Effects Of Vegetation Restoration On Soil Carbon And Nitrogen Sequestration And Their Responses To Climatic Gradient In A Subtropical Karst Region,southwest China

The karst area in Southwest China is one of the main fragile areas of the ecological environment in China.Vegetation restoration is an effective way to improve the ecological environment and enhance the retention of soil carbon(C)and nitrogen(N)in fragile ecosystems.However,the dynamic changes of soil C and N in different vegetation restoration methods in the karst region and the understanding of the response to water and heat gradients are still insufficient.In this paper,karst cultivated land and mature forest are used as controls.Natural restoration(natural restoration of vegetation after cultivated land abandonment)and artificial restoration(artificial afforestation)are the research objects,analyze the characteristics of C,N,and dissolved organic carbon(DOC)in the underlying soil(0-15 cm)with water and heat gradients of the four land use methods,clarify the effect of regional water and heat conditions on artificial restoration and natural restoration of soil carbon and nitrogen fixation.Combining the changing characteristics of topographic factors,soil physical and chemical factors and microbial factors,the main controlling factors of soil carbon and nitrogen fixation and the control mechanisms of water and heat conditions of different vegetation restoration methods are revealed.The main findings are as follows:(1)Vegetation restoration has a significant impact on the basic physical and chemical indicators of the soil.Compared with cultivated land,vegetation restoration reduced the soil p H value,specifically as cultivated land>plantation forest>abandoned land>mature forest.At the same time,compared with cultivated land,after vegetation restoration,the surface soil sand content increased,clay content decreased,bulk density decreased.This change is conducive to increasing soil porosity and permeability,improving the living environment of soil organisms and improving soil fertility.The content of exchangeable Ca²⁺and Mg²⁺in plantation forest and abandoned land is significantly higher than that in cultivated land,but lower than that in mature forest.It shows that vegetation restoration can significantly increase the content of Ca²⁺and Mg²⁺,but there is no significant difference between the plantation forest and abandoned land.(2)Vegetation restoration significantly increases soil C and N content and reserves,soil C and N contents and stocks are shown as mature forest>abandoned land>plantation forest>cultivated land,and mature forest is significantly higher than abandoned land and plantation forest,abandoned land and plantation forest is significantly higher than cultivated land,between abandoned land and plantation forest,the difference was not significant.The soil DOC performance is consistent with C and N,but there is no significant difference between cultivated land,plantation forest and abandoned land,and mature forest is significantly higher than the three.After the vegetation is restored,the surface litter and animal and plant residues of the plantation forest,abandoned land and mature forest will be much higher than the cultivated land,so that the active organic carbon that can be directly decomposed and used in the surface soil increases,which is significantly higher than the cultivated land.Although there is no significant difference between abandoned land and plantation forest,compared with the single vegetation type of plantation forest,the abandoned land has a higher plant diversity and a higher amount and type of surface litter,which is more conducive to the improvement of active organic carbon.DOC:SOC is expressed as mature forest>abandoned land>plantation forest,the difference is that cultivated land is higher than plantation forest and abandoned land.Cultivated land under the disturbance of long-term human farming activities,soil C and N are continuously lost,vegetation restoration measures can effectively promote the increase of soil C and N content and stocks,but it is still significantly lower than that of mature forests.It shows that vegetation restoration in karst area has great potential for carbon and nitrogen retention.(3)The regression analysis results show that,the content and stocks of soil C and N in karst area are mainly affected by temperature.Among them,the C and N contents and stocks of cultivated land decreased significantly with increasing temperature.The C and N contents and stocks of plantation forest,abandoned land and mature forest were not related to temperature.It shows that the disturbance of farming in the area with higher temperature will aggravate the loss of soil C and N,and vegetation restoration can effectively reduce the temperature sensitivity of soil C and N.Soil activated carbon pool components such as DOC are also mainly affected by temperature,and DOC in abandoned land and mature forest is significantly negatively correlated with temperature,this means that the higher the temperature,the stronger the microbial activity,and the more easily the activated organic carbon can be directly used.(4)The analysis found that except for altitude,the other topographic factors(bare rock rate,soil depth)have no significant correlation with soil C and N,it shows that on the regional scale,topographic factors have little effect on C and N retention,and altitude mainly affects C and N retention by affecting temperature.Further analysis found that the soil C and N content increased with the increase of sand content,the C and N stocks of abandoned land and mature forest also had a significant positive correlation with the sand content.Among the soil exchangeable cations,except for cultivated land,exchangeable Ca²⁺is significantly positively correlated with C and N content and stocks.The exchangeable Ca²⁺of different land use methods was significantly negatively correlated with soil DOC:SOC.Soil bacteria and fungi were significantly positively correlated with soil C and N content and stocks,and significantly negatively correlated with DOC:SOC.Sand content,exchangeable Ca²⁺content and microbial community abundance are the main reasons for the increase of soil carbon and nitrogen,among which microbes dominate.The soil sand content,exchangeable Ca²⁺content,bacterial and fungal abundance of plantation forest,abandoned land and mature forest are higher than that of cultivated land,and it is significantly positively correlated with soil carbon and nitrogen.These are conducive to improving the stability of the soil carbon pool and increasing the source of soil carbon input,which can offset the decomposition of soil carbon pool by microorganisms in areas with higher temperature to a certain extent.In summary,when the karst area is disturbed by farming,soil carbon and nitrogen are largely lost,and it decreases significantly with increasing temperature.Both artificial restoration and natural restoration can significantly improve the soil carbon and nitrogen retention capacity,and can effectively reduce the sensitivity of soil carbon and nitrogen to temperature changes.Soil sand content,exchangeable Ca²⁺content,abundance of bacteria and fungi are the main reasons for the significant increase of soil carbon and nitrogen and the decrease of temperature sensitivity after vegetation restoration.Therefore,vegetation restoration is a practical ecological restoration measure in the karst area.