Acidity Characteristics And Acid Buffering Performance Of Typical Purple Soil In Sichuan Basin

Purple soil is a typical non-zonal soil in southwest China.However,in recent years,it has been found that,due to the influence of human activities and natural factors,the purple soil is acidified seriously.Some studies found that the acidification characteristics of purple soil are different from those of yellow soil,red soil,and other zonal soils.The systematic,quantitative studies on the acidification mechanism and acid buffering characteristics of purple soil are rare in the literature.For preventing acidification of purple soil and ameliorating acidity of acidic purple soil,further studies are needed to fully understand the acid buffer mechanism of purple soil.In this work,the measurement method of soil acid buffer capacity was optimized.Then,the acidity characteristics and acid buffering capacity of three typical purple soil profiles（calcareous purple soil developed from red-brown-purple mudstone of Jurassic Suining Formation（J₃sn）,neutral purple soil developed from gray-brown-purple mudstone of Jurassic Shaximiao Formation（J₂s）and acid purple soil developed from red-purple sandstone of Cretaceous Jiaguan Formation（K₂j））were studied.Finally,the potential acidification risk of three typical purple soils was evaluated by the electrodialysis method.The main conclusions of the experiment were as follows:1.The calculation method of soil acid buffer capacity was optimized.The method of data fitting had a great influence on the accuracy of acid buffer capacity measured by acid-base titration.The data fitting methods include polynomial curve fitting of all data（PA）,linear fitting of data between two inflection points（LT）,polynomial curve fitting of acid titration data（PAT）,and linear fitting of acid titration data（LAT）.Among these data fitting methods,PA produced the best fit for the acid-base titration data.There was a great difference of soil acid buffer capacity obtained by different fitting methods.The correlations between soil p HBCs calculated by different fitting methods and soil exchangeable cations（r₁）or effective cation exchange capacity（ECEC,r₂）were as follows:PA（r₁=0.486**,r₂=0.525**）>LT（r₁=0.223,r₂=0.245）>PAT（r₁=0.183,r₂=0.220）>LAT（r₁=-0.219,r₂=0.002）.There was a significant positive correlation between soil p HBC calculated by the PA method and soil exchangeable cations or ECEC.It revealed that the p H buffering ability of tested soils at current p H was mainly controlled by exchangeable cations and weathering of silicate minerals.Given the importance of soil p HBC,an accurate method for its measurement is essential.In conclusion,the accuracy of soil acid buffering capacity calculated by the polynomial curve fitting of all date was better than the linear fitting method in the jump range of all date.2.The acidity and acid buffer characteristics of typical purple soil were identified.The p H of purple soil developed from different parent materials varied greatly.All layers of acid purple soil were acidic.The surface soil of neutral purple soil was acidified.The p H values of calcareous purple soils were above 8.0.The acid buffering capacity of purple soil developed from different parent materials was different.The acid buffer capacity of the three kinds of purple soil varied greatly and followed this order:calcareous purple soil>neutral purple soil>acidic purple soil.This is mainly attribute to the buffer materials in these soils,which has a different ability to resist soil acidification.The buffer capacity of acidic purple soil to exogenous acid was mainly controlled by exchangeable base cations,and the acid buffer capacity of this soil was weak.The buffer capacity of neutral purple soil to exogenous acid was mainly controlled by carbonate（p H 6.2～8.6）and silicate minerals（p H>5）.There was abundant of silicate minerals in the neutral soil,which induced the buffer capacity of neutral purple soil was much stronger than that of acidic purple soil.Since the high content of carbonate in the calcareous soil,the calcareous soil had the highest value of p H and acid buffer capacity among three kinds of purple soils.3.The acidification risk of three kinds of purple soils was evaluated by electrodialysis treatment.After 4 days of electrodialysis treatment,except for the parent material,the other layers of neutral purple soil acidified rapidly.The p H value of neutral soil decreased to less than 5,and the contents of exchangeable acid and Exchangeable Al³⁺of these neutral soils increased sharply.The exchangeable acidity in the 0-10 cm layer increased rapidly from 0.83 cmol·kg^-1 to 4.41 cmol·kg^-1.After 4 days of electrodialysis,the p H of calcareous purple soil did not decrease but slightly increased.After electrodialysis,the contents of exchangeable acidity and Exchangeable Al³⁺in acidic purple soil decreased greatly.There values decreased in the range of 2.52～4.67cmol·kg^-1 and 2.23～4.89 cmol·kg^-1,respectively.The contents of exchangeable acidity in acidic purple soil were lower than those in neutral purple soil.Calcareous soils mainly rely on carbonate to buffer exogenous acids.After electrodialysis,the content of exchangeable base cations in the surface soil of neutral purple soil decreased significantly.The amount of exchangeable base cations in 0-10 cm and 10-20 cm soil decreased from32.99 cmol·kg^-1 and 38.63 cmol·kg^-1 to 11.42 cmol·kg^-1 and 16.89 cmol·kg^-1 before and after electrodialysis treatments.Although the potential acidification risk of neutral purple soil increased obviously after electrodialysis,it still contained high content of base cations,which was much higher than that of acidic purple soil under the same acidified condition.Therefore,the acid buffering characteristics of tested purple soils was different.Compared to other purple soils,neutral purple soil had a higher CEC value,which lead to a higher content of exchangeable acidity and increased the soil potential acidification risk greatly.Due to the deep acidification of the acidic purple soil,the content of exchangeable base cations in the soil was low.During the further acidification process,the positive charged iron and aluminum oxides can decrease the soil acidity by overlapping the electrical double layer of oppositive charged Fe/Al oxides and soil clay.So,the potential acidification risk of acidic purple soil was lower than that of neutral purple soil.