| Soil acidification of farmland has been one of the global agricultural and ecological problems in a long term.According to statistics data,nearly half of the world’s arable land area is acidic,and some limed areas have begun to reacidify.However,the current researches about soil acidification focus on the effect and mechanism of different amendments on remediating soil.There is still a lack of attention to the soil reacidification process and the variation of the substances in the progress.Considering the increasingly serious phenomenon of soil acidification,especially in farmland,the effect of reacidification on the exchangeable base cations and buffering capacity of soil were discussed in this study.And based on the above,this study researched the morphological changes of iron and aluminum oxides,which have aluminum toxicity effect on plants,and the morphological characteristics availability of inorganic phosphorus that promoted plant growth in the process of soil reacidification.In this study,the experimental method of indoor culture was adopted,and the reliming and reacidification processes of acidic soils were achieved by repeatedly adding CaO(3.1 g/kg)and HCl(206.5 mL/kg).Five levels of reacidification times were set up,including 0(CK),1(R1),3(R3),6(R6),10(R10)times.After soil cultivation,the soil pH,exchangeable cations,mineral and free Fe2O3/Al2O3,Ca-P,Olsen-P,Al-P,Fe-P were determined in all treatments.The main findings are as follows:(1)Multiple reacidification reduced soil acid buffering ability.The results showed that soil pH value was significantly reduced by reacidification.There was a linear negative correlation between the soil pH variation and the number of reacidification(r=-0.98).For each increase in the number of reacidification,the soil pH will decrease by 0.033 units compared with the last reacidified soil,and H+concentration was 1.08 times that of the previous treatment,so that the H+concentration increased continuously.The increase of H+concentration caused the base cations lose gradually with the increased number of reacidification.There were significant negative correlations between the number of reacidification and the variations in soil exchangeable Mg(r=-0.79).With the times of soil reacidification increases,the exchangeable will decrease gradually at rates of 0.75 mg/kg.As the content of exchangeable base cations decreasing,soil exchangeable aluminum also released simultaneously.There was a significant linear positive correlation between the variation of soil exchangeable aluminum and the number of reacidification(r=0.87).With the increase of soil reacidification numbers,the exchangeable aluminum content will increase by 0.26 cmol/kg in each time.Ultimately,the aluminum toxicity influence produced by the exchangeable aluminum in the soil will become more serious.With the loss of base cations and gradual generation of exchangeable aluminum,soil buffer capacity also decreased.But the soil buffer capacity was significantly lower than that of the control treatment(48.76 mmol·kg-1·pH-1)until reacidify 10 times(45.31 mmol·kg-1·pH-1).Therefore,the depletion of based cations in the reacidification process has a cumulative effect after multiple reacidification treatments,resulting in the decrease of soil buffer capacity in R10,which weakened the soil buffer performance against acid.(2)Reacidification accelerated the progress of soil weathering,and resulted the transformation of mineral Fe2O3/Al2O3 to free Fe2O3/Al2O3,and crystalline Fe2O3/Al2O3 to amorphous Fe2O3/Al2O3.A In this study,there was a significant linear positive correlation between the number of soil reacidification and the variation in the percentage of freedom(r=0.74)and the percentage of activation(r=0.92)of Fe2O3.With the increase of the times of reacidification,the content of mineral Fe2O3 in the soil decreased significantly,and the content of crystalline and amorphous Fe2O3 increased significantly,so that the percentage of iron oxide freeness and activation increased at a rate of 0.14%/time and 0.09%/time.Compared with the control(205.62 g/kg),the soil mineral aluminum(199.2 g/kg to 203.41 g/kg)under all reacidification treatments gradually decreased significantly.However,due to the large solubility of aluminum in acidic solution,the crystalline and amorphous aluminum were dissolved in the acid solution,without showing differences between these reacidification treatments.However,there was a positive correlation between the amount of soil aluminum activation percentage and the number of reacidification(r=0.73),and it increased at a rate of 0.61%/time.The increase in the freeness and activation of soil iron and aluminum oxides indicates that soil reacidification promoted the transformation of mineral iron and aluminum oxides to crystalline iron and aluminum oxides,as well as the conversion of crystalline iron and aluminum oxides to amorphous iron and aluminum oxides,which increased the degree of soil weathering.(3)Reacidification promoted the conversion of Olsen-P to Al-P,improving the bioavailability of soil phosphorus.The results of the study showed that reacidification didn’t affect the content of total phosphorus and inorganic phosphorus.The occurrence of reacidification decreased the content of Olsen-P and Ca-P in the soil.The content of soil Olsen-P(146.98-184.98 mg/kg)in all reacidification treatments were significantly lower than that of control(232.37 mg/kg).Compared to control(36.26 mg/kg),soil Ca-P content in the remaining treatments(23.54～29.17 mg/kg)were also significantly reduced except in R3.At the same time,soil reacidification gradually increased aluminum phosphorus content,and soil Al-P content(37.02～131.15 mg/kg)from R1 to R10 was significantly higher than that of the control(24.93 mg/kg).By analyzing the correlation between the components of inorganic phosphorus,it can be found that reacidification promoted the conversion of Olsen-P and Ca-P to amorphous Al-P,which improved the bioavailability of soil phosphorus finally.In summary,the results of this study showed that reacidification process reduced the content of elements such as Ca,Mg,K and Na in the soil,and weakened soil acid buffering ability,which will lead to the risk of nutrient deficiency in the reacidified soil.Cause it is difficult to provide sufficient nutrients for the plants,thereby increasing the cost of supplying mineral fertilizers in the crop production process and the cost of improving the acidic arable soil.At the same time,the reacidification process led to the activation of iron and aluminum oxides,which improved the content of soil exchangeable aluminum,deepened the aluminum toxicity influence of the soil,and further reduced the bio-suitability of the reacidification soil.In addition,the formation of amorphous aluminum phosphorus in the process of reacidification was conducive to plants obtaining available phosphorus from the soil,but it also increased the risk of effective phosphorus loss caused by factors such as irrigation and precipitation.These results suggest that soil reacidification will seriously endanger soil ecosystem productivity and animal and plant health. |
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