Mechanism Of Calcium In Soil Nitrogen Transformation Process Under Different Land Use Types In Karst Regions

Limestone soil in karst regions has a Ca-rich alkaline geochemical environment.Calcium is one of the key elements in determining the structure and function of karst ecosystems.Carbon cycle,water cycle and calcium cycle are the three major material cycles of karst ecosystems.Previous studies have a deeper understanding of the carbon and water cycle,but the study of calcium cycle is relatively weak.Nitrogen is an important nutrient element in soil.The process of soil nitrogen transformation is an important process of nitrogen biogeochemical cycle.At present,most studies on the rate of nitrogen conversion focus on the net conversion rate or the total conversion rate,but lack of in-depth understanding of nitrogen conversion process.Changes in land use types will lead to changes in soil physical and chemical properties,thus affecting the distribution of soil calcium forms and the transformation process of soil nitrogen.The soil samples were collected from grassland,artificial forestland,cornfield and pitaya land in Bisezhai Village,Xibeile Country in Mengzi City,Yunnan Province,all of which were characterized by typical karst landform.The BCR three-step sequential extraction procedure was used to determined the calcium content of different forms.Determination of soil nitrogen primary conversion rates by ¹⁵N tracing study with Markov Chain Monte Carlo sampling algorithm.Discussing the mechanism of calcium influence on nitrogen transformation under different land use types will help us to understand the distribution and movement characteristics of calcium and the regularity of soil nitrogen mineralization in karst environment.The main results were showed as follows:?1?The limestone soils contain a large amount of total concentration of calcium.However,due to the influence of land use types,the contents of total calcium and various forms of calcium are significantly different.With the increase of disturbance intensity,the total calcium content of soil decreased significantly,especially exchangeable calcium and acid-soluble calcium.Total calcium and various forms of calcium were significantly positively correlated with organic carbon,total N,total Mg,CEC,and pH.The results indicate that the total calcium and the various forms of calcium are interrelated to the physical and chemical properties of the soil to a certain extent.?2?The soil nitrogen transformation process is significantly different under different land use patterns.Compared with grassland without human disturbance,the application of large amount of organic fertilizer and chemical fertilizer,soil nitrogen mineralization rate and autotrophic nitrification rate are the direct causes of soil nitrate nitrogen accumulation in the pitaya field.Corn fields are only applied fertilizer,the rate of autotrophic nitrification and inorganic nitrogen supply is significantly reduced.The ammonium nitrogen adsorption rate of the unfertilized plantation soil increased significantly and inhibited the autotrophic nitrification rate,which reduced the soil nitrate nitrogen content.?3?The characteristics of calcium-rich soil in limestone soil in karst areas affect soil nitrogen transformation.The high content of calcium in the soil is conducive to the accumulation of organic matter and promotes the rate of labile organic nitrogen mineralization.High content of available calcium can promote soil nitrogen conversion,and organic matter bound,residual calcium can promote soil NH₄⁺adsorption rate.In the study area,there was no significant relationship between calcium and total mineralization rate,total nitrification rate,I_NH4H4 and I_NO3O3 also have no significant relationship.Calcium may not be the dominant factor affecting soil nitrogen transformation process.The effects of P and K are more pronounced.