Effects Of C:N Stoichiometry Of Plant Litter On Soil Carbon Balance

Soil is the largest organic carbon pool in terrestrial ecosystems,and its small changes may cause great changes in atmospheric CO₂concentration.The input of plant litter can affect the decomposition rate of soil organic carbon（SOC）through a positive or negative priming effects,which has a great impact the balance of soil carbon pool.At present,few studies have investigated the effects of litter stoichiometry on soil priming effect and soil carbon balance.In this study,we sampled the soil from coniferous and broad-leaved mixed forest in Wuyishan and investigated the influence of different C/N stoichiometric ratios after fertilization and no fertilization treatment derived from roots,stems,leaves of Cyclobalanopsis glauca and Phoebe bournei on the priming effect and soil carbon balance.This study contained 13 treatments（i.e.,2 plants types×3 plant tissues×2 C/N ratio+1 control=13）.The effects of different C/N stoichiometric of plant on soil priming effect and soil carbon balance were studied.Moreover,we also determined the influences of C/N stoichiometric of plant on soil microbial biomass,soil enzyme activities and soil available N to clarify the underlying mechanism.The results showed that:（1）Inputs of plant litter to the soil increased the total soil CO₂emissions,and the total CO₂ emissions caused by plant leaf treatment were greater than those of plant stems and roots.In Cyclobalanopsis glauca,the cumulative CO₂ emission from high C/N ratio Cyclobalanopsis glauca leaves was the highest,while the cumulative CO₂ emission from high C/N ratio Cyclobalanopsis glauca roots was the lowest.In Phoebe bournei,the cumulative CO₂ emission from low C/N ratio Phoebe bournei leaves was the highest,while the cumulative CO₂ emission from low C/N ratio Phoebe bournei roots was the lowest.From the perspective of litter mineralization,the CO₂ emission of Phoebe bournei leaves with low C/N ratio was the highest,and the CO₂ emission of Cyclobalanopsis glauca roots with high C/N ratio was the lowest.The mineralization rate of each organ of the two plants was in the order of leaf>stem>root.Plant-derived CO₂ emissions accounted for about 16%-29%of the total amount of soil CO₂ emissions.（2）During the whole incubation period,the input of Cunninghamia lanceolata leaves induced a positive priming effect.while the input of other litters induced a positive priming effect at the early stage of incubation,and then shift from positive priming effect to negative priming effect,and inhibited the decomposition of native soil organic carbon.Soil cumulative priming was affected by plant species and plant tissues,but not by different C/N ratios.The cumulative priming effect among different tissues was in the order of root>stem>leaf.From the perspective of the intensity of the negative priming effect,the high C/N ratio Phoebe bournei leaves significantly reduced the mineralization of SOC,and the intensity of the negative priming effect was greatest compared with other treatments.The cumulative priming effect from Cunninghamia lanceolata tissues were greater than that of Phoebe bournei.（3）The input of Cunninghamia lanceolata leaves caused a positive priming effect,and’microbial nitrogen mining’may be its mechanism;other litter input caused negative priming effect,and’preferential substrate utilization’may be the main mechanism.Litter input generally reduced the content of soil available nitrogen were generally reduced.Soil cellulase activity,soilβ-glucosidase activity and soil catalase activity were affected by plant tissues.MBC,¹³C-MBC,soil NAG activity and soil phenoloxidase activity were not only affected by plant tissues,but also affected by plant species.From the structural equation model,we found that changes in ¹³C-MBC,soilβ-glucosidase,C/N,soil cellulase,soil catalase and soil available N could nitrogen explain 73%of the variation of soil positive priming effect.In this study,the positive priming induced by Cunninghamia lanceolata leaves may be that exogenous carbon cannot meet the N requirements for microbial growth and metabolism,which may lead soil microbes to decompose nitrogen-containing soil organic matter to meet N requirements,thus accelerating the decomposition of native soil organic carbon,causing positive priming effect,so the’microbial nitrogen mining’may be the mechanism.Other litter input changed ¹³C-MBC,soilβ-glucosidase,MBN,soil phenoloxidase,soil cellulase,lignin/N ratio and soil available nitrogen could explain 65%of the negative priming effect.This indicated that the input of other litters can accelerate the decomposition of litters and reduce the decomposition of native SOC.Therefore,’preferential substrate utilization’may be the main mechanism.（4）From the perspective of carbon balance,litter addition is beneficial to the storage of soil organic carbon.The soil carbon balance under the root amended treatments were greater than that under the stems and leaves amended treatments.Therefore,plant root input is more favorable for soil organic carbon sequestration than other tissues.From the perspective of the distribution of exogenous carbon in each carbon pool,the proportion of exogenous carbon remaining in the soil was the largest（18%-39%,moreover,most of the exogenous carbon remained in the soil was as particulate organic carbon）,followed by their emitted from the soil in the form of CO₂（16%-36%）.There were about 9%-24%of exogenous carbon was incorporated into microbial biomass.