Mechanisms Of Microbial Effects Of Water Change On Organic Carbon Mineralization Of Typical Soil On The Loess Plateau

Soil moisture,as an important limiting factor for biological growth on the Loess Plateau,has a significant impact on soil microbial community composition and organic carbon mineralization.However,there is still a lack of systematic understanding of soil water-driven microbial community change and its effect on organic carbon mineralization.Based on this,this study took the typical Huangmian soil and Lou soil on the Loess Plateau as the research object via the combined methods of field investigation and indoor simulation.After systematically simulating various moisture change scenarios,including drying and rewetting（DRW）with different intensities of wetting and drying,step-by-step wetting process,single-step wetting and single-step drying,the responses of microbial community and organic carbon mineralization to water change were investigated applying high-throughput sequencing,quantitative q PCR and other technologies.The influence patterns of water-induced microbial community change to organic carbon mineralization were revealed.The obtained results can provide scientific basis for improving the carbon cycle theory of terrestrial ecosystem.The main findings are shown as follows:（1）Responses of soil microbial community to water change processes.The results showed that water variation had no significant effect on fungal community structure.As the cycle number increased,the interaction between bacterial communities became more complex,while the complexity of interaction between fungal communities decreased.However,for drying and rewetting with different drought intensities,the interactions between bacterial and fungal communities became more complex with the cycle number increased,and the complexity of interactions between bacterial communities was greater than that of fungi.In the process of step-by-step wetting,the dominant bacteria group changed from Actinobacteria to Proteobacteria and Acidobacteria.Actinobacteria and Proteobacteria were the dominant bacteria in the process of single-step wetting and single-step drying.When the initial water value increased from 30%WHC to 100%WHC,the complexity of the interaction between the active bacterial communities increased.The complexity of co-occurrence networks for treatments with 5%WHC and 100%WHC as initial moisture values was similar.Thus,water enhanced the potential for interaction between bacterial communities.The results showed that bacteria were more sensitive to the dynamic changes of soil moisture than fungi.（2）Study on the influence of water variation process on microbial diversity.The results showed that the soil wetting treatment had a significant negative effect on bacterial diversity and richness,but different soil types exhibited different declining degrees of bacterial diversity.As for Huangmian soil,different wetting treatments imposed no significant effect on soil bacterial diversity.However,bacterial diversity in Lou soil showed a significant negative response to different wetting treatments,and the declining extend of bacterial diversity was even greater with the increase of wetting intensity.In contrast,drying treatment had no significant effect on bacterial diversity and richness.The bacterial richness and diversity of abandoned land and Robinia pseudoacacia forest were lower than that of the corresponding constant water treatment for the step-by-step wetting treatment with the initial value of 5%.However,there was no significant difference in soil bacterial richness and diversity between 30%constant moisture and 30–45%wetting process.Single-step wetting and single-step drying treatments exhibited no significant effect on bacterial diversity and richness.Various treatments including DRW,step-by-step wetting,single-step wetting and single-step drying had no significant effects on fungal diversity.Therefore,different water treatments had little effect on fungal diversity.The moisture changes with low water content as the initial value induces the decrease of bacterial diversity,while the mosture changes with medium initial water value had little effect on bacterial diversity.（3）The effect of water variation processes on organic carbon mineralization.The results showed as follows:in terms of Lou soil,the higher the wetting intensity of DRW treatments was,the larger the organic carbon mineralization rate pulse was,and the pulse value increased with the increase of cycles.For Huangmian soil,the pulse values of mineralization rate of organic carbon for low wetting intensity decreased with the increase of cycles,while the pulse values of mineralization rate of organic carbon for high wetting intensity were similar with different cycles.DRW treatments reduced soil carbon emission,as for Huangmian soil,the higher the wetting intensity,the lower the net carbon emission,while Lou soil showed the opposite patterns.For DRW with different drying intensities,the soil organic carbon mineralization rate pulse of organic carbon mineralization increased with the increase of drought intensity and decreased with the increase of cycle number.The pulse value of soil organic carbon mineralization rate with a step-by-step increase in moisture（i.e.,5–30–45%）was greater than that with a one-step increase in moisture（i.e.,30–45%）.For Robinia pseudoacacia forest,excessive water content inhibited the rate of organic carbon mineralization.For abandoned land,excessive moisture would increase the mineralization rate of organic carbon.The pulse value of organic carbon mineralization rate increased in both single-step wetting and single-step drying processes,and the larger the difference between the initial moisture and the target moisture,the larger the pulse value of organic carbon mineralization rate.While single-step drying treatment increased carbon emissions,the effects of single-step wetting treatment on soil carbon emissions depended on the values of initial water and target water.Therefore,water changes can reduce carbon emissions on the Loess Plateau.（4）Study on the influence mechanisms of water variation process on organic carbon mineralization.For Huangmian soil,the richness of Chao 1 was an important driving factor for the increase of CO₂ emissions under DRW with different wetting intensities.For Lou soil,Methylomirabilota and Chloroflexi played an important role in CO₂ emissions under different wetting intensities.Myxococcota was likely to be the most critical species for cumulative CO₂ emissions for DRW with different drought intensities.Fungal beta diversity（PCAs）and bacterial Chao 1 richness were important limiting factors affecting organic carbon mineralization during step-by-step wetting process.Firmicutes and Gemmatimonadota were the two most important bacteria groups affecting single-step wetting and single-step drought.Therefore,the influence of different soil moisture changes on organic carbon mineralization was mainly driven by different microbes.The DRW decreased the diversity of bacteria,and,the interaction potential of the bacterial community was enhanced with the increase of the cycle number,by which the caused increase of organic carbon mineralization rate was still lower than that caused by constant water treatment.Therefore,the change of water could reduce the carbon emission on the Loess Plateau.This study provides theoretical support for soil water regulation and microbial-mediated carbon cycling in the Loess Plateau after vegetation restoration or in farmland.