| Strengthening the protection and restoration of ecosystems and enhancing the carbon sequestration capacity and efficiency of soils have become one of the important measures for ecological carbon sequestration.Mid-Yunnan is a key soil and water conservation area in the upper reaches of the Yangtze River,and is also an ecologically sensitive area with prominent human disturbance and severe ecological degradation.Ecological restoration projects have been implemented so far have improved forest land coverage,however,the impact of different ecological restoration communities on soil organic carbon(SOC)and its carbon sequestration processes are still unclear.Therefore,five vegetation types:shrub-grass land(SG),coniferous Pinus forest(PF),coniferous and broad-leaved mixed forest(MF),natural secondary forest(NSF),and natural forest(NF,reference vegetation)were selected,and the distribution of soil carbon pools in different plant communities was analysed combined with soil physico-chemical attributes and soil microbial communities.Meanwhile,soil carbon sequestration potential and processes under different vegetation types were also comprehensively evaluated.The main results of this study are as follows:(1)The concentrations of total organic carbon(TOC)and carbon fractions(DOC,dissolved organic carbon;LFOC,light fraction organic carbon;HFOC,heavy fraction organic carbon;POXC,permanganate oxidizable carbon;POC,particulate organic carbon)were strongly affected by vegetation types and soil depths.The concentrations of all five soil carbon fractions on the surface layer(0-10 cm)were higher than other three soil layers and were generally increased along the vegetation complexity.Pearson correlations and redundancy analysis showed that fractions of soil glomalin-related soil protein(GRSP)and Fe oxides(Fep and Feo)as well as soil bulk density were the most important factors that were significantly correlated with soil TOC and carbon fractions,suggesting soil biochemical and physicochemical processes are among the most important mechanisms contributing to SOC persistence.Considering the sensitive index of soil carbon variables and the results of PCA,soil POXC was considered the most sensitive indices for differentiating effects of vegetation types.(2)Significant effects of vegetation restoration types were found on soil aggregate composition,soil aggregate organic carbon content,aggregate nutrient stoichiometric ratio,and GRSP fractions distribution.The concentrations of carbon in soil macroaggregates(>250 μm)and clay and silt(<53 μm)were higher than those in microaggregates(53-250 μm).Under the same soil aggregation,soil aggregate organic carbon concentration under naturally restored community was higher than that under artificial plant community.Moreover,correlation analysis showed that there were the significant correlations between mass proportion of soil macroaggregate and concentrations of carbon and GRSP in microaggregate and silt and clay,and between soil carbon content in soil aggregation and nutrient stoichiometric ratio and GRSP fractions in aggregation.With the increase of nutrients,the carbon content in soil macro-aggregation increased more than those in soil micro-aggregation and clay and silt,and soil phosphorus and GRSP played the important roles in promoting the accumulation of organic carbon in soil aggregations.Isotope analysis results showed that the δ13C content in naturally restored soil was significantly lower than that in artificially restored soil in all soil layers,indicating that litter carbon input in plantations is one of the main limiting factors affecting the accumulation of soil aggregation carbon.(3)Comparing to control(CK),proportions of soil macro-aggregation and carbon contents in soil aggregation displayed the decrease trends for all three plant carbon input treatments(litter removal,root removal,litter+root removal)under PF,EF,and NSF.Litter removal significantly reduced the carbon content of both macro-and microaggregates.Soil microbial biomass also showed a downward trend,with the degree of decline being litter+root removal>root removal>litter removal.Response values of soil carbon and nitrogen root removal and litter removal in PF and EF were higher than those of NSF,indicating that plant carbon input was more important to soil carbon and nitrogen content in man-made forests.(4)There were significant differences in soil microbial communities(bacteria and fungi)under different vegetation restoration types.Significantly correlations between the dominant phyla(Ascomycota and Basidiomycota)in soil fungal community and SOC and most carbon fractions,while species diversity of soil bacterial microbial community was associated with soil carbon fractions.The carbon content of fungal residues was significantly higher than that of bacterial residues,and the carbon content of fungal residues in the natural restoration type soil was higher than that of plantation communities.The carbon content of microbial residues accounted for more than 40%of SOC,and was significantly correlated with soil carbon content,indicating that microbial residues played an important role in the accumulation and stability of soil carbon.The SEM model results showed that changes of soil enzyme activities and soil fungal communities induced by litter in different vegetation types were the main influencing factors to soil microbial residue carbon content.(5)The SEM model results showed that litter biomass of different plant communities had a higher impact on SOC than root biomass,indicating that plant litter was the main aboveground factor regulating soil carbon accumulation under different vegetation restoration conditions in central Yunnan.Soil TN content is the main soil physicochemical property that affects SOC by regulating soil microbial enzyme activity and concentrations of soil GRSP.The results indicated that GRSP and microbial residue carbon played an important role in SOC accumulation,and the soil fungal community,which was the content of GRSP and soil microbial residue carbon,was the key biological factor regulating GRSP and soil microbial residue carbon content.(6)Litter biomass and fine root biomass under the natural restoration community were significantly higher than those under the man-made restoration community.The results of redundancy analysis showed that SOC was closely related to soil microbial community and GRSP,indicating that soil carbon stock in natural remediation types was mainly controlled by soil biological processes;while SOC in man-made remediation type was closely linked with soil physico-chemical properties,indicating that SOC was mainly control by soil physico-chemical properties.In conclusion,the above research results provided evidence and a new theoretical basis for exploring the contribution of plant-derived carbon and microbial-derived carbon to soil organic matter and its driving mechanism,and also provides data support and technical guidance for the improvement of carbon sink capacity of forest ecosystems in this region. |
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