Effects Of Fertilization On Soil Acidification In Upland And Paddy Soils

Soil acidification is a serious problem in south China,and the application of chemical fertilizers is one of the main factors enhancing soil acidification.Excessive application of nitrogen fertilizer can change the gene abundance and community structure of microorganisms in soil nitrogen cycling,causing a series of environmental problems,which may seriously threaten people’s health.The physiological acidity of compound fertilizer may contribute protons to soil acidification,but it has not been quantitatively evaluated compared with the contribution from nitrogen transformation.Different nitrogen fertilizer types have certain effects and contributions to the changes in acidity and nitrification in acidic soil.The paddy ecosystem plays an important role in agricultural production and the global nitrogen cycle.Although paddy soil acidification is weaker than that in upland soil,the acidification problem of paddy soil in China is more serious due to long-term fertilization.However,the related mechanisms of paddy soil acidification by fertilizer are still unclear.This thesis treated upland and paddy soils with different acidity in Jiangxi and Hunan provinces as the research object and to quantify the contribution of acidity characteristics of compound fertilizers to soil acidification.Also,we used fluorescence quantitative PCR to analyze the effects of nitrogen transformation and ammonia-oxidizing microorganisms on soil acidity in the upland and paddy soils under different water conditions.The main findings are as follows:（1）In this study,12 compound fertilizers with N,P and K were chosen from domestic and foreign companies.Results indicated that 11 compound fertilizers were acid,and the fertilizers with pH between 4.0 and 5.0 accounted for more than 58%of soil acidification,and the maximum acid content was 1.95 mol kg^-1.When they directly were applied to soil,they could significantly reduce soil pH and increase exchangeable acidity in one day.Therefore,acid compound fertilizers could significantly promote soil acidification due to their high acid content.When they were applied for incubation,the ratio of protons from fertilizers directly input to nitrogen conversion could reach up to 2.05 on the 5th day of incubation,while the maximum ratio was only 0.04 on the 60th day.The results also suggested that the relative contribution of direct proton input to H⁺by strong acid compound fertilizer was great at the early stage of fertilizer application,while protons from nitrification played a major role in the later stage.Nevertheless,the contribution of the long-term application of acid compound fertilizer to soil acidification should be given more attention.（2）In the short term,the application of nitrogen fertilizers led to soil pH reduction in acid upland soils.The pH of treatments decreased significantly by 0.37 to 1.23 units compared with the initial soil pH.The higher the initial upland soil pH,the stronger the nitrification.During the experiment,the nitrification and acidification in paddy soils were weaker than that in upland,and the maximum average daily nitrification rate was 0.44 mg kg^-1 d^-1.The main form of inorganic nitrogen in paddy soils was ammonium nitrogen,and the mineralization rate reached 2.81 mg kg^-1 d^-1.Soil ammonia-oxidizing microorganisms had different responses to nitrogen fertilizers.Compared with day 0,nitrogen fertilizers significantly stimulated the growth of Ammonia-Oxidizing Archaea（AOA）and Ammonia-Oxidizing Bacteria（AOB）,and the gene abundance of AOB in paddy soils was slightly lower than that in upland soils,which might lead to the difference in nitrification between upland and paddy soils.Therefore,the different nitrification intensity caused by the different activities of ammonia-oxidizing microorganisms and the land use types would be the main reasons for the different soil acidity characteristics of these soils under aerobic conditions.（3）In the short term,the transformation of Fe,Mn and S was the main factor causing the soil pH change,and the contribution rate was about 90%,while the maximum contribution rate of nitrogen transformation was only 8.1%.However,long-term accumulation of gaseous losses such as ammonia volatilization would accelerate soil acidification.In the long term,nitrogen in paddy soils promoted soil acidification mainly through ammonia volatilization and denitrification while in upland soils,soil acidification was mainly caused by nitrification and nitrate leaching.Therefore,the acidification promotion rate of paddy soils by nitrogen was weaker than that in upland soils and this would be the main reason for soil acidification difference between upland and paddy soils under nitrogen application.