Characteristics And Affecting Factors Of Nitrogen Transformation In Calcareous Soil In Karst Regions Of South China

The karst ecosystems with fragile ecological environment is widely distributed in the south of China,soil nitrogen（N）supply is an important factor affecting the productivity and stability of plant communities in the karst regions.It is particularly important to investigate the N transformation process which controls soil inorganic N supply.The main soil type in karst regions of south China is calcareous soil which characterized by high calcium and magnesium materials,high p H and relatively sticky soil texture,these factors may lead to the obvious differences in N transformation compared to red soil in the same latitude.But at present,there are few studies on N cycling in calcareous soil.Calcareous soils under natural forests were sampled from different karst regions of south China（Hainan Province,Guangxi Province,Yunnan Province,Guizhou Province）.In addition,natural forests calcareous soils with different development degrees were also sampled of test station in Maocun in Guilin,Guangxi Province,and the adjacent red soils under natural forest were collected as control.Soil N transformation rates were determined using a ¹⁵N tracing technique combined with the N model.The characteristics and influencing factors of N transformation in calcareous soils were studied to evaluate the inorganic N supply and retention capacity and N availability of natural forest soils.The main results of the study were as follows:1.The inorganic N was dominated by nitrate（NO₃^-）in calcareous soils,which was significantly higher than in red soils.Compared to red soils,calcareous soils exhibited significantly higher organic carbon and total N content,cation exchange capacity,WHC and p H,but lower C/N ratio and aluminum concentrations.Compared to red soils,calcareous soils had a high mineralization rate and autotrophic nitrification rate.The rates of microbial ammonium（NH₄⁺）immobilization and dissimilatory NO₃^-reduction to NH₄⁺in calcareous soils also increased significantly.Moreover,the above N transformation rates were significantly and positively related to the organic carbon contents,total N contents,calcium contents and p H,indicating that these four factors were responsible for N transformation in calcareous soils.2.The inorganic N of calcareous soils in different karst regions was mainly NO₃^-,but the content was between 22.8～60.7 mg N kg^-1.The autotrophic nitrification dominated total nitrification in all calcareous soils,but the rate was between 8.02～33.0 mg N kg^-1d^-1.The contents of total nitrogen,organic carbon and other N transformation rates also vary with the study karst regions,which may be due to the the different development degree of calcareous soil in different karst regions.3.With increasing development degree of calcareous soils,the contents of organic carbon,total N,total calcium,sand and cation exchange capacity decreased significantly,while the contents of iron,aluminum,iron-aluminum oxides and clay increased significantly.The inorganic N in calcareous soils with low development degree was dominated by NO₃^-.With the increase of development degree,the inorganic N changed to be dominated by NH₄⁺.With the development degree increasing,the mineralization rate and autotrophic nitrification rate in calcareous soils decreased significantly,while the microbial NH₄⁺immobilization rate increased significantly,resulting in the decrease of NH₄⁺supply capacity.In addition,although the heterotrophic nitrification rate in calcareous soils increased with the increase of development degree,the reduced degree of autotrophic nitrification rate was significantly higher than the increased degree of heterotrophic nitrification,which led to the decrease of NO₃^-supply capacity.The rate of dissimilatory NO₃^-reduction to NH₄⁺and microbial NO₃^-immobilization in calcareous soils also decreased with the increase of soil development.Correlation analysis indicated that the contents of calcium,organic carbon,total N,iron-aluminum oxides and soil texture were the key factors affecting the N transformation of calcareous soils with different developmental degrees.Taken together,compared to red soils,calcareous soils had higher inorganic N supply capacity but lower retention capacity.The trend of N transformation was similar in different karst regions,but it may affect the physical and chemical properties of soil due to the different development degree,resulting in the difference of N transformation rate.With the increase of the development degree of calcareous soils in the same region,the inorganic N supply capacity decreased and the retention capacity increased,indicating that with the increase of the development degree,calcareous soils can also develop N cycle characteristics similar to that of red soils.