Spatial Distribution And Influencing Factors Of Soil Activity And Inert Organic Carbon In Cultivated Land In Sichuan Basin

The content of soil organic carbon(SOC)in cultivated land is relatively low,so increasing SOC in cultivated land is of great significance for reducing global warming and improving soil quality.Active organic carbon(POXC)and inert organic carbon(ROC)are two important components of SOC,and their content differences are important indicators of soil SOC stability.Due to environmental variability and human activity differences,the contents of POXC and ROC in cultivated soil and their influencing factors were significantly different in different regions.Based on this,this study took the Sichuan Basin with complex environmental background and intense human activities as a case area,and analyzed the content characteristics and spatial distribution rules of cultivated soil POXC and ROC in this area through 871 measured soil sample data.Anova and random forest model were used to reveal the effects of natural environment(climate and terrain),anthropogenic activities(agricultural land use)and soil physical and chemical properties on the spatial variation of POXC and ROC,and to clarify the roles and relative importance of different factors.The main research results are as follows:(1)The average contents of POXC and ROC in the study area were 2.74g/kg and12.05g/kg,respectively,in which ROC was between 2.68g/kg and 26.4g/kg,accounting for the largest proportion(81.18%),which was 4.31 times that of POXC,indicating that surface soil organic carbon mainly existed in the form of ROC.The variation coefficients of soil POXC and ROC contents and their distribution ratios were less than 50%,indicating moderate spatial variation.Soil POXC and ROC were positively correlated with total organic carbon(P<0.01),and the correlation between ROC and SOC was the strongest(r=0.990),while the correlation between POXC and total organic carbon was relatively weak(r=0.823).There was a significant negative correlation between POXC proportion and SOC content(r=0.134,P<0.001),that is,POXC proportion in soil with higher SOC content was lower,while ROC proportion and SOC content were positively correlated(r=0.134,P<0.001).The higher ROC proportion,the higher SOC.This indicates that soil carbon sequestration depends on ROC ratio.(2)The spatial distribution trend of SOC and ROC in the study area was similar,showing a trend of decreasing first and then increasing from the west to the east,with the highest content in Leshan,Meishan,Ya ’an,Guanghan and Deyang in the southwest and Chongqing,Yibin and Luzhou in the southeast.POXC decreased from the west to the east,with the highest content in Yibin,Leshan,Chengdu and Guanghan.The high SOC content(>23.0g/kg)was mainly distributed in the western part of the Sichuan Basin,a small amount was distributed in the southern and southeastern parts of the basin,and the low SOC content(<8.5g/kg)was mainly distributed in the central and eastern parts of the basin,and a small amount was distributed in the southwestern margin areas.The high POXC content(>3.5g/kg)was mainly distributed in the western and southern parts of the basin,while the low POXC content(<2.0g/kg)was mainly distributed in the central and eastern parts of the basin.The area with high ROC content(>14.5g/kg)was mainly distributed in the west of the basin,a small amount was distributed in the south and southeast of the basin,and the area with low ROC content(<8.5g/kg)was distributed in the central area.(3)Agricultural land use is the most important factor controlling soil POXC and ROC in Sichuan Basin,and its relative importance to POXC and ROC is much greater than other factors.Average annual temperature,sand content and net primary productivity are the most important factors after agricultural land use mode,and topographic factors have less influence than other factors.The main controlling factors of soil POXC and ROC varied according to agricultural land use.For dry land,net primary productivity and average annual mild soil texture were the most important controlling factors for POXC and ROC,while average annual rainfall,topographic factors and average annual fertilizer application rate also had significant controlling effects on POXC and ROC(P<0.05).For drought-water rotation land,POXC was significantly correlated with average annual temperature,slope,silt grain,sand grain and average annual rainfall(P<0.05),and the correlation coefficient between average annual temperature and slope was large.ROC was significantly correlated with average annual temperature,slope,soil texture and net primary productivity(P<0.05),and the correlation coefficients of average annual temperature,soil texture and net primary productivity were larger.Soil POXC and ROC were significantly correlated with soil texture and net primary productivity(P<0.05),and the correlation coefficient of soil texture was larger.In general,soil POXC and ROC were more affected by external environment variables,and soil texture had greater influence on ROC than POXC.As the second controlling factor,the influence of average annual temperature on POXC and ROC content was different in different agricultural land use modes.Correlation analysis showed that the significant correlation between average annual temperature and POXC and ROC existed only in dry land and drought-water rotation land(P<0.01).The relationship between annual mean temperature and POXC and ROC was not significant(P>0.05).The correlation between average annual temperature and POXC and ROC was higher in dry land than in drought-water rotation land.This study analyzed the spatial distribution characteristics of cultivated soil POXC and ROC in Sichuan Basin,and revealed the relationship between cultivated soil POXC and ROC and various influencing factors in this area,which could provide scientific basis for regional development of effective management measures to achieve sustainable agricultural development and cultivated soil carbon sequestration.