Distribution And Interaction Mechanism Of Soil Organic Matter Composition And Microbial Communities In Settling Fractions

Globally,soil erosion is estimated to mobilize on the order of 750 Gt soil and 1-5 Gt soil organic carbon(SOC)per year from hillslope soil profiles,which profoundly affects the carbon cycle of the terrestrial ecosystem.Soil particles are the carriers of SOC and microhabitats for microbes,and the size of soil particles strongly affect the transportation and spatial distribution of eroded soil and associated SOC and microbes.The chemical composition of soil organic matter(SOM)determines the quality and stability of SOC,and the microbial community composition plays pivotal role in SOC turnover.However,the distribution of SOM composition and microbial community structure in differently sized soil particles and their combined effects on the fate of eroded SOC have not been fully elucidated.In this study,we collected two types of soil(Black and Loess soil)from the typical water erosion regions in northern China.The two soils were then fractionated into three size classes using a settling tube apparatus according to their settling velocities: >250 ?m,63-250 ?m,and<63 ?m,out of which fractions of smaller size are presumed to be transported further than those of larger size.The individual particle size fractions and unfractionated soil were analyzed to determine(1)the diversity and composition of the soil microbial communities via 16 S r RNA and internal transcribed spacer amplicon sequencing,(2)the SOM composition using mid-infrared diffuse reflectance spectroscopy and(3)the SOC mineralization through incubation.In addition,the coupling relationships between SOM composition and microbial community in particle size fractions were further analyzed.Finally,a typical eroded landform(autonomous,transitional and depositional zone)was selected to analyze the distribution of SOM composition and microbial communities,to verify the impacts of soil particle size on SOM composition and microbial communities observed from settling fractionation.The main results are as follows:(1)The distribution of SOC quantity and SOM composition varied in particle size fractions.The SOC concentration of EQS >250 ?m in Loess soil is 13.0% and 42.7% higher than that of 63-250 ?m and <63 ?m,respectively;the SOC concentration of EQS 63-250?m in Black soil is 17% and 21.0% higher than that of >250 ?m and <63 ?m,respectively.In Loess soil,the labile organic matter(C-H/C=O)in EQS >250 ?m is 64.6% and 87.5%higher than that of 63-250 ?m and <63 ?m,respectively.but the C-H/C=O in EQS 63-250?m is 81.3% and 51.6% higher than that of >250 ?m and <63 ?m in Black soil,respectively.The correlations between the ratio of SOM functional groups and SOC concentration,or SOC mineralization varies in particle size fractions,and their relationships in EQS 63-250 ?m and<63 ?m fractions are relatively stable and consistent,while those in ESQ >250 ?m fractions are more complex.(2)Different sized soil particles create a unique microenvironment that support the colonization of contrasting trophic and functional groups of bacterial and fungal community.Compared with other size fractions,the relative abundance of Proteobacteria,Firmicutes and Actinobacteria are higher in EQS >250 ?m,63-250 ?m and <63 ?m fraction,respectively.Bacterial ? diversity increases with the decreasing soil particle size.The most complex bacterial co-occurrence network was observed in EQS 63-250 ?m fractions in Black soil and EQS >250 ?m in Loess soil,both of which indicate relatively stable community structure than other particle size fractions.The function of bacterial communities involved in soil carbohydrate and energy metabolism decreased significantly with the decreasing particle size,while function for heterologous biodegradation and metabolism increased significantly with the decreasing particle size.Fungal communities in EQS >250 ?m play a crucial role in soil aggregation,while fungal communities within EQS 63-250 ?m and <63 ?m fractions are more abundant with saprotrophic fungi,which have a strong capacity to degrade recalcitrant substances such as cellulose and chitin.The fungal ? diversity in two soils increases with the decreasing particle size,and this pattern is more pronounced in Loess soil.The most complex fungal co-occurrence network was observed in EQS 63-250 ?m fractions in Black soil and EQS <63 ?m fractions in Loess soil.The relative abundance of pathogenic and saprophytic fungi in two soils increase as the particle size decrease.(3)The SOM composition in soil particles significantly affects the composition of bacterial and fungal communities.The labile SOM plays a pivotal role in SOC mineralization and stability,while recalcitrant SOM shows greater impact on microbial diversity compared with labile SOM.Different bacterial/fungal phyla are related to specific organic functional groups,and these relationships are varied in different particle size fractions.Within each particle-size class,the most prevalent and low abundant microbial groups show opposite correlation relationships with SOM composition across the mid-IR spectra.Compared with the recalcitrant SOM,labile SOM seems to be the key drivers of microbial community structure.Proteobacteria,Bacteroides,Ascomycetes and Basidiomycetes are significantly positively correlated with C-H/C=O.The fast-growing r-selected species are more abundant in EQS >250 ?m and 63-250 ?m fractions with higher quantity and quality of SOC.Sphingomonadales and Mortierellales are significantly positively correlated with C=C/COO,and these two species all share a strong ability to decompose complex organic compounds and prefer to inhabit EQS <63 ?m.Soil particle size affects the activity of microorganisms(SOC mineralization)by regulating the chemical composition of SOM and the structure of microbial community.(4)Erosion-induced changes in particle size distribution led to distinct distribution of microbial community composition and SOM composition among the autonomous,transitional and depositional zone in the eroding watershed in the southern Loess Plateau.The labile SOM in the depositional zone is significantly lower than that of the autonomous,transitional zone.The bacterial ? diversity of in the depositional zone was significantly lower than that of the autonomous,transitional zone,while the fungal ? diversity showed opposite pattern.These results are partially support the findings obtained from settling fractionation.In addition,the unique microenvironment in the autonomous,transitional and depositional zone also contributes to distribution of soil microbial communities and SOM composition.Based on the settling fractionation,this study quantitatively evaluated the spatial distribution of different soil particles in an eroding environment and revealed that the SOM composition in particle size fractions drives effect the distribution and function of the microbial community.The labile SOM in particle size fractions affects SOC mineralization and stability,while recalcitrant SOM is more associated with microbial diversity.These results deepen our understanding of the transport and transformation process of SOC in soil eroding landscape.