Effects Of Degraded Forest Restoration On Soil Aggregate Stability And Organic Carbon In Subtropical Region

The restoration of degraded forests has always been the focus of attention in forestry,ecology and other fields.Good forest soil structure not only helps forest growth and development,but also plays an important role in maintaining soil fertility and carbon sequestration.As the basic structural unit of soil,the composition and stability of soil aggregates are important indicators of soil fertility.The binding agents of aggregate are one of the necessary conditions for the formation of aggregates.However,the combined mechanism of their influence on the stability of aggregate and organic carbon still unclear.Subtropical forests in China have the widest distribution area in the world,with unique vegetation types and structures,but some forests have been degraded.Therefore,in this study,the effects of restoration on soil nutrients,soil aggregates composition and stability,organic carbon of aggregates and their main binding agents(soil Fe and Al oxides,mycorrhizal density and glomalin-related soil protein contents,etc.)in dry and wet seasons were investigated in different restoration types of degraded forests in subtropical region,that is,natural forests,coniferous plantations,broad-leaved plantations and mixed broadleaf-conifer plantations.The aim was to provide theoretical and practical basis for optimizing soil structure,soil fertility and carbon sequestration in degraded forests,and mitigating climate change.The main results were as follows:(1)Artificial restoration of degraded forests in subtropical region significantly affects soil moisture,bulk density,organic carbon,carbon nitrogen ratio and available phosphorus content compared to natural restoration(P<0.05).The soil bulk density,organic carbon,carbon nitrogen ratio and available phosphorus in broad-leaved plantations and mixed broadleafconifer plantations were significantly lower than natural restoration,but the soil moisture and carbon nitrogen ratio of coniferous plantations were significantly higher than natural restoration(P<0.05).There was a significant positive correlation between soil moisture,bulk density,organic carbon,carbon nitrogen ratio,total phosphorus and available phosphorus(P<0.05).It is shown that the restoration type of degraded forest in subtropical region significantly influences soil physicochemical properties and artificial restoration is beneficial to improve soil laxity.(2)Compared with natural restoration,artificial restoration of degraded forests in subtropical region significantly increased the proportion of >2 mm aggregate and the mean weight diameter of soil aggregate(P<0.05).The trend of forest types was broad-leaved plantations > mixed broadleaf-conifer plantations > coniferous plantations > natural forests,and the proportion of 2 mm aggregate in wet season was significantly higher than that in dry season(P<0.05).Meanwhile,artificial restoration of degraded forests significantly affected soil Fe and Al oxides,mycorrhizal density,and easily extractable glomalin-related soil protein(EEG)(P<0.05).Among the forest types,soil Fe and Al oxides were the highest in coniferous plantations,mycorrhizal density and total glomalin-related soil protein(TG)were the highest in broad-leaved plantations,and EEG was the highest in natural forests.Soil Al oxides,mycorrhizal density and TG in wet season were significantly higher than in dry season(P<0.05).Correlation analysis showed that the mean weight diameter of soil aggregate was significantly positively correlated with Fe oxides,mycorrhizal density and TG,and significantly negatively correlated with EEG(P<0.05).Redundancy analysis further indicated that there was interaction between inorganic and organic binding agents in the aggregate.In general,artificial restoration can improve the soil aggregate stability of degraded forests in subtropical region through inorganic and organic binding agents.The aggregate stability is affected by inorganic and organic binding agents in dry season,but mainly by organic binding agents in wet season.(3)Compared with natural restoration,artificial restoration of degraded forests in subtropical region significantly affected soil >2 mm and 1-2 mm aggregate organic carbon(P<0.05),but had no significant effect on soil >0.25 mm and <0.25 mm aggregate organic carbon(P>0.05).The >2 mm and 1-2 mm aggregate organic carbon and the contribution rates of >2 mm aggregate organic carbon to soil organic carbon in artificial restoration were significantly higher than natural restoration(P<0.05).With the decrease of aggregate size,the soil aggregate organic carbon and the contribution rates of aggregate organic carbon to soil organic carbon in all forest types were decreased first and then increased.In addition,the ratio of soil aggregate organic carbon to soil organic carbon in broad-leaved plantations and mixed broadleaf-conifer plantations was significantly higher than that in natural forests.The 1-2 mm and 0.25-1 mm aggregate organic carbon and their contribution rate of aggregate organic carbon to soil organic carbon of forest types in wet season was significantly higher than in dry season,but the proportion of aggregate organic carbon to soil organic carbon was significantly lower than in dry season(P<0.05).Correlation analysis and redundancy analysis showed that the aggregate organic carbon of each particle size was significantly positively correlated with the soil aggregate stability,soil Fe oxides,mycorrhizal density and EEG(P<0.05),suggesting that the soil aggregate stability mainly depended on the adhesion of aggregate binding agents and organic carbon,and the aggregate organic carbon was inseparable from the aggregate structure.In conclusion,the use of broad-leaved plantations or mixed broadleaf-conifer plantations in artificial restoration of degraded forests in subtropical region can improve the stability of soil structure and increase the organic carbon content of soil aggregates and the carbon sequestration capacity of aggregates in the study area.In conclusion,artificial restoration of degraded forests in subtropical region can help to increase the soil aggregate stability,especially broad-leaved plantations or mixed broadleafconifer plantations can improve the number of large aggregate and the carbon sequestration capacity of aggregate,and can be used as a better forest restoration type in the study area.The main mechanism is that artificial restoration can significantly affect soil Fe oxides,mycorrhizal density and extractable glomalin-related soil protein.These binding agents not only improve the aggregate structure of soil,but also strengthen the fixation of soil organic carbon by aggregate.