The Response Of Soil Respiration To The Turnover Of Soil Aggregates And Its Microbiological Mechanism In The Riparian Zone Of The Three Gorges Reservoir

Soil respiration is a main component of global ecosystem carbon cycling,which plays a critical role in regulating atmospheric CO₂ concentration and climate dynamics in the earth system.Soil aggregates are the carrier of soil organic carbon protection and the main space for microbial activity.Microbes are the most active part of the ecosystem and directly affect the soil nutrient cycle and the formation of aggregates.The processes of soil aggregate formation and turnover are closely related to soil respiration.Under the periodic disturbance of water level fluctuation,the riparian zone will be the potential hot spot for the carbon cycle.However,the response of soil respiration to the turnover of soil aggregates and its microbiological mechanism in the riparian zone of the Three Gorges Reservoir are not clear.This thesis targeted on the riparian zone of the Three Gorges Reservoir.The soil respiration was continuously and systematically measured using the LI-6400XT portable photosynthesis measurement system and static opaque chamber-gas chromatography method in the study area.Daily and monthly variations of soil respiration and their regulating factors were analyzed.The distribution and stability of soil aggregates in the riparian zone were measured,and the effects of dam-triggered flooding intensity and land-use types on soil aggregate stability were quantified.The variation characteristics of soil aggregates and soil respiration in the riparian zone after four times dry-wet cycles and the response of soil respiration to the turnover of soil aggregates were studied.The respiration rate and microbial community characteristics of soil aggregates in different aggregate sizes were measured in laboratory incubation experiments.Meanwhile,the regulating factors and microbiological mechanisms for aggregate respiration in the riparian zone were analyzed.The main results are as follows:?1?Significant differences in diurnal variations were found to be mainly affected by soil temperature at 10 cm depth,while seasonal variations were mainly regulated by ST and rainfall events?i.e.10 cm depth soil water content?.Importantly,croplands were found to contain higher soil organic carbon but lower soil respiration efflux and Q₁₀values than those for non-cropping fields.The results imply that soil respiration was strongly affected by land-use types rather than flooding intensity in the TGR riparian zone.Furthermore,this study highlights the significant impacts of tillage in stabilizing soil organic carbon and reducing soil respiration efflux in the area that is highly influenced by hydrological regime shift.Finally,from the perspective of controlling the soil carbon dioxide emission,we suggest that local government should manage and guide farming activities in the riparian zone.?2?The longer the flooding time was,the more destructive the soil aggregates were in the TGR riparian zone.Compared with land-use types,the influence of dam-triggered flooding intensity on soil aggregate stability was much stronger in the riparian zone.Maintaining higher soil organic carbon and nitrogen content was conducive to promoting the formation and stability of soil aggregates in the riparian zone.The artificial ecological restoration?such as arbor,lotus?measurement at 168 m or below168 m did not significantly promote the stability of the soil aggregates and the organic carbon content.Unsuitable ecological restoration will adversely affect the soil aggregate stability and lead to the degradation of soil.Therefore,the natural restoration without artificial interference may be more conducive to the stability of soil aggregates in the riparian zone.?3?Soil respiration and the evolution characteristics of aggregate structure in 2014and 2018 under two land-use types?paddy field and corn field?in the riparian zone of the Three Gorges Reservoir were studied.The results show that:1)the stability of soil aggregates at 173 m elevation with less flooding intensity was much stronger.Adversely,the stability of soil aggregates at elevations below 173 m with stronger flooding intensity decreased;2)Dry and wet cycles increased soil respiration;soil respiration rate in corn fields was higher than in paddy fields;additionally,soil respiration rate gap decreased among strong flooding intensity areas;3)The response of soil respiration to the evolution of soil aggregate structure in the riparian zone was mainly manifested in the following:the stability of soil aggregates had the direct negative effects on soil respiration?mitigating CO₂ emission?,and the negative effects will weaken with the times of dry-wet cycles;the soil aggregate fractions and its organic carbon content directly or indirectly affect soil respiration,especially the proportion of micro-aggregates,their nitrogen and organic carbon had the greatest effect on soil respiration.From the perspective of controlling soil CO₂ emissions and promoting the stability of aggregates,it is recommended to avoid farming in low-altitude elevation areas where the flooding intensity is stronger,and micro-aggregates should be protected particularly.?4?It shows that the soil aggregate respiration flux decreased as the aggregate size declined.Bacterial diversity had significant negative effects on soil aggregate respiration,but Fungi diversity has significant positive effects on the respiration of soil aggregates.After the submerged soil drained,the bacterial diversity declined and the increased fungal diversity resulted in the soil aggregate respiration flux raising.Soil bulk density and pH had stronger negative effects on soil aggregate respiration,but soil organic carbon had strong positive effects on soil respiration,which indirectly affected soil aggregate respiration by affecting the soil structure characteristics and activity of soil microbial communities.The results of this study contribute to improving our understanding of the key processes of ecosystem soil respiration under extreme environmental changes.So as to provide a basis theoretical support for scientific management of land use in the riparian zone of the TGR.