Effects Of Exogenous Carbon And Nitrogen Addition On Turnover And Stability Of Soil Organic Carbon In Cunninghamia Lanceolata Plantation

The soil organic carbon（SOC）pool is the largest carbon pool in terrestrial ecosystems,with a storage capacity about 2.5 times that of the atmospheric carbon pool or 3 times that of the plant carbon pool.Plant litter is the main source of SOC formation in forests,and the chemical composition is complex,mainly composed of compounds such as carbohydrates,cellulose,hemicellulose and lignin.The different chemical structure of carbon and C:N ratio of litter remarkably affect SOC turnover and stability formation.However,the relative importance of the influence of the chemical structure complexity and quality of litter on SOC turnover and stability is still unclear.Therefore,in this study,exogenous carbon（glucose,cellulose）with different chemical structures and inorganic N were added to soils under indoor incubation conditions,and totally 7treatments were tested:control（CK）,glucose（GN0）,glucose+low level N（GNL）,glucose+high level N（GNH）,cellulose（CN0）,cellulose+low level N（CNL）,and cellulose+high level N（CNH）.Firstly,based on the ¹³C tracer method to distinguish CO₂ release accumulation and priming effects between exogenous added organic carbon and soil-sourced,in order to investigate the dynamic characteristics of soil CO₂ release rate..Secondly,we quantified the contribution of exogenous carbon accumulation in SOC,dissolved organic carbon（DOC）,microbial biomass carbon（MBC）,particulate organic carbon（POC）and mineral organic carbon（MAOC）pools.In addition,the amino sugar content of soil microbial debris markers and soil microbial enzyme activities were measured to explore the microbial mechanisms of the effects of exogenous carbon and nitrogen additions on SOC turnover.The main results and conclusions were as follows:（1）Exogenous carbon addition significantly increased the rate and accumulation of soil CO₂ emission,and the addition of cellulose reduced the accumulation of total soil CO₂ emission compared with glucose addition,but the accumulation of CO₂ emission from SOC sources was higher,showing a greater positive priming effect.In the glucose addition treatment,total soil CO₂ emission accumulation and priming effect were increased by 7.44%and 49.26%,respectively.In the treatment with added cellulose,the total cumulative CO₂ release and priming effect of the CNH treatment soil was 16.38%and 105.47%higher,respectively,compared to the GNL treatment..Therefore,the addition of high quality exogenous carbon can increase the total accumulation of soil CO₂ emission and priming effect.（2）The exogenous carbon type and C:N ratio could remarkably affect the exogenous carbon content and contribution ratio of different active carbon pools in soil.The accumulation and contribution ratio of exogenous glucose carbon in soil DOC,MBC,SOC,f POC and MAOC pools were significantly higher than those of added cellulose.However,there were great variations in the effect of N addition on the accumulation of glucose and cellulose carbon among the different soil carbon pools.Under glucose addition,the exogenous carbon source carbon content of DOC pool was in the order of GN0>GNL>GNH,while the exogenous carbon source carbon contents of DOC pool and MBC pool were significantly higher in CNL treatment than in CN0 and CNH under cellulose addition.Although Exogenous glucose C:N ratio had no significant influence on the accumulation and contribution ratio of exogenous carbon in different soil particle organic carbon pools,the exogenous carbon source carbon content of MAOC pool of CNL and CNH treatments was significantly higher than that of CN0 treatment.（3）The type of exogenous carbon significantly affected soil total amino sugar,muramic acid（Mur N）content and soil enzyme activities,with the order of Mur N content was glucose>cellulose>control.At 110 day of incubation,cellulose addition significantly increased soilβ-glucosidase and protease activities,but decreased soil cellulase and polyphenol oxidase activities compared to the control.Meanwhile,N addition significantly affected soil amino sugar content and soil enzyme activity,with a decreasing trend of soil total amino sugar and glucosamine（Glu N）content as the C:N ratio decreased.Moreover,exogenous N addition could remarkably increase soil protease activity,but the effects on other soil enzyme activities were closely related to the type and time of exogenous carbon.（4）Correlation analysis showed that total soil CO₂ emission,exogenous carbon source CO₂ and priming effect were mainly related to soil enzyme activity,which showed a significant positive correlation with soil acidic protease activity.However,at 25 day of incubation,the accumulation of exogenous carbon in each soil carbon pool was positively related to soil Mur N content;at 110 day of incubation,the accumulation of exogenous carbon in each carbon pool of soil was mainly influenced by soil acidic protease activity.This study shows that the chemical structural complexity and chemical quality（e.g.,C:N ratio）of exogenous organic carbon jointly regulate soil CO₂ emission and SOC mineralization priming effects.,as well as the contribution of exogenous organic carbon from different active carbon pools in the soil Soil organic carbon mineralization output was more influenced by soil enzyme activity,while the accumulation of exogenous carbon in each soil carbon pool was more correlated with soil microbial residual carbon content and composition.