Dynamics In Soil And Microbical Carbon And Nitrogen Of Five Temperate Plantations

Tree species with different plant traits profoundly influence soil carbon(C)and nitrogen(N)cycling.Nearly all tree species can be associated with either arbuscular mycorrhizal(AM)or ectomycorrhizal(ECM)fungi.It has been reported that tree species associated with different mycorrhizal associations have different impacts on the biogeochemical progresses,but the underlying mechanisms remain uncertain.This study used a common garden experiment that included two AM(Fraxinus mandshurica Rupr.,Juglans mandshurica Maxim.)and three ECM(Betula platyphylla Suk.,Larix gmelinii Rupr.,and Pinus sylvestris var.mongolica Litv.)monocultures to explore the effects of mycorrhizal associations on the related indeces of soil C and N cycling(i.e.,plant traits,soil microbial biomass,microbial extracellular enzymes,microbial metabolic quotient(qCO2),soil respiration,soil organic C content(Csoil,and total N content(Nsoil).The objectives were to examine the effects of AM and ECM tree species on soil C and N cycling and the underlying mechanisms so as to provide scientific bases for forest management and tree species selection for afforestation.The main results were as follows:(1)Most of the plant traits were significantly different between the AM and ECM stands.Litterfall production,forest floor mass,fine root C/N ratio(C/Nroot)in the AM stands were significantly lower than those in the ECM stands,whereas the fine root biomass and decomposition rate of leaf litter in the AM stands were significantly higher than those in the ECM stands.There was no significant difference in leaf litter C/N ratio between the AM and ECM stands.Differences in these plant traits can directly and/or indirectly affect the microbial growth and activity,soil respiration,and then dynamics in the soil C and N cycling,which diverge the biogeochemical syndromes between AM and ECM stands.(2)Both the soil microbial biomass carbon(Cmic)and nitrogen content(Nmic)for the AM stands were significantly greater(50.0%)than those for the ECM stands.The Cmic was significantly and positively correlated with the fine root biomass,soil total dissolved N content(Ndis),temperature,water content,and pH,but negatively correlated with the fine root C/N ratio(C/Nroot),soil dissolved C/N ratio(C/Ndis),and forest floor mass.The best-fitted models of the Cmic explained 68.4%,66.8%,and 85.7%of the variability for the AM stands,the ECM stands,and the combined datasets,respectively.However,the factors contributing to the Cmic varied with mycorrhizal groups.The Cmic within the AM was mainly influenced by joint effects of fine root biomass,Ndis and pH,while that within the ECM ones was majorly influenced by soil water content and pH.These findings indicate that AM trees are conducive to the microbial growth,which may promote the formation of soil organic matters.(3)The activities of ?-Glucosidase(BG),N-acetyl-?-glucosaminidase(NAG)and qCO2 of soil microbes in the AM stands was significantly lower(21.2%,38.6%,and 44.1%,respectively)than those in the ECM stands,while the BG/NAG in the former was significantly higher than that in the latter.In addition,the Csoil in the ECM stands was significantly affected only by the NAQ whereas the Csoil in the AM stands was affected only by the BG.Such differences in microbial activities between the AM and ECM stands were mainly driven by their distinct effects on soil properties and microbial stoichiometry,which were indirect effects of different mycorrhizal tree species.These results suggest that the investment on N-acquisition of soil microbes for AM stands is lower than that of ECM stands,whereas the C use efficiency for the former is greater than that for the latter.Therefore,the AM tree species is more conducive to the formation of soil organic matters.(4)The Rs in the AM stands was significantly greater(34.3%)than that in the ECM stands.The Rs was significantly and positively correlated with microbial biomass C(Cmic),fine roots biomass,or soil water content,but negatively with forest floor mass or soil dissolved organic C content(Cdis).The best-fitted models of Rs explained 46.3%,38.3%,and 45.4%of the variations in the Rs for the AM stands,the ECM stands,and the combined dataset,respectively.However,the factors contributing to the Rs varied with mycorrhizal groups.The Rs of the AM stands was mainly influenced by Cmic,fine root biomass,C/Nroot,forest floor mass,and Cdis,whereas the Rs of the ECM stands was mainly affected by litter C/N ratio,Cdis,Ndis,soil temperature and water content.These findings highlight the significance of shifts in AM or ECM tree abundance due to forest management and global change in forest C cycling.(5)During the first 11 years after afforestation,both Csoil and Nsoil of the topsoil(0-10 cm)in the AM and ECM stands decreased as the stand ages increased,and their relative decreasing rates were significantly correlated with each other.The amount of change in Csoil(2.6-4.8%yr-1)was significantly greater than that in Nsoil(0.8-2.8%yr-1).During the experimental period,the decrements of Csoil and Nsoil for the AM stands were significantly lower than those for the ECM stands.The tree-species traits and microbial properties together explained 68.5%and 90.9%of the variability of the relative changes in Csoil and Nsoil during the experimental period,respectively.Specifically,the rate of changes in Csoil and Nsoil decreased with the C/N of leaf litter and C/Nmic,but increased with the fine root biomass,Cmic,and BG/NAG.The rate of change in Nsoil also decreased with leaf litter production and microbial metabolic activity.These findings indicate that the net C and N contents in the topsoil of these temperate plantations decreased during the 11 years after afforestation,while the rate of change differed with different mycorrhizal tree species through differentiating their vegetation and soil microbial properties.Collectively,AM tree species plantations are conducive to the growth of soil microbes and formation of soil organic matter and thus have a greater ability of soil C sequestration than ECM stands,because they provide more available substrates and investments in extracellular enzymes for N-acquisition and greater C use efficiency.Our findings provide evidence that mycorrhizal associations play an important role in soil C and N cycling,and are of significance for tree species selection for afforestation and prediction of the responses of forest biogeochemical cycling to forest management and global change.