| The Songnen Plain is an important base for animal husbandry production in China,but the soda saline-alkaline soil in this area greatly restricted sustainable development in grass industry,and only a few plants with strong salt-alkali resistance,such as Suaeda(Suaeda salsa),are able to survive in serious saline-alkaline soil.Planting alfalfa(Medicago sativa)is an efficient way to improve the saline-alkaline soil.However,in the process of saline-alkaline soil by naturalsuccession-restoring and planting-alfalfa-restoring,the interaction between plant and soil microbial communities,especially it is still unclear that the specific mechanism in changes of both soil microbial community and the abundance and community of nitrogen cycle functional genes mediated by soil microorganisms.Also,there is no sufficient theoretical and scientific basis for the improvement effect of different alfalfa quality on saline-alkaline land.In this study,Sifangshan Farm,a typical serious soda saline-alkaline soil area in the Songnen Plain,was used as the experimental base,while the bare land without vegetation but with alkali spot was used as control(referred to as ’bare land’).In order to evaluate the dynamic characteristics of different soil nutrients and the limitation of available resources for microbe,we investigated the changes of different physical and chemical factors in the soil which restored by both Suaeda growing of natural succession(referred to as ’Suaeda’)and planting alfalfa with 10 varieties for 18 years(referred to as ’planting alfalfa’),respectively.The restoration of soil microbial community structure and function was evaluated from the perspectives of average carbon source utilization,community diversity,community composition,functional prediction,network interaction,nitrogen cycle functional gene abundance and functional community.On this basis,the suitability of different alfalfa varieties to improve saline-alkaline land was clarified,which provided a reference for the improvement and use of saline-alkaline land resources in Songnen Plain.The specific conclusions are as follows:1.The pH and electrical conductivity(EC)of saline-alkali soil reduced by planting alfalfa,while Suaeda and bare land had little change in pH and EC.Among different alfalfa varieties,the pH of the rhizosphere soil of both Zhaodong and Barricade decreased most significantly,with a decrease of more than 27%.Planting alfalfa increased soil organic carbon(SOC)and available potassium(AK)content,decreased soil available phosphorus(AP)content,and increased soil alkali hydrolyzable nitrogen(AHN)content,indicating that planting alfalfa could restore the nutrient level of saline-alkaline soil.It indicated that planting alfalfa increased the carbon utilization metabolism intensity and the α diversity of carbon utilization metabolism function of microbial community,thus changing the metabolic activity and community function of soil microorganisms in saline-alkaline land.2.The difference of α diversity of soil bacterial community showed that the Shannon index of bacterial community in alfalfa-planting soil was higher than that of Suaeda soil and bare land soil,and the Shannon index of Suaeda soil was higher than that of bare land soil.The soil bacterial community of saline-alkaline land was mainly composed of three dominant phyla,Proteobacteria,Actinobacteria and Acidobacteria,and Proteobacteria and Acidobacteria were significantly correlated with soil pH,EC,SOC and AHN by correlation analysis.The relative abundances of dominant genera Subgroup 6＿norank,RB41 and Gaiellales＿norank in the soil restored by Suaeda and planting alfalfa were higher than those in the bare land soil.Through the modular analysis of the co-occurrence network of soil bacterial community,the soil bacterial community was divided into three modules,and the populations in the three modules mainly belonged to Acidobacteria and Proteobacteria.The study found that planting alfalfa played a role in nitrogen cycle and stimulated the microbiota of soil bacterial community,and it could promote the nitrogen cycle in saline-alkaline land.3.The differences of Shannon index and Pielou index among the fungal communities of bare land,Suaeda and alfalfa-planting soil showed that the α diversity of fungal communities in alfalfa planting soil was better than that in Suaeda soil and bare land,but there was no significant difference between Suaeda soil and bare land.This indicated that the increase of vegetation coverage promotes the increase the α diversity of soil fungal community.Ascomycota was the dominant phylum in the composition of soil fungal community,and the relative abundance of Fusarium,the dominant genus of Ascomycota,in the fungal community of bare land soil was higher than that of Suaeda and alfalfa soil,and the relative abundance of Fusarium in the