The Magnitude Of Rhizosphere Priming Effect And Its Underlying Mechanisms Along Succession Of Subtropical Evergreen Broad-leaf Forest

Roots and the associated rhizosphere microbial activities crucially regulate the mineralization of native soil organic carbon（SOC）,which is referred to as the rhizosphere priming effect（RPE）.The RPE has emerged as a critical driving factor for dynamics of SOC pool.However,how the RPE modulate soil C accumulation along forest successions and its underlying mechanisms are still not fully comprehended,which seriously hindering the accurate assessment of terrestrial ecosystem carbon dynamics and their response to future climate change.In this study,three treatments were set with different pore-size nylon nets in the growth rings to differentiate co-growth of plant roots and mycorrhizal fungi,only mycorrhizal fungi growth and no plant roots and mycorrhizal fungi growth along two successional gradients of subtropical evergreen broad-leaf forests in Tiantong national forest ecosystem observation and research station.We investigated the changes of soil respiration and its components along forest successions combined with long-term field monitoring of CO₂flux,and the RPE would be quantified using the soil column replacement and ¹³C natural abundance method.We investigated the relationships between RPE and biotic factors such as root traits and soil microbial community structure.Meanwhile,we also explored how the abiotic factors,including soil physicochemical properties and soil aggregate stability regulate the RPE.The main results are as follows:（1）The soil respiration rate gradually decreased along the succession of subtropical evergreen broad-leaved forests,from 7.14μmol CO₂·m^-2·s^-1 in the early stage to 6.66μmol CO₂·m^-2·s^-1 in the late stage.The heterotrophic respiration rate decreased from 6.24μmol CO₂·m^-2·s^-1 in the early to 4.76μmol CO₂·m^-2·s^-1 in the late stage,and its proportion to soil respiration also decreased accordingly.As the forest succession progressed,the direction of the RPE changed from negative priming effect（-12.86%）in the early stage to positive priming effect（7.13%）in the late stage,which had an important impact on the changes in soil respiration and its components along forest succession.Mycorrhizal fungi played an important role in the RPE.（2）Along the succession of subtropical evergreen broad-leaf forest,the mean weight diameter（MWD）of soil aggregate increased from 2.31 mm in early stage of succession to 2.65 mm in late stage of succession,and the ratio of dissolved organic carbon to nitrogen increased from 1.14 in early stage of succession to 2.07 in late stage of succession.Both of them were significantly positively correlated with the RPE.The biomass of AM fungi was the largest(0.2 nmol g^-1)at the early stage and showed a downward trend along forest succession,which was significantly negatively correlated with the RPE,while the ratio of gram-positive and negative bacteria showed an opposite trend,which was significantly positively correlated with the RPE.The content of fine root nitrogen increased gradually from 5.5 mg g^-1in early stage to 11.54 mg g^-1in late stage,which was positively correlated with the RPE,while the root diameter showed a downward trend,with a minimum of 0.44 mm in the late stage of succession,which was negatively correlated with the RPE.In general,along the succession of subtropical evergreen broadleaved forests,the RPE was mainly regulated by abiotic factors such as soil physicochemical properties and soil aggregate stability,as well as biotic factors such as root traits and soil microbial community structure.At the early and middle stages of forest succession,the RPE was mainly regulated by microbial driving mechanism,while at the late stage,the importance of physical protection mechanism and root regulation has increased.In summary,this study focused on the dynamic changes of soil respiration and its components,as well as the RPE along the succession of subtropical evergreen broad-leaved forests.The main regulatory mechanisms of the RPE were explored from the perspectives of biotic and abiotic factors.This study can help us better understand the underlying mechanisms of soil organic carbon accumulation and stability,and improve the performance of the terrestrial ecosystem models.These findings also offer scientific basis for forest C sequestration management.