Study On Distribution Of Fixed Ammonium And Its Influencing Factors In Paddy Soil Profile

Fixed ammonium is an important NH₄⁺-N form in the soil.It can transform to exchangeable and soluble NH₄⁺,which to supply enongh ions of NH₄⁺ in growth of crops.The distribution regulations of fixed ammonium in profiles different granularity samples clearly reflect the state and behavior characteristics of fixed ammonium.The component and content of clay mineral play an important role on the distribution of fixed ammonium.Hence,it is necessary to study clay mineral of paddy soil profiles and the distribution of fixed ammonium systemically,further to understand the relationship between them,which will provide a theoretical basis for the rational use of nitrogen fertilization and the improvement of fertilizer applying.This would be important theoretical and practical significance in the ammonium regulation and nitrogen transformation in the paddy soil.At present,there are mainly a few questions in the studies of soil fixed ammonium in and abroad:1) lack the further study in the clay mineral of soil profile.The component and content of clay mineral have large differences in soil layers,and many information would be lost if we treat the different soil layers as the same;2) lack the systematic work on fixed ammonium of soil profile.Most work only restricted on the plowed layer except,while few related to the whole profile,in addition,the sampling interval is too large which cannot expose all the informationThis paper toke the hydromorphic paddy soil profiles in Nanjing(Changchenggang,Quanshui and Dahutou profiles) as the research objects,and toke the understanding of distribution regulations of fixed ammonium as the research content.The main research aim was to explore the mechanism of controlling the fixed ammonium content This study analyzed the distribution regulations of fixed ammonium in the profile and assignment features in clay particle,silt particle and fine sand,and finally summarized the main factors that influenced the fixed ammonium content.The results showed that:(1) The vertical distribution regulations of fixed ammonium were not identical in the three profiles.The trends from top to down were:Changchenggang profile-from the surface layer to 80cm,the value gradually decreased,and then gradually increased in 80～100cm;Quanshui profile-the value showed gradually increase but with a slight increment;Dahutou profile-the value were nearly the same with a slight changes.The value of fixed ammonium/TN all showed the gradually increasing trend in the three profiles.(2) The main component of the free sand was quartz,and the main mineral components of the silt particle were mica and quartz.The average values of main the clay particle were illite (61.74%),kaolinite(29.25%),and smectite(9.01%) in all samples.(3) The assignment regulations of fLxed ammonium in different granularity samples were remarkable.The average contents of fixed ammonium in the three size fractions(clay particle,silt particle and fine sand.) were 371.4 mg/kg,242.1 mg/kg,and 130.1 mg/kg,respectively.(4)The fixed ammonium content of paddy soil was related to cation exchange capacity,clay particle content,clay particle mineral component,TN content,organic matter.The results of principal component analysis showed that the degree order that influence the fixed ammonium content was:clay particle content＞illite content＞TN content＞organic matter＞cation exchange capacity.