Distribution Of Photosynthetic Carbon And Microbial Formation Mechanism Of Soil Organic Carbon Of Populus Davidiana And Quercus Wutaishanica

Forest ecosystem is an important terrestrial carbon reservoir,storing 73%of the world's terrestrial soil carbon and 71%of the global vegetation carbon,therefore,the turnover and retention of forest ecosystem carbon(C)have a profound impact on the global C cycle.Under the background of climate change,it is of great significance to further understand the C sequestration capacity of forest ecosystems to study the distribution of photosynthetic C of trees in plant-soil.Litter decomposition is the main source of forest SOC and microorganisms play a pivotal role in the process of litter decomposition.It is of great significance to clarify the mechanism of microbes on the formation of SOC in the process of exploring the turnover and retention of SOC and predicting global climate change.This study used ¹³C pulse labeling method to label the typical arboreal forest of Populus davidiana and Quercus wutaishanica in Ziwuling on the Loess Plateau in situ.After labeling,the labeled plants and soil samples were collected to explore the allocation of recent photosynthetic C in the plant-soil system.The leaf source carbon in SOC were measured to explore the effect of leaf decomposition on SOC budget.The labeled leaves were used to conduct the decomposition experiment in the laboratory to analyze the SOC conversion process and microbial turnover,combined with molecular biology(¹³C-PLFA and ¹³C-amino sugar)technology,to explore the microbial-mediated decomposition process of Populus davidiana and Quercus wutaishanica.The main research conclusions are as follows:1.The average annual net rhizosphere C of P.davidiana and Q.wutaishanica was estimated to be 4.2 g C m^-2 and 28 g C m^-2,respectively.After labeling,the leaves?¹³C of P.davidiana and Q.wutaishanica were decreased(the leaf?¹³C was 568‰and 1037‰,respectively),roots?¹³C gradually increased with sampling time.Compared with the day after labeling,the ¹³C recovery in the leaves of P.davidiana and Q.wutaishanica gradually decreased,and the ¹³C recovery in the branches,roots and soil gradually increased.The ¹³C-SOC in 0-10 cm reached the maximum value 5 days after labeling,indicating the timeliness of photosynthetic carbon transport to underground.2.With the leaves decomposition,the fluctuation of leaf carbon enrichment in SOC of P.davidiana increased,while that in Q.wutaishanica increased first and then decreased.Using the ¹³C labeled leaves to conduct the decomposition experiment,as the leaves decomposed,the ¹³C in the leaves decreases as a logarithmic form.The content of the ¹³C in SOC and humus carbon of P.davidiana and Q.wutaishanica increased with decomposition,the content of the¹³C in soil microbial biomass carbon of P.davidiana increased with decomposition.The content of the ¹³C in the soil microbial biomass carbon and dissolved organic carbon of Q.wutaishanica increased sharply and then gradually decreased.At the end of decomposition,the leaf carbon enriched in Q.wutaishanica soil is higher than that of P.davidiana.3.Leaf decomposition had positive priming effect on SOC mineralization,which increased first and then decreased with leaf decomposition of P.davidiana and Q.wutaishanica.With leaf decomposition,soil mineralization of P.davidiana and Q.wutaishanica increased first and then decreased,both of which were higher than that of the control without leaves,and soil mineralization of P.davidiana was higher than that of Q.wutaishanica soil.The proportion of CO₂ generated from leaf litters gradually increased and then gradually stabilized to about 45%.There were significant correlations between leaf residues and the rate of mineralization,and the proportion of CO₂ derived from leaf litter of P.davidiana and Q.wutaishanica.4.The soil microbial necromass carbon was mainly derived from the utilization of the leaves difficult to decompose by microorganisms of P.davidiana,while the soil microbial necromass carbon was mainly derived from the utilization of the leaves easy to decompose by soil microorganisms of Q.wutaishanica.The content of ¹³C enriched in soil fungi PLFAs,bacteria PLFAs,actinomycetes PLFAs of P.davidiana and the content of ¹³C enriched in soil bacteria PLFAs of Q.wutaishanica increased with leaf decomposition.At the end of decomposition,the maximum content of ¹³C enriched in soil fungi PLFAs in P.davidiana was higher than that in Q.wutaishanica.While,the content of ¹³C enriched in soil bacteria PLFAs of Q.wutaishanica was higher than that of P.davidiana in the 163 days decomposition process.The content of ¹³C in total microbial necromass C of P.davidiana and Q.wutaishanica ranged from 24.6 to 40.5 mg kg^-1 and 38.0 to 51.2 mg kg^-1.In terms of ¹³C content in soil microbial total necromass carbon,P.davidiana was higher at the late stage of leaf decomposition,while Q.wutaishanica was higher at the early stage of leaf decomposition.The activities of soil extracellular enzymes related to carbon transformation increased gradually with leaf decomposition,and the activities of extracellular enzymes related to nitrogen and phosphorus transformation were the highest during the middle stage of leaf decomposition of P.davidiana.The extracellular enzyme activities related to C,N and P transformation increased first and then decreased with leaf decomposition of Q.wutaishanica.The soil microorganisms of P.davidiana and Q.wutaishanica were limited by both carbon and nitrogen,and soil microorganisms of Q.wutaishanica were also limited by phosphorus at the late stage of leaf decomposition,and the soil microbial biomass of carbon,nitrogen and phosphorus were in homeostasis.The extracellular enzyme activities related to carbon transformation and the ratio of extracellular enzyme activities related to carbon and limited element transformation were the significant factors affecting the ¹³C content of PLFA in soil of P.davidiana and Q.wutaishanica.5.The leaf decomposition source carbon enriched by soil microorganisms in Q.wutaishanica was higher than that in P.davidiana.The ¹³C-PLFA accounted for 0.0006%-0.0027%and 0.001%-0.006%of ¹³C-SOC,the ¹³C in soil total microbial necromass C accounted for 2.72%-4.19%and 2.65%-20.08%of ¹³C-SOC of P.davidiana and Q.wutaishanica,respectively.The SOC formation efficiency was obtained by proportional the increase of SOC content in the process of leaf decomposition to the decrease of leaf litter C content.The soil SOC formation efficiency of P.davidiana and in the early stage of leaves decomposition of Q.wutaishanica were basically consistent with the ¹³C change of microbial enrichment.While,in the late decomposition of Q.wutaishanica stage,the SOC formation efficiency increased sharply but the ¹³C enriched in microorganisms did not increase.The structural equation model results showed that during the decomposition process of P.davidiana leaf(high C:N),soil organic carbon and soil C:N are the main factors that guide the action mechanism of soil microorganisms in the process of leaf decomposition,while,during the decomposition process of Q.wutaishanica leaves(low C:N),the leaf litter remaining mass and the ¹³C content in the leaves are the main factors affecting the action mechanism of soil microorganisms.In the leaf decomposition process of P.davidiana and Q.wutaishanica,the formation of soil organic carbon was dominated by the theoretical model of"carbon pump".The leaves of Q.wutaishanica mainly promote the formation of soil organic carbon by physical transport at the later stage of decomposition,so it can be explained by the"biochemical and physical pathways"model.