Influences Of Acidification To Microbiological Characteristics Of Garlic Cropping Soil And Its Restoration Effect

Chengdu Plain is an important vegetable production area in China,and garlic is an agricultural product with geographical indication in the region.In recent years,soil acidification and garlic cropping obstacle have become increasingly serious problems in Chengdu Plain.Microorganisms are important drivers of soil nutrient cycling and are essential for plant growth,disease control and the maintenance of soil ecosystem functions.Changes in microbial diversity and community structure can reflect the effects of environmental stress on soil quality sensitively.The responses of microbial community structure and function to soil acidification in garlic cultivation are not clearly characterized,the relationships between soil acidification and garlic cropping obstacles are still unclear,and the mechanisms of garlic cropping obstacles need to be further investigated.In this study,we investigated the responses of soil chemical properties,enzyme activities and microbial communities to soil acidification in garlic cropping soils with different degrees of acidity in Chengdu Plain.Furthermore,the changes in and driving factors of microbial community structure in soils with garlic cropping obstacles were further studied,and a structural equation model was employed to reveal the mechanisms behind garlic cropping obstacles.Subsequently,we explored the characteristics of garlic plants and rhizosphere and bulk soil microbial communities in restoration soils and clarified the effects of amendments and the underlying response mechanisms.The primary research results are as follows:（1）The physicochemical and biological characteristics of the acidified soil changed significantly.With the decrease of soil p H,the contents of soil available phosphorus,nitrate nitrogen,ammonium nitrogen,exchangeable acid,exchangeable Al³⁺and exchangeable H⁺increased gradually,and the contents of soil organic carbon and exchangeable base ions,together with the base saturation,decreased.The soil organic carbon,exchangeable Ca²⁺,exchangeable Mg²⁺and exchangeable Na⁺contents and base saturation correlated with soil p H positively.The activities of soil urease,catalase,sucrase and neutral phosphatase correlated positively and that of acid phosphatase negatively with soil p H.（2）Soil acidification altered the bacterial diversity and communities greatly.Alpha diversity of bacterial communities decreased at soil p H<5.5.Proteobacteria were the dominant bacteria,with 26.71-41.33%relative abundances.The relative abundance of Proteobacteria and Firmicutes increased with the decrease of soil p H.At the genus level,with the increase of soil p H,the relative abundances of acidophilic Al³⁺-resistant bacteria such as KF-JG30-C25,JG30-KF-AS9,Subgroup＿2 and Rhodanobacter increased,and those of Geobacter,Subgroup＿6 and Nitrospira decreased.Soil fungal communities were very tolerant to soil acidification;αandβdiversity decreased clearly only at p H<5.0.Ascomycota was the dominant fungal phylum,with the highest relative abundance（53.19%）in excessive acidic soils.The relative abundance of Mortierellomycota decreased significantly by 81.79%as the p H changed from neutral to excessive acidity.At the genus level,the relative abundances of plant pathogenic fungi such as Penicillium and Stemphylium increased,and those of beneficial fungi such as Mortierella,Oidiodendron and Talaromyces decreased as the soil p H decreased.（3）Soil acidification inhibited nitrogen cycling in garlic cropping soil,and changed the related functional microbial diversity and community composition.With soil excessive acidity（p H<5.5）,the abundance of nif H gene,nitrogen fixation capacity andαdiversity of diazotrophs decreased significantly,and the community structure was clearly altered.Proteobacteria（93.24-97.12%）was the dominant diazotroph phylum.As the soil p H decreased,the relative abundances of Geobacter,Bradyrhizobium and Pseudacidovorax decreased,and those of Desulfovibrio,Halorhodospira and Burkholderia increased.Soil acidification significantly decreased the abundances of ammonia oxidizing archaea（AOA）and ammonia oxidizing bacteria（AOB）and inhibited the potential nitrification in garlic cropping soil;the abundances of AOA and AOB correlated positively with p H.The abundance of AOA gene was significantly higher than AOB gene in the study area.The detected AOA and AOB belonged to the phyla Thaumarchaeota and Proteobacteria,respectively.In excessive acidic soils（p H<5.5）,αdiversity of AOA and AOB decreased significantly.With the decrease of p H,the relative abundance of Nitrososphaera in AOA gradually decreased to 21.94%,and the that of unidentified taxa increased to 74.67%.However,the AOB were all in the genus Nitrosospira and there was no significant change in the AOB community composition.