Abstract: Soil microorganisms and enzyme activities play a crucial role in the quality of the soil environment and the sustainable use of soil nutrients. In order to further reveal the mechanisms of soil microbial biomass, soil enzyme activity, and their vector characteristics in response to litter addition and removal during the natural succession stage of the Loropetalum chinense community in karst areas, this study used a spatial instead of temporal research method to select the understory soil of different stages of natural succession of the L. chinense community as the research subjects. Litter addition and removal experiments were conducted in the early stage (shrub stage), middle stage (arbor stage), and late stage (aged forest stage) of natural succession of the L. chinense community. The effects of apophyte addition and removal on soil microbiomass carbon, nitrogen and phosphorus, soil accessible carbon, nitrogen and phosphorus enzyme activities and their stoichiometric ratios and vectorial characteristics in fringe flower communities were analyzed, and the correlation with soil physico-chemical properties was also analyzed to explore the main driving factors affecting changes in soil enzyme activities and their stoichiometric ratios and vectorial characteristics. The results showed that: the MBC content and QMB value in the late successional stage of fringe flower community were significantly higher than those in the pre-successional and mid-successional stages; the MBN and MBP contents in the pre-successional stage were significantly lower than those in the mid-successional and post-successional stages; apomictic and root removal significantly reduced the soil water content in the pre-successional stage of the fringe flower community as well as the organic carbon and total nitrogen contents in the three successional stages when compared with the control; the root and contents of the root removal treatment were lower than those of the napier removal; and the doubled napier addition significantly increased the SOC, TN, microbial biomass carbon, microbial biomass nitrogen, and microbial biomass phosphorus contents of the three successional stages; NL, NR, NI, and DL treatments significantly reduced the NAG activity in the late stage of succession and the AP activity in the early stage of succession; NI treatment significantly increased the AP activity in the middle stage of succession, while NR treatment significantly increased the AP activity in the later stage of succession; The average value of soil extracellular enzyme C/N was 0.32, the average value of C/P was 1.21, and the average value of N/P was 4.47. The vector angle of soil extracellular enzymes in the karst fringe flower community was lower than 45° under all treatment conditions, indicating that N was the main limiting factor affecting soil nutrients in this community. The redundancy analysis showed that the main factors affecting the activity of soil enzyme β-1,4-glucosidase were SOC, TN, pH and QMB; the main factors affecting the activity of soil enzyme NAG were the microbial quotient, SOC and TN; and the main factors affecting the activity of soil enzyme AP were soil MBP, MBN and TP. This study showed that the natural succession stage of fringe flower community and apomictic treatment had certain influence on soil microbial amount, enzyme activity and soil nutrient limitation in karst area.

Key words: litter, soil enzyme activity, microbial biomass, stoichiometric characteristics, Loropetalum chinense