Response Patterns Of Soil Microbial Community And Its Influencing Factors At Erosion-deposition Settings

Soil erosion,the one of most widespread form of land degradation,presents a profound impact on the biogeochemical cycles of terrestrial ecosystem.Globally,?75billion tons of soil is redistributed by erosion per year,of which 70%–90%is deposited in various low-lying areas.Soil microbes are ubiquitous and abundant in soil ecosystems and play a key role in driving the flow of energy and matter,hence they are crucial in maintaining the health of terrestrial ecosystems and the balance of biogeochemical cycles.The variation of soil environment derived from soil erosion-deposition may trigger dramatic responses in microbial communities because of the dependence of microbial communities on soil environment.However,it remains large unclear in the response mechanisms of soil microbial communities to soil erosion and deposition.To take a more accurately estimating for the shift of microbial ecological function during soil erosion-deposition process,it is urgent to identify the spatial variations of soil microbial communities in erosional and depositional sites.In this study,the erosion-deposition plots were constructed based on local eroding landscape pattern,including erosional sites of three different slope gradients of 5°,10°and 20°,each connected with a depositional site.During the experimental period(2015–2019),the runoff and sediment were collected under natural rainfall,and the soil moisture,soil temperature and soil CO₂ emission rate were monitored in situ.In meanwhile,microbial community and microbial extracellular enzyme activities were measured by using the methods of high-throughput sequencing and microplate fluorescence respectively.Additionally,we simulated different soil erosion processes based on an artificial rainfall simulator system.During the simulated rainfall,the runoff and sediment were collected and then the sediment was used to analyze the distribution of different size particles,organic matter functional groups and microbial community in sediment.The objectives of this study were to:1)identify the distribution of bacterial and fungal communities in the erosional and depositional sites;2)observe the changes of microbial community of eroded sediment and its influence factors during the process of soil erosion;3)investigate the compromising effects of soil erosion and deposition on soil CO₂ emission and its influencing factors.Main results are as follows:1)The divergences of bacterial Shannon index,community composition and community stability were found between erosional and depositional sites.The Shannon index was markedly 13.3%higher in the depositional sites than in the erosional sites of20°slope.The significant differential in bacterial community composition was found between erosional sites and depositional sites.In meanwhile,soil bacterial network was more complex within depositional sites than erosional sites,indicating the high community stability in the depositional sites.Moreover,bacterial Shannon index presented different responses to soil erosion and deposition when slope gradients increased.In detail,soil bacterial Shannon index in the 10°and 20°erosional sites had no variations when compared to the 5°erosional sites.However,soil bacterial Shannon index in the 10°and 20°depositional sites increased by 1.6%–2.6%when compared to the 5°depositional sites.In term of soil fungal community,the differentials of fungal richness and community composition and community stability were observed between erosional sites and depositional sites.Particularly,soil fungal richness in the 20°depositional sites increased by 43.9%than that in the 20°erosional sites.The difference of soil fungal community composition was observed between erosional sites and depositional sites.The fungal network in the depositional sites was more complex compared that in the erosional sites,suggested fungal community stability of depositional sites was higher than that in the erosional sites.The relative abundance of saprotrophic fungi in the erosional sites increased by 42.6%than that in the depositional sites,but the relative abundance of pathogenic fungi was 122.2%lower in the erosional sites than that in the depositional sites.Additionally,when compared to the 5°erosional sites,fungal richness in the 10°and 20°erosional sites decreased by 11.8%–28.2%obviously.However,fungal richness in the 10°and 20°depositional sites had no changes comparing with 5°depositional sites.The spatial redistribution of soil moisture,pH,soil organic carbon,dissolved organic carbon and nitrate nitrogen with the migration of runoff and sediment were the main reason for the changes of bacterial community in the erosional and depositional sites.Changes of fungal community in the erosional and depositional sites were mainly attributed to the spatial variations of soil moisture,pH,nitrate nitrogen,dissolved organic carbon and clay.2)Changes of sediment particle composition presented a significant influence on microbial Shannon index and microbial network during the process of soil erosion.Compared with the 5°slope,the bacterial Shannon index of eroded sediment generated from 10°and 20°slopes significantly increased by 6.4%and 3.8%.The fungal Shannon index on 10°slope had no obvious variation but declined by 15.4%on the 20°slope when compared to the 5°slope.The community compositions of both bacteria and fungi were similar in eroded sediment across 5°,10°and 20°slopes.Moreover,the bacterial network was more complex and more extensive bacterial interactions and greater community stability potential with increasing slope gradients.In contrasting,the fungal network changed in opposite way to the bacterial network.The changes of bacterial community in eroded sediment might be related to the shifts of the bacterial community of the fraction with<0.053 mm,and organic matter stability in eroded sediment.In addition to the impact of organic matter stability of eroded sediment on fungal community,the fungal community of the fraction with0.053–0.25 mm and<0.053 mm could be mainly responsible for the changes of fungal community in sediment of different slope gradients.3)The cumulative soil CO₂ emissions of depositional sites was significantly higher than that in the erosional sites,and the differences were enlarged by increasing slope gradients(5°:0.4%–16.7%;10°:20.1%–32.6%;20°:31.9%–51.5%).When combining the erosional sites and depositional sites,the total amount of CO₂ emissions from the10°and 20°erosion-deposition plots decreased by 17.3%-187.7%and 122.7%-198.6%than that from the 5°erosion-deposition plot(1640-2006 g CO₂-C y^-1).Soil microbial extracellular enzyme activities,positively linear to soil CO₂ emissions,acted a key role in regulating the total amount of CO₂ emissions in the erosion-deposition plot.In meanwhile,changes of soil moisture,organic carbon and mineral elements caused by the migration of runoff and sediment had significant effects on the total amount of CO₂emissions.In the study,the migrations of runoff and sediment changed the spatial distributions of soil physical-chemical properties,and the thus derived the influences on soil microbial alpha diversity,community compositions and the community stability,were the important force in driving the microorganisms to regulate soil CO₂ emissions.Furthermore,the study have clarified the response patterns of microbial alpha diversity and community stability to the changes of sediment particle composition during soil erosion process.Our results would provide direct evidence for the accurate evaluation of the shifts of microbial community and microbial ecological function in the eroding landscapes.The research in the future should consider the complicated changes of soil microbial communities during different stages of water erosion,and microbial ecological function should be given much attention.It would be help to accurately evaluate the effects of erosion-and deposition-induced variations of microbial community on terrestrial ecosystem.