Effects Of The Burial Layer Caused By Shallow Loess Deposition On Organic Carbon Mineralization And CO₂ Emission

Soil erosion-deposition is one of the most important processes affecting the biogeochemical cycling of organic carbon.Unlike traditional natural soil-forming profiles,sedimentary profiles are deposited by interlacing coarse and fine particles,with a distinct cyclonic hierarchy,and abrupt changes in coarse and fine particles and pore spaces between layers can lead to CO₂ concentration and diffusion patterns in sedimentary profiles that no longer conform to Fick’s first law.Therefore,to understand the influence of the inhomogeneous layer structure of the shallow sediment profile on the rate of organic carbon mineralization and CO₂ emission is the key to quantitatively assess the carbon emission intensity at the soil-gas interface in the erosion landscape.In this study,based on the typical erosion-deposition process of the Loess Plateau,we simulated different typical sedimentary soil structures by backfilling fine loess particles and coarse sand particles in a staggered manner,as well as different methods of sedimentation sorting and cyclonic deposition of loess particles,and combined with¹³C-labelled glucose tracing method to quantitatively investigate the effects of burial level,hierarchical structure and cyclonic number on organic carbon mineralization and CO₂ emission patterns under inhomogeneous layer structure.The main results are as follows:（1）The mineralisation of ¹³C-labelled glucose increases with deeper burial levels,but the efficiency of CO₂ transport to the soil surface can be significantly inhibited.The degree of mineralisation was 38.6%,65.1%and 50.9%for the top,middle and bottom layers,respectively,after the addition of low concentration of ¹³C-labelled glucose;and the closer to the bottom the glucose layer was added,the smaller the peakδ¹³C was,with a lag of 1 to 2 days at each addition concentration.The results suggest that the sedimentary burial effect of loess can be effective at 9 cm depth of organic carbon burial.（2）The non-homogeneous hierarchical structure formed by the interlacing of coarse and fine particles significantly inhibited CO₂ diffusion,but not glucose mineralisation.During constant temperature and humidity incubation,the mean value of apparent CO₂ release from the homogeneous mixed soil structure H0 was 84.5μg CO₂-C·g^-1,while the apparent CO₂ emission from the two non-homogeneous soil structures H1 and H2 decreased to 31.6μg CO₂-C·g^-1 and 27.3μg CO₂-C·g^-1,a significant reduction of 62.6%and 67.7%,and the gas CO₂peak ofδ¹³C also decreased and lagged;however,at the end of the 10-day incubation,soil residualδ¹³C was significantly lower in H1 and H2 than in the homogeneous mixed structure H0（with a difference of about 3-6‰）,i.e.the non-homogeneous soil structure H1 and H2 had a higher degree of ¹³C-labelled glucose mineralisation,which may have resulted in a higher CO₂ enrichment in the profile.This result indicates that the apparent CO₂emission rate of the non-homogeneous hierarchical structure is still lower than that of the homogeneous mixed soil structure in the presence of CO₂ enrichment in the profile,suggesting that the non-homogeneous hierarchical structure affects apparent CO₂emissions mainly through the inhibition of CO₂ diffusion by the fine soil layer.（3）The number of deposition couplet significantly affects the apparent CO₂emission rate.Under the same soil quality and deposition height,the higher the number of deposition couplet,the stronger the impediment to CO₂ diffusion in the CO₂ profile caused by the superposition of fine layers and the abrupt change in grain size between layers,resulting in a lower apparent CO₂ emission rate.The cumulative CO₂ emission during 20 days of constant humidity incubation was 34.2,34.0 and 29.1μg CO₂-C·g^-1for 1,2 and 3 coarse to fine deposition couplet,respectively;and after three deposition couplet,a fine layer of<32μm particle size on the surface was only 0.3 cm thick to effectively inhibit CO₂diffusion to the soil surface.In general,this thesis is based on the sedimentary sediment particle sorting and cyclotron deposition perspective,based on simulated stratification or sedimentation velocity sorting deposition,with the help of isotope tracing method,to quantitatively distinguish the influence of the typical inhomogeneous layer structure of shallow sedimentation on organic carbon mineralization and CO₂ emission,and find that different layers,hierarchical structure and cyclotron number mainly affect the CO₂emission at the soil-gas interface of the sedimentary area by inhibiting the diffusion of CO₂ profile,which can provide data support and methodological reference for quantitative assessment of the carbon source-carbon sink effect in the sedimentary area of the Loess Plateau.