Distribution characteristics of soil organic carbon and its influencing factors in cultivated land in south subtropical region of Guangxi

【Objective】To explore the distribution characteristics and influencing factors of soil organic carbon in the south subtropical cultivated land of Guangxi，which could provide scientific basis for promoting regional carbon neutralization and sustainable agricultural development.【Method】Taking Rongxian County and Tiandong County，which were typical counties with eastern and western natural geographic features of south subtropical region in Guangxi，as sample counties，soil samples were collected in the field，soil organic carbon related data were obtained by testing，and classical statistical methods and spatial autocorrelation analysis were applied to analyze the spatial distribution characteristics of soil organic carbon，and to reveal important factors affecting the density of organic carbon.【Result】The average surface soil organic carbon density of cultivated land in the south subtropical region of Guangxi was 64.20 t/ha in the east and 36.75 t/ha in the west，which was much higher in the east than in the west. The Moran’s I index for organic carbon density in the east was -0.154，indicating a spatially negative correlation，whereas in the west，the Moran’s I index was -0.002，suggesting a more random distribution. From the perspective of land types，the organic carbon density of the paddy field in the east was 67.21 t/ha，and that of dry land was 51.15 t/ha. The organic carbon density of paddy field was 36.18 t/ha in the west，and that of the dry land was 36.65 t/ha. The overall distribution pattern was high in the east and low in the west，and there was a big difference. In the vertical distribution，the soil organic carbon density in the east and west reached the highest value in the profile of 0-30 cm. Correlation analysis showed that soil organic carbon density was significantly（P<0.05，the same below）or extremely significantly（P<0.01，the same below）positively correlated with precipitation，soil water content and total nitrogen，and significantly negatively correlated with altitude，soil bulk density. The correlation between ground temperature and slope and soil organic carbon density of arable land was not significant （P>0.05，the same below），but the two showed extremely significant negative correlation and significant positive correlation with paddy soil organic carbon density respectively. Soil pH was extremely significantly and negatively correlated with organic carbon density in the east，but not significantly correlated in the west.The results of random forest regression model analysis showed that the relative importance of precipitation（19.3%），soil water content（17.0%）and total nitrogen（15.8%）to organic carbon density was higher.【Conclusion】The distribution of soil organic carbon in the south subtropical cultivated land of Guangxi is high in the east and low in the west in the longitudinal direction，and decreases with the increase of soil depth in the vertical direction，which is mainly affected by topography，climate，soil physical and chemical properties，cultivated land and crop types.