Microbial Characteristics And Geochemical Response Of Loess Under Dry And Wet Environment

The loess had recorded rich Geoscience and Biology information for the past millions years,among which the continuous loess paleosol sequence represented by Luochuan Loess Section had recorded the process of climate and environment change of the Loess and Plateau since about 2.5 million years ago.The alternation of loess and paleosol in Luochuan soil profile is essentially the material performance of different intensity weathering and chemical element migration with the participation of microorganisms.The functional diversity of microorganisms stem from the abundant physiological activities in soil,and the interaction mechanism with the environment could feed back the changes of climate and soil environmental conditions accurately.The characteristics of microorganisms distribution can also indicate the development information of vegetation on the soil.Therefore,it has great theoretical significance to explore the loess paleosol alternation in the Quaternary loess profile with the new ideas of ecology and biogeochemistry,and to carry out the research on the biological information contained in soil,so as to the climate evolution and the paleoenvironmental records are further understood,At the same time,this study has far-reaching practical value for exploring the impact of climate change on biology.Within this study,soil samples were collected from the typical loess section in Luochuan,and the data of environmental factors and geochemical components in soil were obtained by indoor experiments.The data of species composition,distribution diversity and abundance composition of soil bacteria after classification and annotation were obtained by microbiological experiments.SPSS?R language and other methods were used to analyze the statistics of soil factors,geochemical components and soil microbiology in different layers.On this basis,the Structural Equation Model(SEM)is established according to reality theory,and analyze the path relationships among them.Compared with other research results,the environmental evolution in different layers of the study area is discussed,and then the high-resolution paleoclimate change records of Luochuan Loess and its surrounding areas in recent 500000 years are further studied.The main results are as follows:(1)The differences of environmental factors and geochemical components between Loess layer(L)and Paleosol layer(S)are significant in general.Most of the factors in the group decrease with the increase of profile depth.SWC%is larger in paleosol layer,which is conducive to the development and activity maintenance of microorganisms;TOC,TN,TP,NH₄⁺-N,NO₃^--N can provide basic nutrients for the survival and reproduction of microorganisms,organic carbon content gradually decreases in S layer,and organic elements such as N and P are higher in paleosol layer;In general,particle size distribution and frequency susceptibility content The contents of Ca CO₃?Fe²⁺?Mn²⁺in Loess layer were higher than Paleosol group.(2)In the Loess and Paleosol layers,as the dominant groups,Acidobacteria,Proteobacteria and Actinobacteria,have the highest relative abundance in the S and L groups.Compared with different Loess layers,the proportion of dominant bacteria in the inner layers of Paleosol decreases,and the proportion of bacteria classified as Othors increases.It can be seen from the change of microbial community between bacterial community?and?diversity analysis samples:except for the ancient loess layer L1,which is greatly influenced by human activities in the upper layer of the sampling site,the soil microbial richness and community diversity index of group S are higher than group L;The difference within the?diversity group is obvious,in group L,except for the L1 layer,the average value of samples is slightly different and evenly distributed,and in group S,the composition of microbial community are similar of samples S4 and S5.The difference of distribution in the inner group can reflect the difference of microbial community diversity among different samples properly.(3)RDA is used to explore the sequence diagram of community species composition constrained by environmental variables.According to the RDA map established by Canoco software,the correlation of soil environmental factors to the distribution of microbiota is TN>NO₃^--N>SWC%>p H>TP>TOC>C/N>NH₄⁺-N,which shows that in this study,the content of TN is the main factor of microbial development in the profile,and the related species are positively correlated with NO₃^--N and SWC%.Combined with RDA analysis of key species and groups,the main environmental impact factors of bacteria in Luochuan Paleosol and Loess layer are different due to the change of horizon.The structure of bacteria in Paleosol layer is dominated by TN,and the species composition of Loess layer is dominated by TP.In the distribution of species composition under the influence of environmental factors,the microbiota in S group is more closely developed.(4)Taking all the soil factors measured in this experiment as geochemical components,the changes of each factor in the Loess-Paleosol sequence can be well consistent with the changes of microorganisms,and the correlation analysis of microbial development characteristics in the soil can be established to obtain the response law of soil microbial development to geochemistry:the community index is negatively correlated with the C/N value;TP with Shannon indexes are extremely significantly negatively correlated;TN was positively correlated with Shannon index,and other nutrient factors such as NO₃^--N were positively correlated with ACE and Shannon index.SWC%was positively correlated with bacterial index,this result show that soil moisture is very important to determine the connectivity between soil microorganisms with discrete horizon.The contents of Ca CO₃,Fe²⁺,Mn²⁺in soil have significant negative correlation with the diversity and abundance index of community.And the development degree of microbial community in Loess layer is less than that in Paleosol layer,which is consistent with the changes of the contents of Ca CO₃and reducing substances in different horizons.The distribution reflects the climate change of the original soil forming environment.The path coefficient of soil microbe and soil environmental factors was 0.53,and the indirect effect of soil microbe with climate change was 0.51.Based on the structural equation model,we can confirm the relationship between the assumed potential variables and the consistency with the observation indexes,and verify the theory proposed in this paper that the soil environment in the Loess-Paleosol sequence has been changed due to the changes of paleoclimate,with the result that the development and composition of micro organism between the Loess layer and Paleosol layer had created significant difference with a long time.