Characterization Of Soil Inorganic Carbon Transformation In Different Types Of Soils Under N Fertilizer Application

Carbon is stored in soils in two main forms:Soil organic carbon（SOC）and Soil inorganic carbon（SIC）,which play an important role in the maintenance of soil fertility,soil health,and food production.It is generally recognized that SIC is an inert carbon pool with a long turnover time.Recently,excess nitrogen fertilizer application has been linked to soil acidity,which has been observed to considerably speed up the turnover rate of SIC.Although SIC stocks are continuously lost as CO₂ by neutralization of N-fertilization-induced soil acidification,the contribution of SOC has been considered in quantifying and assessing the soil carbon pool on atmospheric CO₂ release and the global carbon cycle.Few studies have focused on SIC transformation after nitrogen application.Therefore,in this study,we used a laboratory incubation experiment to identify and quantify the contribution of soil inorganic carbon to CO₂emissions（SIC-CO₂）after N treatments.A soil column experiment was also carried out to explore the discrepancies in SIC caused by different types of N fertilizers.The effects of field cropping patterns and nitrogen application rates on the soil carbon pool and soil buffering capacity were explored based on two long-term field studies.Finally,the soil carbon pool and buffering capacity were investigated using two long-term field experiments based on nitrogen application rates.We also looked at how the minerals in the soil changed and how the elements in the soil microstructure changed after long-term nitrogen application from a microscopic point of view.The main findings are:（1）To assess the contribution of SIC-CO₂ in six soil types,three N treatments were used:control（N₀）,0.2 g kg N application(N_0.2),and 0.2 g kg N application with the addition of a nitrification inhibitor(N_0.2+DMPP).The results showed that ammonium N fertilizer significantly increased soil CO₂ emissions.When comparing the(N_0.2)and(N_0.2+DMPP)treatments to the（N₀）treatment,a significant linear positive correlation was observed between the cumulative soil CO₂ emissions and the soil carbonate contents,with increases of 42.5%-99.7%and 22.5%-84.2%,respectively.In 13 days on Lou-soils,the proportion of soil SIC-CO₂ was 33-37.2%,45.6-48.6%,and 40.1-40.3%for N₀,N_0.2,and N_0.2+DMPP treatments,respectively.The fraction of SIC-CO₂,increased by 11.4-12.6%and 3.24-7.23%compared with the N₀ treatment,respectively.（2）Ammonium sulfate,urea,and potassium nitrate N fertilizers were used in soils with varying inorganic carbon content(S1:3.32 g kg^-1 and S2:33.69 g kg^-1).The amount of Ca²⁺and Mg²⁺leaching in soil leachates was greatly increased by the application of N fertilizer.The application of ammonium sulfate and urea treatments significantly reduced soil p H and Ca CO₃,with the greatest reduction in the 0-5 cm soil layer,and the different N fertilizer types revealed the following:ammonium sulfate>urea>potassium nitrate=no N application.In S1 soil,the soil Ca CO₃ of ammonium sulfate treatment has been exhausted.At 0-5 cm depth,the decrease in soil p H and Ca CO₃ was more pronounced.When X-ray diffraction was used,the intensity of calcite was lower in the ammonium sulfate treatments,and the weight percent content of soil Ca elements was also reduced compared to the control treatment.This shows that soil carbonates decreased after the application of ammonia nitrogen fertilizers.（3）Yangling and Zhouzhi long-term experiments were conducted to examine the effects of cultivation modes and N fertilizers on SOC,SIC pool,and soil buffer capacity in 0-20 and20-40 cm soils.In both experiments,the straw mulching and straw return modes significantly increased SOC and SIC storage at different soil layers.The results showed that the straw mulching and straw return modes significantly increased the SOC and SIC storage at different soil layers in the two experiments.For the Yangling experiment,compared to furrow planting,soil inorganic carbon stock in the conventional mode and straw mulching increased by 5.25and 6.51 t ha^-1 at 0-20 cm soil,respectively.Compared with the N₀ treatment,the soil inorganic carbon stock in the conventional mode decreased by 1.2 and 3.99 t ha^-1 at 0-20 cm,respectively.But a significant decrease was observed under the N₂₄₀ treatment in the straw mulching.Soil p H decreased significantly with the increase of nitrogen application rate,and soil buffer capacity decreased under high nitrogen treatment.For the Yangling long-term experiments,soil p H decreased by about 0.1 units.However,for the Zhouzhi long-term experiments,soil p H decreased by 0.54 and 0.78 units under N₁₆₈ and N₃₃₆ treatments,respectively.It is demonstrated that calcareous soil effectively buffers soil acidification caused by long-term nitrogen application by dissolving carbonate,while the p H of soil without carbonate decreases rapidly and the acidification process accelerates.