Spatial Variation And Scale Effect Of Soil Organic Matter And Total Nitrogen In Cultivated Land In Shaanxi Province

As the global climate continues to warm,China,as a major emitter of carbon and nitrogen from agriculture,faces many challenges to achieve its "dual carbon" goal as scheduled.As important "source-sink" of carbon and nitrogen,the proportion transformation and imbalance of SOM(soil organic matter)and TN(total nitrogen)in cultivated soil have a great impact on the global biochemical cycle.In addition,the study on spatial structure and influencing factors of cultivated land soil organic matter(SOM)and soil total nitrogen(TN)are the basis of regional sustainable utilization of cultivated land and food security.It is conducive to the formulation of regional precision agriculture strategy,farmland ecological environment protection and agricultural structure adjustment and other macro decisions.Based on the high-density sampling data of cultivated land fertility survey and quality evaluation project and soil testing and formula fertilization project in Shaanxi Province,this study applied classical statistics,geo-statistics,GEO-D,Geographically Detector),Geographically Weighted Principal Component Analysis(GWPCA),Multi-scale Geographically Weighted Regression(MGWR),geographically weighted Principal Component analysis(GWR)The methods were adopted to study the content level and spatial structure characteristics of cultivated soil organic matter and total nitrogen in Shaanxi province.The effects of environmental factors on the spatial distribution of soil organic matter and total nitrogen content and their scale effects were analyzed,and the optimal models for the simulation of soil organic matter and total nitrogen content at different scales were determined.The main conclusions are as follows:(1)The average levels and coefficient of variation of soil organic matter and total nitrogen contents at different scales were different.In the same region,the variation coefficients of soil organic matter and total nitrogen content at provincial scale were generally higher than those at regional scale and county scale.The average contents of soil organic matter and total nitrogen in Shaanxi Province were 15.63 g/kg and 0.91 g/kg,respectively.The geographical weighted average contents were higher in the south and lower in the north.The average contents of soil organic matter and total nitrogen in Guanzhong Plain were 17.65 g/kg and 1.01 g/kg,respectively.At the county scale,the average contents of soil organic matter and total nitrogen in Wugong County were 19.97 g/kg and 1.26 g/kg,respectively.The geographical weighted average contents showed a trend of low in the east and high in the west,but the difference was small.(2)Soil organic matter and total nitrogen of cultivated land showed significant spatial autocorrelation and obvious spatial agglomeration characteristics at provincial and regional scales in Shaanxi Province and its sub-regions.The spatial agglomeration effect of soil organic matter and total nitrogen content was not significant at the county scale.The lump-gold coefficients of soil organic matter content and soil total nitrogen content decreased with the increase of the scale,the influence of structural factors on the spatial variation was strengthened,and the random factors were weakened.(3)Total nitrogen content showed the largest single factor influence on soil organic matter content in Shaanxi Province(q was 0.65-0.90).Among the other factors,the annual sunshine hours in Shaanxi Province had the greatest influence(q>0.40),the sub-categories of the Han River Basin and the Blown sand region had the greatest influence(q = 0.26-0.50),the C/N ratio in Qinling Mountains had the greatest influence(q = 0.58),and the sunshine hours in the Loess Plateau had the highest influence(q = 0.36).The total soil nitrogen in the whole region and sub-region of Shaanxi province was most affected by soil organic matter content(q was 0.55-0.89).Among other factors,the annual sunshine hours in Shaanxi Province and the Loess Plateau region had a greater influence,reaching more than 50%.The C/N ratio showed the greatest influence in Guanzhong plain(q = 0.41).Soil classes and subclasses had a high influence on soil total nitrogen in the Han river Basin(q = 0.48,0.54).The influence of each factor in other regions is low.The driving factors had a greater combined effect on soil organic matter and total nitrogen variation than single factor analysis,showing mutual enhancement or non-linear enhancement effect,but the interaction between soil type and sub-type reduced it.(4)The correlation between soil organic matter and total nitrogen content with environmental factors was the strongest at the short range scale,followed by the long range scale and the block gold scale.High correlations were found between soil organic matter and total nitrogen content at all scales.At the block gold scale,the correlations between soil organic matter and climate factors,total nitrogen and climate factors,which have strong correlations in the ordinary correlation analysis,were not obvious.At the short range scale,the correlations between soil organic matter content and climate factors,pH value,soil total nitrogen and climate factors were significantly higher than those at the block gold scale,but generally lower than the ordinary correlation analysis.At the long range scale,the correlation coefficients between soil organic matter and total nitrogen content and climate factors were relatively high,ranging from 0.37 to 0.53.Moreover,the correlation coefficients between soil organic matter,total nitrogen content and related factors decreased in the short range scale.(5)GWPCA-MGWR was the best regression model for the spatial distribution regression of soil organic matter and total nitrogen and identifying the dominant driving factors at regional and provincial scales,while PCA＿MGWR was the best regression model at county scale.At the scale of Shaanxi Province,the northern region was mainly affected by soil pH and climate conditions,the central region was affected by ripening and climate conditions,and the southern region was affected by crop ripening and land use type.At the regional scale,land use type,soil type and geomorphic type were the main factors affecting soil organic matter content in Guanzhong Plain.At the county scale,climate conditions,geomorphic types and soil pH were the main factors affecting soil organic matter content in Wugong County.Soil total nitrogen in northern Shaanxi Province was mainly affected by soil types and human activities,soil pH,climate conditions and human activities in central Shaanxi province,and geomorphic types in southern Shaanxi Province.At the regional scale,the Guanzhong Plain area was mainly affected by soil pH and human activities.At the county scale,soil total nitrogen in Wugong County was affected by climatic conditions,geomorphic types,pH and soil types.