Variations Of Soil Microbial Communities Under Erosion Environment

As a key component of terrestrial biosphere,soil organisms drive the mineralization of soil organic carbon（SOC）and regulate soil CO₂ efflux,being strongly sensitive to variation of soils.Soil microbial communities were profoundly affected by soil temperature,soil moisture and organic matter,which occur strongly differentiation in the environment of water erosion.Different types of erosion environment exert distinct effect on the formation and development of soil microbial communities,thus resulting in contrasting responses of soil microbial communities as well their regulated soil carbon dynamics.To research soil microbial communities under different environment erosion has critical significance in accurately understanding and estimating soil CO₂ efflux of terrestrial ecosystem as well its response to altered environment.The loess region has fragmented terrain and widespread sloping land where suffers severe soil erosion.Runoff and sediment strongly affected soil physicochemical and microbial properties along the eroded slopes.Soil profile inversion occurs when continued erosion of an upslope position transports material that originates from progressively deeper in the profile and layers it on top if previously deposited soils downslope.Ultimately,subsoils are left exposed at the surface in both eroded and depositional positions,thus bringing in large uncertainty to the soil microorganisms that residing in it.Vegetation restoration treatments also have strong effect on the distributions of soil water,heat,sediments,organic matter,thereby influencing soil microbial communities.Based on the the Changwu State Key Agro-ecological Experimental Station（shorten as Changwu Station）,three field experiments were conducted at the typically managed Wangdonggou small watershed.1)microbes-slope positions ex.:soil microbial community among slope positions（upper,middle and bottom slope positions）were compared along a sloping grassland;2)microbes-vegetation ex.:soil microbial community between vegetation types（forest vs.grassland）were compared at various slope positions;3)microbes-soil exposure ex.:soil microbial communities between the exposed and control soils were compared at various depths（topsoil:0–20 cm,subsoil:20–60 cm and deepsoil:60–100 cm）.During the experiment period from 2014 to 2016,the soil microbial community composition,activities of enzyme involved in soil carbon cycling,soil respiration and its temperature sensitivity(Q₁₀),soil temperature,soil moisture,runoff,sediment,SOC,dissolved organic carbon,soil mineral nitrogen content and fine root biomass were measured.The aims of this study were to:1)characterize soil microbial community composition and enzyme activities under different erosion environments（different slope positions,vegetation types and different depths of exposed soils）;2)identify the influencing factors of erosion environments on the soil microbial communities and enzyme activities;and 3)develop possible relationship between soil microbial community and soil respiration,Q₁₀.The main conclusions were as follows:1).Divergent responses of soil bacterial and fungal communities among slope positions were observed along the steeps slope.Soil bacterial diversity were greater at middle and bottom slope positions than at upper slope positions while fungal diversity varied little among slope positions.The bacterial class Betaproteobacteria was 25.3%and 20.7%lower but unidentified_Acidobacteria was 31.5%and 35.8%greater at bottom-than at upper-and middle-slope positions.The fungal community transitioned from being Agaricales-dominant（relative abundance of 23.7%）at upper slope position to being Mortierellales-dominant（relative abundance of 35.3%）at bottom slope position.Theβ-D-glucosidase activity generally declined down the slope whileβ-D-xylosidase and cellobiohydrolase activities hiked at middle slope position.All the enzyme activities were suppressed at bottom slope position.Soil water,root biomass and soil carbon/nitrogen content were the main factors affecting variation of soil microbial communities among slope positions.2).Similar composition of soil microbial communities between forest and grassland revealed much stronger effect of soil types than that of vegetation types.However,vegetation exerted weak effect on soil bacterial community through different erosion intensities.The bacterial diversity of grassland was smaller,similar and greater than that of forest,respectively at upper,middle and bottom slope positions.The Proteobacteria was greater in grassland than in forest regardless of slope positions,while soil Acidobacteria was lower in grassland than in forest at upper and middle slope positions and similar between them at bottom slope position.Theβ-D-glucosidase activity was greater in grassland than in forest at upper slope position,and the cellobiohydrolase activity was greater in grassland than in forest at middle slope position,both of which were lower in grassland than in forest at bottom slope position.Soil temperature,soil carbon/nitrogen contents were the main factors influencing the variation of soil microbial communities between vegetation types at various slope positions.3).Under the in situ condition,differences in the nutrients and organic matter among soil layers resulted in the enrichment of microbial biomass and enzyme activities,as well the variations of soil bacterial diversity and community composition.Moreover,soil bacterial community transitioned from being copiotrophs-dominant in the topsoil to being oligotrophs-dominant in the deepsoil.After being exposed,soils have similar microbial community composition with that of in situ,limited by the experimental time.However,subsoil exposure exerted effects on bacterial abundances,not fungal abundances.At both 20–60 and 60–100 cm,the copiotrophic groups within phyla Proteobacteria and Actinobacteria increased and the oligotrophic groups within phyla Thermotogae and Nitrospirae decreased in the exposed soil than in the control soil.Both number of species differentiated the two soils and magnitude of the increased abundances increased with depths.The increased soil mineral N（0–20 cm）,soil mineral N（20–60 cm）,soil mineral N and DOC（60–100 cm）were the factors controlling variation of bacterial community composition between the control and the exposed soils.4).Contrasting distribution patterns of soil respiration and Q₁₀ were observed along the steeps slope,with soil respiration increased by 49.1%and the Q₁₀ decreased by 13.2%over the slope.Soil respiration positively correlated with bacterial diversity.The copiotrophic groups（unidentified_Acidobacteria and Mortierellales）negatively and oligotrophic groups（Betaproteobacteria and Agaricales）positively correlated with Q₁₀.Although different vegetation types have equivalent soil respiration and Q₁₀,different erosion intensities resulted in different pathway by which the soil microbial communities in grassland and forest involved in soil carbon cycling.As the only soil factors affecting the Q₁₀ of forest,the NO₃-N have postive effect on the Q₁₀ through influencing the microbial communities involved in soil N cycling,such as Sphingomonas,Blastococcus and Rubrobacter.In contrast,the NO₃-N have postive effect on the soil respiration and negative effect on the Q₁₀ of grassland through influencing the microbial communities involved in soil N cycling,such as Rubrobacter.After being exposed,soils have the significantly decreased soil respiration and Q₁₀ with increasing depths.That is,the subsoil and deepsoil have reduced soil respiration（by 38.5%and 33.9%）and Q₁₀（by 21.8%and 24.1%）relative to the topsoil,due to the limited microbial biomass and enzyme activities.Both soil respiration and Q₁₀ have positive correlations with the copiotrophic groups within phyla Proteobacteria,Actinobacteria and Ascomycota（Blastococcus,Microvirga,Exserohilum and Pyrenochaetopsis）,which at the same time positively correlated with all the microbial biomass and enzyme activities.The main conclusions:1)The extremely strong spatial variation of soil microbial communities were revealed,as well as the difference between the responses of bacterial and fungal communities.2)The soil microbial communities were directly related to the soil respiration and its temperature sensitivity,thus providing evidence for the assumption that soil microbial driving soil carbon dynamics.The findings in this study are of great importance to improve our understanding of the dynamics of soil carbon cycling in terrestrial ecosystem,so as to accurately estimate the variations of the soil CO₂ flux of the erosion area.