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Soil Organic Carbon Mineralization And Sequestration And Its Microbial Influencing Mechanisms Under The Driving Of Water Erosion And Vegetation Restoration On The Loess Plateau

Posted on:2020-06-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:H B XiaoFull Text:PDF
GTID:1360330590477948Subject:Soil science
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Water erosion?erosion for short?and vegetation restoration of degraded cropland?vegetation restoration for short?are the important driving factors of soil carbon?C?dynamic,which have significant impacts on the sequestration and mineralization of soil organic carbon?SOC?.In the past decades,SOC dynamic induced by erosion and vegetation restoration have been extensively studied,but there is still a lack of understanding of the role of soil microorganisms in them.Therefore,further study on the SOC mineralization and fixation potential dominated by soil microorganisms in erosional and vegetation restoration environemnts of the Loess Plateau and its intrinsic relationship with soil biotic and abiotic factors are of great importance for revealing the role of soil erosion in the global C cycle and exploring the role of microbes in the soil C dynamic.In this study,the typical dam-controlled small watershed-Qiaozigou watershed was selected as the research object.The qPCR,high-throughput sequencing and 13C stable isotope labeling were used to study the SOC mineralization and fixation potential dominated by soil microorganisms in the erosional and vegetation restoration system.The main conclusions are as follows:?1?The influence mechanism of soil microorganisms on SOC mineralization in slope erosional and depositional areas was elucidated.The results showed that lower bacterial abundance was observed in the down-slope site?depositional site?relative to the upper-and middle-slope sites?erosional sites?,while there was no significant difference in bacterial species diversity and community composition.The SOC mineralization rate in down-slope site is 19.02 mg CO2-C kg-1 d-1,which is 1.26 and1.07 times of that in upper-and middle-slope sites,respectively.The SOC mineralization ratio shows middle-slope(0.082 g CO2-C g-1 SOC)>upper-slope(0.070g CO2-C g-1 SOC)>bottom-slope(0.064 g CO2-C g-1 SOC).Erosion-induced destruction of soil aggregates increases the risk of SOC mineralization,but the lateral migration of SOC leads to a decrease in CO2 release rate.Multiple stepwise regression analyses showed that available nitrogen was the main explanatory factor for the variation in SOC mineralization?60.2%,P=0.009?.Relative to bacterial abundance and species diversity,labile organic matter is the more important regulator for SOC mineralization.Microbial communities in soil appear to be characterized by a high functional redundancy,and erosion-induced moderate changes in microbial abundance and species diversity did not significantly affect SOC mineralization.?2?The study pointed out spatial redistribution of labile organic carbon induced by gully erosion can effectively change the autotrophic bacterial community composition,and then determine the microbial C-fixing potential in erosional and depositional sites.The results showed that autotrophic bacterial abundance and species diversity in slope cropland is 1.70 and 1.10 times of those in check dam,respectively.Gully erosion induced deposition of nutrient-poor soils significantly reduced the total autotrophic bacterial abundance and species diversity in check dam,while the relative abundance of obligate autotrophic bacteria,such as Thiobacillus,were significantly increased.In addition,the microbial C-fixing rate in check dam is 5.002 Mg C km-2 yr-1,which is 4.67 times of that in slope cropland.Microbial C-fixing rate was negatively correlated with SOC,relative abundances of most facultative autotrophs,total autotrophic bacterial abundance and Shannon index,while positively correlated with the relative abundances of most obligative autotrophs.Obligative autotrophs may be the major‘contributor'to microbial CO2 fixation.Multiple stepwise regression analyses showed that dissolved organic carbon?DOC?was the main explanatory factor for the variation in microbial C-fixing rate?72.0%,P=0.000?.Gully erosion can effectively change the autotrophic bacterial community composition?such as the proportion of obligative autotrophs?by affecting the spatial distribution of labile organic carbon,and then determine the microbial C-fixing potential in erosional and depositional sites.?3?It reveals that soil microbe is the main‘undertaker'of SOC mineralization rather than the key regulator of it in the process of vegetation restoration.The results showed that bacterial abundance in the secondary garssland(1.47×107 copies g-1)was significantly lower than that in the cropland(8.39×108 copies g-1),while the fungi:bacteria in the secondary grassland was 7.68 times higher than that of cropland.Microbial communities transitioned from bacteria-dominant to fungi-dominant communities during vegetation restoration.Additionally,SOC mineralization rate in the secondary grassland was 1.29 times higher than that in the cropland,while the soil C mineralization ratio showed the opposite trend.Although vegetation restoration reduces the risk of SOC being mineralized,the increase in SOC during vegetation restoration significantly enhances the CO2 release rate from soil.Multiple stepwise regression analysis showed that DOC explained up to 68.5%of the variation in SOC mineralization.Labile organic matter is the primary rate-limiting factor for SOC mineralization rate,and soil microbe may be the main‘undertaker'of SOC mineralization rather than the key regulator.?4?It clarified that great transpiration loss induced by vegetation restoration inhibits autotrophic bacteria growth,and which further reduced the potential of CO2fixation by autotrophic bacteria.Although the large input of plant debris and root exudates during vegetation restoration contributed to the improvement of soil C and nitrogen?N?pools,the huge transpiration loss induced by Coronilla varia plant reduced the soil moisture content.The reduction in soil available water inhibited the rapid growth of autotrophic bacteria,resulting in the abundance and species diversity of autotrophic bacteria in vegetation restoration sites were significantly lower than those in erosional sites.Microbial C-fixing rate in abandoned cropland(1.114 Mg C km-2 yr-1)was significantly?P<0.05?higher than that in secondary grassland,and it was positively influenced by soil moisture,autotrophic bacterial abundance and diversity.Overall,vegetation restoration of eroded agricultural land has negative impacts on autotrophic bacterial community and microbial C-fixing potential.In arid and semi-arid regions,great transpiration loss induced by vegetation restoration may inhibit autotrophic bacteria growth,and which further reduces the potential of CO2 fixation by autotrophic bacteria.In general,SOC mineralization in erosional and vegetation restoration system is mainly regulated by labile organic carbon,and microorganisms show obvious functional redundancy.soil microorganism is the main‘undertaker'of SOC mineralization rather than the key regulator.Furthermore,microbial C-fixing rate is positively correlated with the relative abundances of obligate autotrophs.Obligate autotrophs may be the main‘contributor'to microbial CO2 fixation.Great transpiration loss induced by vegetation restoration inhibits autotrophic bacteria growth,and which further reduces the potential of CO2 fixation by autotrophic bacteria.This study changed the traditional concept that microbial biomass determines the rates of SOC mineralization,and confirmed the microbial functional redundancy and the key contribution of soil autotrophic bacteria in SOC fixation,which provides a new idea for the simulation and research of SOC dynamic in erosional and vegetation restoration system.
Keywords/Search Tags:Soil erosion, Vegetation restoration, Microbial community, Soil organic carbon mineralization, Microbial CO2 fixation
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