fungal community of Suaeda soil was significantly higher than that of alfalfa soil.Principal component analysis(PCA)of soil fungal community function showed that the fungal functions of Suaeda,bare land and alfalfa soil were significantly different.The modular analysis of co-occurrence network showed that the soil fungal community was divided into 5modules,and the populations mainly belonged to Ascomycota.Furthermore,there were significant differences in the relative abundance of fungal modules in bare land,Suaeda and alfalfa soil.4.There was no significant difference in the α diversity index of soil archaeal community among bare land,Suaeda and alfalfa.However,PCA analysis showed that there were significant differences in soil archaeal community structure between bare land and both Suaeda and alfalfa soil,indicating that restoration ways affected the β diversity of soil archaeal communities.Compared with the bare land,the relative abundance of the dominant phylum Thaumarchaeota and the dominant genus Candidatus Nitrocosmicus in the Suaeda and alfalfa soils was significantly increased.The co-occurrence network of archaeal communities in bare land,Suaeda and alfalfa-planting soil was mainly composed of 5 modules.In the cooccurrence network of soil archaeal community,5 modules were mainly composed of Thaumarchaeota.The relative abundance of ammonia-oxidizing archaea Nitrososphaera and Nitrocosmicus in Suaeda and alfalfa restored soil was higher,indicating that these two restoration ways enhanced the soil ammonia oxidation process and played a positive role in the functional restoration of soil archaea.In addition,the FAPROTAX database was used to predict the function of archaeal community in bare land,Suaeda and alfalfa rhizosphere soil.It was found that the relative abundance of archaea involved in soil carbon and nitrogen cycle in rhizosphere soil of planting alfalfa was significantly increased.5.The copy numbers of nifH,nirS,nirK and nosZ genes of nitrogen-fixing microorganisms and denitrifying microorganisms in alfalfa-planted soil were significantly higher than those in bare land and Suaeda soil.The analysis showed that EC,pH and SOC had significant effects on nifH gene abundance;AHN,AK,AP and SOC had significant effects on AOB-amo A gene abundance;AP had a significant effect on the abundance of nirS gene;AHN,SOC,pH and EC had significant effects on nirK gene abundance.SOC and EC had significant effects on nor B gene abundance.TP,AP,SOC,pH,EC,AP/TP had significant effects on nosZ gene abundance.On the whole,the Shannon index and Chao index of nifH gene,AOB and nosZ gene functional microbial communities in alfalfa rhizosphere soil were significantly higher than those in bare land and Suaeda soil.Chao index of nirS gene and nor B gene functional microbial communities in alfalfa soil were higher than that in bare land and Suaeda soil.PCA results showed that the main phylum of nifH gene,AOB,nirS gene,nirK gene,nor B gene and nosZ gene functional community were Proteobacteria.The main genus of soil nifH gene functional community in bare land and Suaeda was composed of Telmatospirillum,while the soil restored by alfalfa was composed of Geobacter.The main genus of AOB community in bare land and Suaeda soil was composed of Nitrosomonas,while the soil restored by alfalfa was composed of Nitrosospira.The dominant genus of nirK gene functional community in bare land and Suaeda soil was Kaistia,while the alfalfa-planting-restoring soil was mainly composed of Bradyrhizobium.The dominant genus of nosZ gene functional community in alfalfa-restored soil was mainly composed of Microvirga.In addition,the microbial communities of nifH gene,AOB,nirS gene,nirK gene,nor B gene and nosZ gene in alfalfaplanting-restoration soil were more connected than those of natural succession soil,which indicated that the nitrogen cycle functional microbial community of alfalfa restoration soil was easier to form a mutually beneficial community.The dominant phylum Proteobacteria was the core phylum in the co-occurrence network of soil nifH gene,AOB,nirS gene,nirK gene,nor B gene and nosZ gene functional microbial community,which indicated that Proteobacteria played an important role in maintaining community stability.In summary,this study compared and explained the reasons for the differences in soil microbial communities and functions between the two ways(planting alfalfa and natural succession)from the perspective of soil nutrient dynamics and microbial community structure and function in the process of saline-alkaline land restoration.At the same time,it also provided a theoretical basis for soil microorganisms to maintain nutrient balance and ecosystem nutrient cycling and microorganism-drived mechanism in the process of salinealkaline land restoration. |
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