Difference Of Soil Carbon Emissions Between Arbuscualr Mycorrhizal And Ecotomycorrhizal Forests And Their Influencing Mechanisms

Forest soil carbon emission is an important carbon efflux in terrestrial ecosystems,which plays a critical role in regulating global carbon balance.Since most tree species are associated with arbuscular mycorrhizal(AM)or ectomycorrhizal(EM)fungi,the differences in resource acquisition and uptake between AM and EM forests may cause distinct soil carbon dynamics(e.g.,soil carbon sequestration,soil organic carbon decomposition,and root exudates).Previous studies mainly focused on effects of mycorrihal type(AM and EM)on soil carbon pool or sequestration potential.However,whether and how mycorrhizal type influences soil carbon emission and its components(e.g.,autotrophic respiration,soil carbon decomposition)in forest ecosystems remains unclear.In this study,we firstly performed a meta-analysis to explore the difference of soil carbon emission between AM and EM forest stands in pure plantations or single-dominant forests at the global scale.To probe the effect of mycorrhizal type on soil carbon emission in subtropical forest,we conducted a field experiment to examine mycorrhizal effects on root and hyphal respiration using nylon net with different meshes in pure plantations located in Tiantong station.At the same time,we collected rhizosphere and bulk soil samples from thirty-one tree species in five subtropical forest sites to investigate mycorrhizal regulation on soil carbon mineralization rates(C_min)through an incubation experiment.The main results were as follows:(1)At the global scale,both soil carbon emission and autotrophic respiration in AM forest stands were significantly higher than those in EM ones,while soil organic carbon decomposition rate had no significant difference between AM and EM forest stands.These results suggested that the difference of soil carbon emission between AM and EM forest stands was mainly driven by the autotrophic respiration process.Moreover,soil carbon emission and autotrophic respiration in temperate AM forest stands were also significantly greater than those in temperate EM ones.However,soil carbon emission and its components had no significant difference between tropical/subtropical AM and EM forest stands owing to great variation,which impeded us to clarify the influencing of mycorrhizal on soil carbon emission.(2)Based on the long-term field monitor experiment at pure plantations in Tiantong station,Zhejiang,root respiration rate(R_r)of EM forest stands(1.13±0.24?mol CO₂ m^-2 s^-1)were significantly higher than those of AM ones(0.55±0.15?mol CO₂ m^-2 s^-1).However,there were no significant difference in R_r and hyphal respiration rates between AM and EM forest stands.Moreover,SOC,TN,p H,ammonium content,gram-negative bacterial,and specific root length(SRL)in AM forest stands were higher than those in EM ones,while root tissue density(RTD)was the opposite.Rr were correlated positively with root uptake ability(SRL and root diameter)and negatively with root(root diameter and RTD)in AM forest stands,but positively with root uptake ability and tissue thickness in EM ones.These results suggested the regulation of root traits on difference of Rr between AM and EM forest stands,but autotrophic respiration had no significant difference between different mycorrhizal forest stands resulting from the variation of hyphal respiration.(3)Based on the incubation experiment about rhizosphere and bulk soil of different mycorrhizal forest stands(AM vs.EM)in five subtropical forest sites,we found that the rhizosphere C_min was significantly higher by 15%in AM forest stands than those in EM ones,while bulk C_min had no significant difference between different mycorrhizal forest stands.Rhizosphere soil availability nitrogen(AN),nitrate,MBC,MBN,bacterial:fungi ratio(B:F),gram-positive:gram-negative bacterial ratio(GP:GN)in AM forest stands were significantly higher than those from EM ones.However,bulk soil physico-chemical properties and microbial community composition and structure had non-significant difference between AM and EM forest stands.Moreover,rhizosphere C_min increased with the decrease of B:F in EM forest stands.For predicting rhizosphere C_min,NH₄⁺·N and NO₃^-·N were most important in AM forest stands,while the phosphor lipid fatty acid(PLFA)content about fungi and saprophytic fungi were most important in EM ones.These results indicated the significant difference of C_minbetween AM and EM forest stands in the rhizosphere soil compared to bulk soil,which was mainly regulated by soil nitrogen availability and microbial community structure.In conclusion,this study examined the difference of soil carbon emission and its components between AM and EM forest stands,and explored its regulation mechanisms through the meta-analysis,long-term field monitor experiment,and incubation experiment of rhizosphere and bulk soil.At the global scale,soil carbon emission and its components had no significant difference between tropical/subtropical AM and EM forest stands.Nevertheless,the difference of root traits between different mycorrhizal forest stands leads to different root respiration rate in single subtropical forest site.The different rhizosphere process between AM and EM forest stands resulted in different rhizosphere C_min in multiple subtropical forest sites.These results would be beneficial to better understand mycorrhizal association of trees in influencing soil carbon emission especially in subtropical regions,which could be used to provide a theoretical base and scientific foundation for the forest carbon management process in the context of global climate changes.