Spatial And Temporal Changes Of Organic Matter Content In Topsoil Of Cultivated Land In Xuchang City And Analysis Of Its Dynamic Relationship With Yield Of Main Food Crops

Cultivated land is the basic material for human survival and development,and the carrier of the growth of various crops,which directly affects national food security.As an important part of soil,soil organic matter（SOM）is an important indicator for evaluating fertility and soil quality,and it is also closely related to the ecological environment.Due to the soil formation process and the influence of human factors,the soil organic matter of cultivated land has strong variability in time and space.At present,the description of SOM spatial variability features mostly adopts the method of ordinary Kriging based geographic-spatial model,and there are few cases of attribute-spatial model that quantitatively fit the relationship between soil properties and various covariates.In addition,in spatiotemporal variation,the same prediction method is often used to process data in different periods,and the prediction results of different models are not compared based on data differences,so as to select the best model suitable for data in different periods.Therefore,this study takes Xuchang City as the research area,based on three representative spatial prediction models of ordinary Kriging,random forest and mixed model（random forest-regression Kriging）,with 29 factors related to soil organic matter As a covariate,the spatial prediction of the topsoil organic matter of the cultivated land in Xuchang City in the past 40 years（1980,2008,2018）was carried out.And verify the optimal model suitable for different periods according to the prediction results.According to the prediction results of the three-period optimal model,the characteristics of temporal and spatial variability of topsoil organic matter in Xuchang City and its driving factors were analyzed,and the relationship between grain yield and soil organic matter in Xuchang City was discussed.The final conclusions are as follows:（1）The optimal prediction models in 1980 and 2008 were both ordinary Kriging,and the optimal prediction model in 2018 was the random forest model.In 2008,when the sample density reached 4 points/km²,the R² of ordinary Kriging interpolation,random forest and random forest-regression Kriging mixed model were 0.41,0.34 and0.32,Ordinary Kriging interpolation method is significantly better than the random forest and random forest-regression Kriging mixed model;when the sample point density is about 1/km² in 2018,ordinary Kriging interpolation,random forest and random forest-regression Kriging mixed model The R² is 0.33,0.34 and 0.33,respectively.There is no significant difference in the prediction accuracy of the three models.In comparison,the random forest model is slightly better than the ordinary Kriging interpolation method and the mixed model model.（2）The average value of the organic matter content of the cultivated land surface soil in the three periods were 10.48 g·kg^-1,15.61 g·kg^-1,and 18.28 g·kg^-1,respectively.It can be seen that the organic matter content of the cultivated land surface soil in Xuchang City has shown an overall upward trend in the past 40 years.According to the organic matter classification standard of the cultivated layer in the second national soil survey,the organic matter content of the topsoil of the cultivated land in 1980 was at the third level,and in 2008 and 2018,it was at the second level.From a spatial point of view,in 1980,the high-value areas of the topsoil organic matter of cultivated land were mainly distributed in the western mountainous area of Yuzhou,Xiangcheng cinnamon soil area and Lime concretion black soil area,and the low-value areas were mainly distributed in the north of Yanling and the east of Changge and Jian’an.Most of the spatial distribution maps in 2008 showed patchy distribution.The high-value areas were mainly distributed in the cinnamon soil area in the west of Changge,the Lime concretion black soil area at the junction of Jian’an and Weidu in the west,and the western mountainous area of Yuzhou,the low-value areas are mainly distributed in the tidal soil area of Yanling and the cinnamon soil area in the north of Xiangcheng.In2018,the overall high-value plots were mainly distributed in the cinnamon soil area of Juci Mountain in the west of Yuzhou,the Ziyun Mountain in the west and south of Xiangcheng,the cinnamon soil area of Shoushan,and the Lime concretion black soil area in the south of Yanling.The value areas are concentrated in the calcareous alluvial soil and de-aquic soil contiguous areas in the east of Changge,the northeast of Jian’an,and the north of Yanling,and the sandy alluvial soil area at the border of Xiangcheng and Weidu.（3）In the past 40 years,the overall organic matter of the surface soil of the cultivated land in Xuchang City has increased to varying degrees.From 1980 to 2008,it rose from 10.42 g·kg^-1 to 15.60 g·kg^-1,an increase of 49.71%,and the content level increased significantly,the average increase is 0.19 g·kg^-1,and the content change range is mainly concentrated between 0～10g·kg^-1,accounting for 98.25%of the total cultivated area;2008-2018 increased from 15.60g·kg^-1 to 18.70g·kg^-1,an increase of19.99%,with an average annual increase of 0.32 g·kg^-1,and the increase in content is mainly concentrated in the range of 0～5g·kg^-1,accounting for 64.82%of the total cultivated land;from 1980 to 2018,the areas with the largest increase are mainly in Yuzhou.The temporal and spatial variability of the organic matter content in the surface soil of the cultivated land in Xuchang City is mainly affected by human factors such as managed production management measures.Different soil types,different land use methods and different levels of cultivated land surface soil organic matter content differences are also mainly caused by human management factors and topographical factors.（4）In the past 20 years,the level of grain production in Xuchang City has been steadily increasing.The level of grain production is closely related to factors such as fertilization,climate,and soil properties.In 2008,there was a strong correlation between grain production and soil organic matter.The grain production in 2018 It is mainly affected by the amount of organic fertilization in the year,so the correlation between soil organic matter and grain yield is concealed by the amount of organic fertilization,and there is no direct correlation between the two.