Acidity Characteristics And Acidification Mechanism Of Soils In Protected Fields

Soil acidification, soil salinization and nutrient imbalance were serious in protected fieldbecause of the intensive cultivation, higher multiple cropping index, fertilizer managementmeasures unreasonably and the lack of rain, which not only affected the sustainabledevelopment of food production, but also had a series of agricultural product quality securityand environmental protection problems. This paper used typical soil of protected field in theliaoning region to study the characteristic, reason, process, mechanism of soil acidificationand to explore the relation between soil salinity accumulating and pH with the help of indoorculture and mathematical analysis. The results were showed as follows:1. The present condition analysis of soil acidification in typical protected field inLiaoning.The areas should be arranged from high to low, according to their soil pH in Liaoningprotected field, as follows: Chaoyang > Qingyuan > Tieling > Dandong > Dongling > Yuhong.At the present time, the soil pH sharply decreased after the field began vegetable cultivationunder protection in most of Liaoning District (except Dandong), and the sampling point that islesser than pH 5.50 consist in most of Liaoning district.The soil pH shows the trend of 0～20cm soil layer<20～40cm soil layer <40～60cm soillayer <60～80cm soil layer in protected field in Chaoyang, Tieling, Dongling and Yuhong.The soil pH increased and closed to open field with soil depth increasing, and topsoil(0～20cm) pH was far below the under layer, Differences among other soil layers were small. Butthe pH of topsoil layer higher than other soil layers in protected field in Dandong andQingyuan. That is to say, the soil pH declined with soil depth increasing, and that's becausethere were related to field manage measures in protected field, geographical location and soiltexture.With replanting time prolonging, the soil pH was continuing decline in protected field.But the soil pH declined at a slow pace even increased in the first 1～4 years, and withreplanting time prolonging, the soil pH declined at a quick pace. With replanting timeprolonging, the soil pH was continuing decline in protected field in general.2. Acidity characteristics of soils in protected fieldsThe content of active acidity increased obviously after the field began vegetablecultivation under protection. Soil acidification displayed a clear rising trend in topsoil thansubsoil. EA, NEA, and EB, in various layers of the soils in the protected fields were muchhigher than those in the open fields, Soil pH was in significant negative correlation with EAand NEA, but in significant positive correlation with EB, Related coefficient and partial regression coefficient of regression equation showd that: Soil pH was determined mainly byEA, while NEA and EB had less impact on that.Exchangeable H⁺ were predominant when the EA<0.75cmol kg^-1, and exchangeable Al³⁺were predominant when the EA>0.75cmol kg^-1 The proportion of exchangeable Al³⁺increased with increasing exchangeable acid, while decreased with increasing OM. And, theproportion of H⁺ in exchangeable acid showed a reverse trend. The content of NEA increasedobviously after the field began vegetable cultivation under protection. The content of NEAwere usually higher than EA. Soil NEA was in significant positive correlation with OM.OM and CEC of the soils in the protected fields were much higher than those in the openfields. Soil CEC was in significant positive correlation with OM, it indicated that with thecontent of OM increasing, the soil CEC was continuing increase in protected field.Exchangeable EB, exchangeable K⁺, exchangeable Mg²⁺ and Exchangeable Na⁺ invarious layers of the soils in the protected fields were much higher than those in the openfields, whereas exchangeable Ca²⁺ in various layers of the soils in the protected fields weremuch lower than those in the open fields, but the difference was not significant. ExchangeableCa²⁺ was predominant in exchangeable EB. Exchangeable K⁺ of the soils has been a dramaticrise in the protected fields, which was the main characteristic of exchangeable EBcomposition variation. Soil Water-solubility Ca, Mg, K, Na were in significant correlationwith exchangeable Ca, Mg, K, Na. Therefore, soil water-solubility EB composition variationwas the main influential factor of the exchangeable EB composition variation.PA and total exchangeable EB of the soils in the protected fields were much higher thanthose in the open fields, but the base saturation (BS%), the saturation of exchangeable Ca²⁺decreased significantly because of the increase of CEC, There is a significant positivecorrelation between the soil pH and the BS (%), which is mainly restrained by the dominantexchangeable base Ca²⁺. Therefore, It was an important factor that the BS(%), especially thesaturation of exchangeable Ca²⁺ decreased which caused soil acidification.3. Effects of protected field vegetable cultivation on soil salinity accumulating and pHThe salinity content and EC value of the soils in the protected fields were much higherthan those in the corresponding open fields. The average of the salinity content was 0.29g kg^-1in open field soil. With replanting time prolonging, the salinity content was continuingincrease in protected field, The average of the salinity content had reached 1.56g kg^-1 after thefield began vegetable cultivation under protection for 10 years. Corresponding EC hadexceeded the scope of that was 0.53mS/cm.The salinity content of 0～80cm soil layer in the protected fields were much higher thanthose in the corresponding open fields. The salinity content declined with soil depth increasing. The salinity content of topsoil was far above the under layer. The salt movingdownwards to the bottom soil and salt accumulating upwards to the topsoil were very obviousin the protected field soils.Apart from HCO₃^-, the content of NO₃^-, SO₄^2-, Cl^-, Ca²⁺, Mg²⁺, K⁺, Na⁺ increasedsignificantly in protected field. Anions are mainly NO₃^- and SO₄^2-, Cations are mainly K⁺ andCa²⁺. The content of NO₃^- may reach 0.66g kg^-1, that was 29 times than in open field.There is a significant logarithmic negative correlation between the soil pH and thesalinity content, that is to say, the soil pH declined with soil salinity content increasing.Correlation analysis, path analysis and stepwise regression results suggested that theproportion of strongly acidic Anions NO₃^-, SO₄^2-, especially NO₃^-, to total salts decreasedwhich caused soil acidification. And it was an important factor that the proportion of HCO₃^-,Ca²⁺ to total salts decreased which caused soil acidification.4. Studies on acidic buffer capacity in protected field soilThe soil pH declined with acid accession in protected fields under different plantingyears. But the rate of pH declined in the shape of "L". The rate of pH declined quickly inopen fields, and the rate of pH declined quickly. The longer the planting years in protectedfield, the slower to the rate of pH declining. When acid accession had once been accumulatedto a certain degree, soil pH in open fields was already below in protected fields.With replanting time prolonging, the soil acid-damage capacity was continuing decreasein protected field, that is to say, the acid bearing capacity of soil was keep decreasing. That'sbecause there were related to soil pH. The higher the soil pH, the more acid to need to reachthe point of crops injured. Therefore, the longer the planting years in protected field, the lowerthe pH and soil acid-damage capacity, the less content of acid to reach the point of cropsinjured, the easier injured.With replanting time prolonging, the soil acid-damage intensity was continuing increasein protected field, that is to say, in the case of equal acid joining, the longer the planting yearsin protected field, the smaller of the degree to which soil pH declined. That's because therewere related to soil OM and CEC. There is a significant positive correlation between soilacid-damage intensity and the content of OM and CEC. That is to say, the higher the contentof OM and CEC, the stronger the ability of the soil to resist acid, it would slow down soilacidification. There were some differences between soil acid-damage capacity and soilacid-damage intensity, Soil acid-damage intensity can better reflect that the soil is sensitive toacid than soil acid-damage capacity.