（4）Soil acidification,in conjunction with changes to the microbial community,constitutes an important factor to garlic cropping obstacles.Acidification was serious in garlic planting obstacles soil,and inhibiting soil enzyme activities.Compared with the control,the activities of catalase,urease,polyphenol oxidase and sucrase were decreased by 41.30%,32.26%,55.78%and 29.41%,respectively.The aboveground and underground biomasses and the root activity of garlic correlated positively with soil p H（P<0.01）,and,compared to the control,decreased in the cropping obstacle soils by 88.58%,91.73 and 16.16%,respectively;Meanwhile,soil p H5.45 is critical in significantly impacting garlic biomass.The richness and diversity of the bacteria were lower in the cropping obstacle soils than in the control.The relative abundances of acidophilic bacteria such as JG30-KF-AS9,Chujaibacter and Rhodanobacter and plant pathogenic fungi such as Stemphylium and Aspergillus increased in the cropping obstacle soils.At the same time,the assembly process of bacteria in garlic cropping obstacle soils changed from deterministic to stochastic.Structural equation model confirmed that soil acidification can inhibit garlic biomass both directly and indirectly by altering soil bacterial community composition and decreasing soil enzyme activities.（5）Organic fertilizer combined with quicklime can effectively ameliorate acidified soil.After amendment,the p H of garlic cropping obstacle soil increased,the soil exchangeable acid,exchangeable Al³⁺and exchangeable H⁺contents decreased by 90.65%,78.57%and93.13%,respectively,and the p H and soil organic carbon content increased significantly.The aboveground and underground biomass of garlic plants increased 8.46 and 7.44 times,respectively.After amendment,soil microbial community composition was clearly restored:in bulk soils,the relative abundances of acidophilic and plant pathogenic bacteria such as JG30＿KF＿AS9,Rhodanobacter,Acidothermus and Candidatus＿Solibacter decreased significantly,and those of beneficial fungi such as Mortierella and Pseudaleuria increased.In garlic rhizosphere soils,the relative abundances of beneficial bacteria such as Actinobacteria,Pseudarthrobacter and Pseudomonas increased,and those of plant pathogenic fungi such as Fusarium and Alternaria decreased significantly.The amendment treatment optimized the soil microbial diversity and community composition by increasing the soil p H,and effectively alleviated the garlic cropping obstacles.In summary,soil acidification affected soil microbial diversity,community composition and functions strongly.When the garlic cropping soils were excessively acidic（p H<5.5）,the microbial diversities and enzyme activities decreased significantly.Through the analysis of the mechanisms behing garlic cropping obstacles,we found that soil p H can directly affect the biomass of garlic,and can also regulate the growth of garlic by changing the microbial community composition and functions,which further implied that the changes in microbial community composition,caused by soil acidification,is the key factor affecting the growth of garlic in Chengdu Plain.The application of amendments can significantly increase the soil p H,thereby alleviating the stress caused by acidification on the microbial communities and the growth of garlic.JG30-KF-AS9,Rhodanobacter,Subgroup＿6 and Nitrospira are suitable microbiological indicators for garlic cropping soil acidification;soil p H 5.5 can be used as the key p H value for the control and prevention of garlic cropping obstacles in Chengdu Plain.Consequently,it is essential to pay close attention to the soil p H status in the cropping of garlic in Chengdu Plain,and appropriate application of quicklime and organic fertilizers can improve the soil micro-ecological environment and effectively alleviate the garlic cropping obstacles.