Effects Of Agricultural Land Use Change On Soil Organic Carbon Stability And Core Microbial Communities In Suburban Area

Urbanization is progressing rapidly.This has led to a loss of soil organic carbon and an increase in emissions of greenhouse gasses,which ultimately leads to global warming and climate change.Suburban soils are considered to be the main components of the entire urban-suburban ecosystem,with functions including logistics supply and ecological buffering.Although the effects of urbanization on the urban soil C balance and global ecosystems have begun to receive global attention,there are few reports on C changes and trend of related microbial communities in the soil of suburban areas.This study was aimed at investigating soil organic carbon fluctuations in the suburbs of Chengdu based on 176soil profile dates(0-100 cm),identifying the related specific factors and quantifying their contribution to soil organic carbon fluctuation to better assess the contribution of urbanization to global warming,and reveals the response of soil microbial communities to changes in organic carbon in suburban agricultural areas.A summary of the primary results are as the follows:1.Rapid urbanization has had a profound impact on the easily oxidized organic carbon(EOC)and total organic carbon(TOC)of the soil profiles in suburban areas.The TOC decreased by 5.4-13.5%from the outer-suburb to the inner-suburb.The quotient of EOC/TOC in the soil profile,which reflects the stability of organic carbon,declined from0.78 to 0.20 from the outer-suburb to the inner-suburb,and resulted the EOC decreased by>30%in the inner-suburb during past 10 years.Principal components analysis(PCA)indicated that the cumulative contribution rate of the four principal components(PC₁-PC₄)related to urbanization(PC1?PC4)>82%,which can reflect changes in economics(economic density,industrialization),urbanization(city size,population growth),farmland(farmland structure,cultivated area)and traffic flows,respectively.Regression analysis further indicated changes in economic(economic density and industrialization)was the main factors contributing to the loss of TOC in whole soil profile and topsoil EOC,with the independent explanatory power of 31.6%on topsoil TOC and 16.0%on subsoil TOC,13.2%on topsoil EOC;changes in farmland(farmland structure and cultivated area)and urbanization(city size,population)were the main factors contributing to the loss of subsoil EOC,with the contribution of 40.4%,the traffic flows also have negative impact on EOC.In addition,the organic carbon loss may persist until the suburbs are completely urbanised.The large losses of EOC and TOC caused by urbanization,and their contribution to global warming,necessitate their consideration in future appraisals of climate change and urban planning projects.2.Based on the above results,28 soil profiles data on the total soil organic carbon(TOC)and labile organic carbon(LOC),including easily oxidizable organic carbon(EOC),dissolved organic carbon(DOC)and microbial biomass carbon(MBC),of the soil profile were analysed for four typical agricultural land use scenarios including rice-wheat/canola rotation,rice-garlic rotation,rice-vegetable rotation(leafy vegetables)and afforested land in the inner suburb area.The impacts of agricultural land use changes on sequestration and nutrient availability of soil organic carbon were assessed in this urban agricultural area using the space-for-time substitution method.Conversion of land use from a traditional agricultural rotation(rice-wheat/canola rotation)to afforestation increased the MBC content and decreased the contents of EOC,DOC and TOC due to the lower input of organic matter,improved aeration of the soil profile and growth of aboveground biomass.Conversion of a traditional rotation to a rice–garlic rotation resulted in a significant increase in topsoil TOC,slight but insignificant decreases in subsoil TOC,and clear increases in LOC because of rice planting,rice straw mulch and reasonable application of chemical fertilizers.In contrast,the conversion of a traditional rotation to a rice-vegetable rotation decreased MBC due to the excessive use of chemical fertilizers that consequently increased EOC,DOC and TOC.We conclude that afforestation on paddy soil has negative consequences for soil carbon sequestration,and a rice-vegetable rotation contributes to carbon sequestration but is detrimental to soil fertility.In addition,the MBC ratio in soil could be the optimal indicator for assessing organic carbon stability and soil fertility,and more attention should be paid to subsoil carbon changes.3.Rapid urbanization has a violent impact on agricultural land uses and soil organic carbon associated with soil microbe.However,trends in soil microbes especially soil fungal