Effects Of Plant Roots On Soil Carbon Accumulation Along Subtropical Ever-Green Forest Successions

Soil organic carbon?SOC?usually accumulates rapidly due to the continuous input of plant-derived residues along forest successions.In this process,plant roots play an important role in the accumulation and stabilization of SOC.Previous studies mainly focused on the relationships between specific root functional traits and soil carbon content along forest successions.However,the mechanistic understanding of how roots affect soil carbon accumulation is largely limited.This study took advantage of three forest successional stages?20²5 yr shrubbery,50⁵5 yr Schima superba forest,and 105¹20 yr Castanopsis fargesii and Castanopsis carlessi?in Tiantong national forest ecosystem observation and research station,Zhejiang Province,China to quantify the input,output,and net accumulation rates of SOC.By investigating the changes in organic bio-marker concentration?i.e.,cutin and suberin?,we examined the relative contribution of above-ground litter and root litter to SOC accumulation and then the influence of root traits on soil carbon accumulation.Meanwhile,the physical-biological protection mechanism of SOC by root system and mycorrhizal fungi was studied.Rhizosphere effects on soil aggregation were quantified by separating rhizosphere from bulk soils.In addition,we used root-mycorrhizae exclusion approach to explore mycorrhizal effects on SOC decomposition.Main results are as follows:?1?This study investigated soil carbon dynamics at different forest successional stages in subtropical forests from 2004 to 2018.Our results showed that the order of soil carbon accumulation rates were at early stage>middle stage>late stage.At the early stage of forest succession,SOC content in the 0-20 cm soil layer increased by41%from 2014 to 2018,and the accumulation rate was 0.82 mg g^-1 y^-1.At the middle and late stages of succession,SOC did not vary significantly over time.?2?By quantifying root functional traits and turnover,we found that root specific respiration,root specific length and fine root nitrogen content significantly increased along forest successions,while fine root biomass and turnover decreased.Meanwhile,annual litter-fall biomass increased along forest successions,whereas microbial respiration rate decreased and soil carbon retention time did not significantly change.SOC accumulation rate was positively correlated with fine root turnover and negatively with litter-fall biomass,but it did not have significant correlation with soil carbon retention time and microbial respiration.The data of bio-marker concentration?i.e.,cutin and suberin?showed that the accumulation of SOC was mainly regulated by fine-root derived C at the early and middle stages of forest succession,while it was jointly regulated by root and litter C at the late stage of succession.In addition,SOC accumulation rate was significantly correlated with root functional traits such as specific root respiration,root mass density,specific root length,and absorptive/pioneer root mass ratio.These results indicated that the changes in SOC accumulation rate along forest successions may be mainly regulated by carbon input and root functional properties,rather than above-ground litter-fall biomass and microbial decomposition process.?3?To explore the effects of roots and rhizosphere on the physical protection of soil aggregates and soil carbon accumulation,our study separated rhizosphere from non-rhizosphere soils at different successional stages of subtropical forests.The results showed that rhizosphere effect significantly improved the stability of soil aggregates at the early stage of succession,but had no significant effect at later stage.Along forest successions,the rhizosphere significantly increased SOC,soil total nitrogen?TN?,C/N,and cation exchange capacity?CEC?,and decreased the soil pH,but had no significant effect on the soil total phosphorus?TP?concentration.Soil aggregate mean weight diameter?MWD?was logarithmically correlated with SOC,TN,TP and CEC.Principal Component Analysis?PCA?showed that SOC was the main abiotic factor driving the change of stability in soil aggregates,while fine root length density and N concentration are two important biological factors.These results indicated the stronger rhizosphere effect on soil aggregates stabilization at the early stage of forest succession,which was beneficial to enhance the physical protection and accumulation of SOC.?4?To pursue the influence of plant root mycorrhizal fungi?MF?on soil carbon accumulation along forest successions,we used a trenching method with ingrowth meshes to investigate the roles of MF in mediating soil C decomposition.The results showed that MF significantly inhibited the decomposition of organic matter at the early stage of forest succession and promoted the accumulation of SOC,but increased soil C decomposition by 13.2%at the middle successional stage and by 29.2%at the late stage.At the late stage of the succession,the presence of MF increased soil respiration by 0.45?mol CO₂ m^-2 s^-1,significantly higher than that at the middle stage(0.2?mol CO₂ m^-2 s^-1),which may be due to the fact that ecto-mycorrhizal mycelia at the late stage of the succession can release more respiratory carbon compared with arbuscular mycorrhizal at the middle successional stage.The inhibitory effect of MF on SOC decomposition at the early stage may be caused by the nitrogen competition between plant mycorrhizae and microorganisms.While the nitrogen competition at the middle and late stages of succession with high soil nitrogen concentration is not significant,mycelia can promote the soil CO₂ emission by their own respiration.The presence of MF significantly inhibited the activity of microbial biomass and carbon degrading enzyme at the early successional stage,but had no significant influence at the middle and late stages.The presence of plant roots and MF significantly inhibited the mineralization of NH₄⁺-N at the early stage of forest succession,had no significant influence on the mineralization of nitrogen at the middle stage,but promoted the mineralization of NH₄⁺-N at the late stage of forest succession.The results suggested that the inhibition of mycorrhizal fungi on the decomposition of SOC may be caused by the nitrogen competition between plant mycorrhizae and microorganisms at early successional stage.In this study,above-and below-ground carbon input,microbial decomposition and SOC content at different succession stages was continuously measured to elucidate soil carbon accumulation rate with forest successions,reveal the important role of root characteristics and root-derived carbon input,and further explore the effects of root and mycorrhizal fungi on soil aggregate stability and carbon decomposition through manipulative experiments.This research further deepens our understanding of SOC accumulation and its mechanism in subtropical forests,and provides reliable parameters for the global model to effectively simulate the SOC sequestration in subtropical forest ecosystems.