Study On Soil Organic Carbon Stability By Land Use Mode In Karst Area

The composition of soil organic carbon and its stability mechanism is the key to understanding the terrestrial carbon sink capacity.The stability of soil organic carbon in the karst ecosystem greatly affects the soil carbon fixation capacity.To understand the impact of human activities on the stability of soil organic carbon in karst areas,the karst valley area of Zhongliang Mountain in Chongqing was selected as an example,and soil samples of four typical land use modes(mixed forest,bamboo forest,grassland,and cultivated land)were collected in layers to analyze the total organic carbon(TOC),heavy faction organic carbon(HFOC)The distribution characteristics of light fraction organic carbon(LFOC),labile organic carbon(LOC),and recalcitrant organic carbon(ROC)are analyzed quantitatively by using structural equation model to provide basic data for soil carbon sink assessment and soil quality protection in karst areas.The results show the following information:(1)Human farming activities have caused an increase in soil bulk density,and a decrease in p H,water content,total nitrogen,and carbon-nitrogen ratio in the study area.The average bulk density of cultivated land was 1.4 g·cm~3,which is 10.71%higher than that of forest land and 8.5%higher than that of grassland.The mean p H value of cultivated land was 6.94,which is 3.47%lower than that of forest land(P<0.01)and 0.85%lower than that of grassland(P>0.05).The average water content was 19.7%,which is 27.57%lower than that of forest land(P<0.01)and 15.26%lower than that of grassland(P<0.01).The mean total nitrogen content was 1.6 g·cm~3,which is 44.83%lower than that of forest land(P<0.01)and 8.04%lower than that of grassland(P>0.05).The carbon-to-nitrogen ratio was 13.75 on average,which is 31.86%lower than that of forest land(P<0.01)and13.3%lower than that of grassland(P<0.05).(2)Reduction of soil organic carbon,labile organic carbon,recalcitrant organic carbon,light fraction organic carbon,and heavy fraction organic carbon caused by land use change in the study area.The mean values of soil organic carbon in mixed forests,bamboo forests,and grasslands are 3.17,2.74,and 1.58 times higher than those in cultivated land,respectively.The mean values of labile organic carbon in mixed forests,bamboo forests,and cultivated land are 3.07,2.83,and 2.16 times higher than those in cultivated land,respectively.The mean values of recalcitrant organic carbon in mixed forests,bamboo forests,and cultivated land are 2.27,1.91,and 1.58 times higher than those in cultivated land,respectively.The mean values of light fraction organic carbon in mixed forests,bamboo forests,and cultivated land are 3.52,4.06,and 0.24 times higher than those in cultivated land,respectively.The mean values of heavy fraction organic carbon in mixed forests,bamboo forests,and cultivated land are 1.45,1.53,and 1.32 times higher than those in cultivated land,respectively.In the same land use,the recalcitrant organic carbon,labile organic carbon,heavy fraction organic carbon,and light fraction organic carbon are all significantly positively correlated with total soil organic carbon(P<0.001),and the correlation between recalcitrant organic carbon content and total soil organic carbon is the highest,with an R~2 of 0.93.The content of soil organic carbon and its components under different land use types exhibits obvious aggregation,with a significant decrease in content and a decrease in differences with increasing soil depth.(3)Under the four land use types in the study area,total soil organic carbon is significantly positively correlated with soil water content and total nitrogen(P<0.01),and significantly negatively correlated with bulk density(P<0.01).Light fraction organic carbon,labile organic carbon,and C/N ratio are significantly negatively correlated(P<0.01),while recalcitrant organic carbon,heavy fraction organic carbon,and C/N ratio are significantly positively correlated(P<0.05).(4)The results of the soil organic carbon stability index(ROCI)show that the order of ROCI variation in the upper and lower soil layers of the four land use types is:mixed forest>bamboo forest>fallow land>cultivated land.The ROCI of organic carbon in mixed forest and bamboo forests is significantly higher than that in fallow land and cultivated land,and the ROCI of organic carbon in cultivated land is the lowest.In addition,there is no significant difference in the ROCI of organic carbon stability between the upper and lower soil layers,indicating that the ROCI of organic carbon stability is relatively stable in the 0-40cm soil layer.(5)The structural equation model analysis shows that human activities promote the decomposition rate of soil organic matter in karst areas.Soil bulk density has a significant negative effect on soil total organic carbon(λ=0.23,P<0.05),soil water content plays a certain promoting role in total soil organic carbon(λ=0.62,p<0.01),and the C/N ratio plays a certain promoting role in recalcitrant organic carbon(λ=0.44,P<0.01).Light fraction organic carbon,labile organic carbon,and C/N are significantly negatively correlated(P<0.01),while recalcitrant organic carbon,heavy fraction organic carbon,and C/N ratio are significantly positively correlated(P<0.05).This study helps understand the process,intensity,and mechanism of the impact of human activities on the stability of soil organic carbon,which is crucial for maintaining the stability of soil ecosystems and restoring fragile ecosystems that have been degraded.