Abstract: To compare with the degree of fluvo-aquic soil (sampled from Daxing and changping located in Beijing suburbs)non-exchangeable magnesium release,the exchangeable Mg was extracted first with 1 mol/L NH4Ac,and then the amount of Mg released from the extracted soil was measured by the same extracted solution at 4,8,12,weeks,respectively.It was found that the amount of Mg released from Daxing soil was higher than that from Changping soil.There was little difference of release between under the conditions of all wet and of the alternative of dry and wet.About 1%-2% of total nonexchangeable Mg could be released at 12 weeks′ measurement,while 30%-35%of the original soil exchangeable Mg could be restored.The dynamic of Mg release from soils by successive extraction with 0.5 mol/L NH₄Ac(pH7.0)was studied.The comparison of the coefficient of correlation(R2)and the standard error (SE)values showed that the parabolic diffusion equation best fitted the kinetics of Mg cumulative release,indicating that the diffusion of Mg out of soils may be the controlling process,but in the beginning periods,this equation wasn′t an excellent description of Mg release,and in the latter periods,process of Mg release might be described by the zero order equation.The first order and power function equation can well fit the Mg release as well.The experiment of exhaustive cropping in the greenhouse showed a remarkable tendency of decrease in soil exchangeable Mg with the increase of crop harvest.The difference between the exchangeable Mg decrease and the total amount of Mg taken up by plants was used to estimate Mg release and its availability plants from soil non-exchangeable Mg forms.After three harvests,the contribution of non-exchangeable Mg to plant uptake was 14% in Changping soil and 35% in Daxing soil.About 50% of the total Mg uptake by plants in six harvests was believed from the soil non-exchangeable Mg forms.

Key words: Fluvo-aquic soil, Magnesium, Release kinectics, Exhaustive crop

CLC Number: 

• S158.3