Effects Of Soil Acidification Induced By Nitrogen Fertilizer On Consumption And Carbon Dioxide Emissions From Inorganic Carbon In Calcareous Soil

Nitrogen fertilizer plays an important role in agricultural production.Recent studies have found that nitrogen fertilization increases CO₂efflux from soils.In calcareous soils,soil inorganic carbon acts as one of the major drivers of CO₂ release.However,it is unclear whether the nitrogen fertilizer application affects the release of CO₂ from soil inorganic carbon.It is generally reported that CO₂ efflux from soil inorganic carbon after application of nitrogen fertilizer is mainly caused by the dissolution of soil inorganic carbon driven by the acidity produced as a result of nitrification.Continuous dissolution of soil inorganic carbon from continuous increase in N fertilization induced H⁺input decreases the acid buffering capacity and reduces soil pH.Whether the soil with different inorganic carbon content be have differently to acidification?What is the acidification potential of different nitrogen fertilizer types?In addition,what is the current situation of soil inorganic carbon reserves and the rate at which they have been dissolved by acidification in China over the past 40 years?We have addressed these questions through different research projects:A closed-jar incubation experiment was carried out to study the effect of application methods of nitrogen fertilizer on the release of CO₂ from inorganic carbon in calcareous soil.Moreover,the dissolution of inorganic carbon in response to different nitrogen fertilizers types was studied by simulation soil column experiment.Besides,based on the data of the Second National Soil Survey,we calculated the loss of inorganic carbon from Chinese croplands during 1980-2020,predictions were made till 2100.The following results were obtained:?1?A closed-jar incubation experiments was conducted to compare the effects of different nitrogen application methods on changes in soil pH and CO₂ efflux.The nitrogen application methods including(applied as one dose?N1?,two doses?N2?,three doses?N3?.The results showed that compared to the no-nitrogen control,soil pH decreased by 0.40,0.38,0.32 units under the treatments of N1,N2 and N3,respectively.Soil pH under the treatment of N1 was significantly lower than the treatment of N2 and N3.The cumulative CO₂emission under N1 treatment of increased by 14.0%and 26.6%compared to N2 and N3,respectively.This study showed that ammonium N fertilizer-induced soil acidification promoted CO₂release from inorganic C in calcareous soils.Therefore,the effect of N fertilizer on inorganic C consumption in calcareous soils needs more attention.?2?This study adopted the simulation soil column incubation experiment;we applied different nitrogen fertilizer types including ammonium sulfate,ammonium nitrate,urea,potassium nitrate on soils with variable inorganic carbon contents(C1,C2,C3,C4,C5,and determined 15 days after each nitrogen application,and the changes of soil pH and CaCO₃were determined after the sixth and the end of culture.The results showed that at the end of culture,nitrogen fertilizer application significantly increased the contents of Mg²⁺and Ca²⁺in the soil leaching solution,maximum in soils where ammonium sulfate was applied followed by ammonium nitrate and urea,and minimum in potassium nitrate.Similar results were observed in case of soil pH as well as on the pH and CaCO₃ contents of the leaching solution,except for the potassium nitrate which had no significant effect on any of the above mentioned parameters.?3?The phenomenon of excessive application of nitrogen fertilizer is common in China's intensive agricultural production system.The problem of soil acidification caused by excessive nitrogen application and atmospheric N deposition gradually increased with time.Soil inorganic carbon?SIC?provides the strong buffering capacity which neutralizes soil acidification at the cost of carbonates dissolution and release CO₂ into the atmosphere.This is the first study to estimate the impact of farmland soil acidification on soil SIC reserves in China from 1980 to 2020,and to predict future impact on soil SIC reserves up to 2100.In1980,the SIC reserve of 0-40 cm farmland soil was 2.16 Pg C(16.3 MgC ha^-1).During1980-2020,7%SIC reserves(0.15 Pg C,1.09 MgC ha^-1)were lost in the cultivated land layer?0-40cm?.By 2100,China's arable land may lose 37%of its SIC reserves if N fertilization continues to increase in the same way.