greatly,which was conducive to the efficiency of crop production and decreases the utilization rate of phosphorus fertilizer.Thus,it is of great significance to explore the dynamic characteristics and mechanism of soil phosphorus release under different long-term fertilization for active reuse of soil phosphorus and improvement of utilization rate of phosphorus fertilizer.Taking the black soil of Harbin as an example,five soil samples with Olsen-P content of 15～217 mg/kg were selected under long-term fertilization in this experimental site.Phosphorus was extracted by 0.5M NaHCO3(pH=8.5)chemical reagent.Continuous extraction(extraction time:2h,12h,24h)and intermittent extraction(extraction time:24h,48h,7d)two experiments were set to observe the release process of Olsen-P in soil at different leaching times.The Hedley method modified by Tissen was used to determine the soil phosphorus morphology after leaching advance.Then,kinetic equations fitting was used to describe its kinetic characteristics.The main research results were as follows:(1)The content of active P and medium active P in high Olsen-P level soil samples were significantly higher than those in low Olsen-P level soil samples,and there was a extremely significant linear positive correlation with Olsen-P level(P<0.01).For each 1 mg/kg Olsen-P increase,the content of active-P increased by 1.85 mg/kg as the content of moderate active-P increased by 1.15 mg/kg.But the content of stable phosphorus had no significant difference,with an average of 196.8 mg/kg.(2)The phosphorus release kinetics curves of both continuous extraction and intermittent extraction showed a trend of rapid release at first,slow release,and finally equilibrium.Michaelis equation,first-order equation,power function equation,Elovich equation and parabola equation can describe the phosphorus release kinetics of continuous and intermittent extraction.But the optimal fitting equation was not unique,so it was not single of the main control mechanism of phosphorus release process,which involved the reverse reactions of phosphate diffusion,competition,dissolution,and chemical fixation of phosphorus,and these reactions were interspersed and simultaneous.(3)There was a significant positive correlation between Olsen-P level and kinetic parameter Qmax,a、α、c(P<0.01).The equation established by those parameters could be used to predict the kinetic process of phosphorus release at any Olsen-P level.The residual phosphorus extraction(Qr)was basically stable.The content of Olsen-P was increased by 1 mg/kg,Qmax was increased respectively by 3.14 mg/kg,3.03 mg/kg,3.48 mg/kg under the conditions of continuous extraction for 2h,12h and 24h,and Qmax was increased respectively by 3.69 mg/kg,3.63 mg/kg and 3.02 mg/kg under intermittent extraction for 24h,48h and 7d.(4)Compared to the contents before the ectraction,the contents of active-P and moderate active-P in soil residue after continuous and intermittent ectraction were reduced significantly and showed a significant decreasing trend with the increase of ectraction time.The contents of active P as Resin-P,NaHCO3-Po and moderate active-P as NaOH-Pi decreased first and then increased with the increase of extraction time.There was no significant difference in the content of stable phosphorus in soil residue after continuous ectraction compared with that in soil sample before ectraction.The content of stable phosphorus in soil residue after intermittent ectraction decreased by 3%～9%compared with that before ectraction,and Conc.HCl-Pi was the most significant decrease.The main influencing factors of phosphorus release were organic matter,pH and total phosphorus.In conclusion,the release of cumulative P in black soil conforms to the Michaelis equation and the first order equation.The release of available P increases with the increase of Olsen-P,and the released P mainly comes from soil active-P and moderate active-P.The main factors affecting the P released were soil organic matter,pH and total P content.The amount of residual phosphorus in black soil was basically stable,and most of the accumulated phosphorus could be released.This provides a theoretical basis for the activation and reusing of accumulated phosphorus in black soil.