community link to organic carbon across agricultural land uses with increasing tillage intensity remain unclear.To address this,we investigate the soil fungal community under four traditional rotation(rice-wheat/canola rotation),rice-vegetable rotation,rice-garlic rotation and afforested land in urban agricultural area of Chengdu,China and reveal the key flora related to organic carbon,based on the high-throughput sequencing dataset.The results showed that there were no significant differences in fungal abundance but a significant uptrend in fungal diversity with soil to depth under rice-wheat/canola rotation.Compared with rice-wheat/canola rotation,fungal diversity was decreased in topsoil(0-20 cm)and the fungal abundance were serious declined in subsoil(20-100cm)under rice-vegetable rotation,rice-garlic rotation and afforested land.The core fungi communities across agricultural land uses were evolved from Sordariomycete.Under rice-wheat/canola rotation,it mainly evolved into similar Dothideomycetes and different norank?Zygomycota;under rice-vegetable rotation,rice-garlic rotation,it mainly evolved into different unclassified?k?Fungi and norank?Zygomycota;under afforested land,it mainly evolved into Tremellomycetes.RDA resulted that MBC and p H were the key factors led to changes in fungal community across agricultural land uses,MBC/TOC and C/N also have contribution to this variation.Moreover,Dothideomycetes responded positively to MBC/TOC and p H,while Sordariomycetes,norank?Zygomycota,unclassified?k?Fungi and Tremellomycetes responded positively to MBC and C/N.Additionally,afforestation on paddy soil has negative consequences for balanced fungal community and led a limited relationship between the fungal community and organic carbon.Altogether,those results indicate that unbalanced fungal community structure in afforested land may be more detrimental to organic carbon accumulation than decreased fungal diversity in cultivated field with intensive farming.4.Compared with changes in fungal communities,the response of bacterial communities to changes in agricultural land uses in suburban agricultural areas presents different characteristics.Changes in agricultural land uses have resulted in significantly higher bacterial abundance(0-20cm and 40-100cm)and diversity(0-20cm)in afforested land than in rice-vegetable rotation,the bacterial diversity of rice-garlic rotation was significantly higher than that of rice-vegetable rotation in 20-40 cm layer.Those further caused changes in the structure of the bacterial community.The core flora of of bacterial communities between topsoil and subsoil is quite different in the suburban agricultural area.The core flora of the topsoil included Actinobacteria,Acidobacteria and Alphaproteobacteria;that in subsoil included Nitrospira,Actinobacteria and Acidobacteria.The core bacterial communities across agricultural land uses in topsoil were evolved from Actinobacteria,it mainly evolved into different Acidobacteria,Alphaproteobacteria and Deltaproteobacteria,and resulted in significantly lower Actinobacteria in rice-garlic rotation than in rice-vegetable rotation and afforested land.The core bacterial communities in subsoil were evolved from Nitrospira,it mainly evolved into different Acidobacteria,Actinobacteria and Deltaproteobacteria;and led Nitrospira of rice-vegetable rotations decreased significantly while Actinobacteria increased significantly in 20-40 cm layer,Actinobacteria of rice-garlic rotation declined significantly in 20-40 cm layer,and a significant decrease in Nitrospira of afforested land in 40-100 cm layers.Soil organic carbon and p H are key factors that cause changes in bacterial communities.Topsoil bacterial across agricultural land uses were mainly positively related to MBC,C/N,and DOC,while subsoil was mainly positively related to p H,MBC/TOC,and EOC/TOC.In the core bacterial community,Alphaproteobacteria,Actinobacteria and Acidobacteria have positive responses to MBC and C/N,Deltaproteobacteria and Nitrospira respond positively to p H,MBC/TOC.These changes imply that the bacterial abundance of topsoil is mainly influenced by C/N and labile organic carbon content(especially MBC);while that in subsoil is more inclined by influences by soil acidification and the ratio labile organic carbon(MBC/TOC).In addition,COG function classification indicated the function unknown,general function prediction only,amino acid transport and metabolism occupied the highest abundance,transcription,carbohydrate transport and metabolism,inorganic ion transport and metabolism also occupied an important position.This implies that the core bacterial communities which perform unknown functions,general functions,and substance transport metabolism are in a highly functional state in the suburban areas.