| Over the years of implementation of the national policy of "returning farmland to forest(returning grass)",the construction of vegetation in the loess hills has achieved fruitful results,forming a typical agroforestry-ecological complex system,which has greatly improved the soil quality in the region.As the main decomposers of litters,the change of microbial community diversity and structural affect the decomposition process of litter,and have certain dynamic response characteristics to the decomposition of litters under seasonal temperature changes,but its response mechanism is still unclear.In this study,we selected the Robinia pseudoacacia forest with different recovery time of 17,27 and 42 years,whch have the same soil type,similar site conditions,as the research object,located in the Wuliwan watershed of Ansai,northern Shaanxi province in loess hilly and gully region.On one hand,we investigated the correlation between litter and soil in Robinia pseudoacacia with different restoration years;On the other hand,we used the method of laboratory culture to simulate the decomposition of litter by different litter adding approach(added litter-RP17,RP27,R42 and unadded litter-CK17,CK27,CK42)which incubated in the seasonal temperature changes,to explore the response characteristics of soil nutrients,enzyme activities and microbial community and the relationship among them.This study could reveal the response relationship between soil nutrients and microbial characteristics of litters in the process of near-natural decomposition and its mechanism,so as to provide scientific basis for further understanding the vegetation-litter-soil cooperative mechanism in the process of ecological restoration.The main results are as follows:(1)The stoichiometric ratio of litter in Robinia pseudoacacia forest in different restoration years has an important effect on soil microbial community structureWith the increase of recovery years,the contents of organic carbon,total nitrogen and phosphorus all increased to different degrees in litter and soil,and their stoichiometric ratios also changed significantly.The litter C:N showed a significant trend of decreasing,while the litter N:P showed a significant trend of increasing.There was no significant difference in the alpha diversity of the microbial community between different restoration years,while the NMDS analysis showed that the beta diversity of Robinia pseudoacacia was significantly different between 42 years and 17 years,27 years.Correlation analysis showed that soil enzyme activity and microbial community diversity were affected by litter and soil factors.The domain bacteria community were Proteobacteria,Actinobacteria,Acidobacteria which occupied more than 75% of the relative abundance,and the domain fungi community were Ascomycota,Basidiomycota,Zygomycota which occupied more than 90% of the relative abundance,the community composition is affected by multiple litter and soil factors,and the litter C:P explained the variation in bacterial community composition of 36.9%(p=0.016),while the litter C:N explained the variation in fungal community composition of 27.2%(p=0.080).(2)Litter addition changed the characteristics of soil nutrient,enzyme activity and their stoichiometric ratioLitter addition have different effect on the contents of soil carbon,nitrogen and phosphorus.Soil organic carbon(SOC)increased by 62.8%,70.3%,57.2%,and total nitrogen(STN)increased by 29.9%,22.7% and 34.0%,respectively in the litter added group compared with the unadded litter group,the impact on the total phosphorus(STP)is relatively weak,and its stoichiometric ratio were also significantly increased.With the change of temperature,the soil p H decreased,and the added litter group decreased more significantly than the unadded litter group.The activities of ?-1,4-glucosidase(BG),?-1,4-N-acetylglucosaminidase and leucine aminopeptidase(NAG + LAP),alkaline phosphatase(AP)in the litter added group increased by 6.6%,33.9%,48.3%,59.1%,104.7%,143.8%,50.0%,114.9%,68.7%,respectively,compared with the unadded group.Correlation and redundancy analysis showed that the factors affecting the change of enzyme activity were different in the litter added and unadded groups,the main factors in the litter added group were soil organic carbon(SOC),soil total nitrogen(STN),soil total phosphorus(STP),nitrate nitrogen(NN),ammonium nitrogen(AN),while the main factors in the litter unadded group were NN,AN,SOC,STN,STP,and specific factor weight changes in each process.(3)Litter addition changed the diversity of soil microbial community and its assembly trend,and the diversity of microbial community was affected by many soil factorsLitter addition slightly decreased soil microbial taxonomic diversity,which are more obvious in RP17 and RP27,but less affected in RP42.Litters addition had little effect on bacterial phylogenetic diversity,but significantly improved the phylogenetic diversity of fungi.The net relatedness index(NRI)and nearest taxon index(NTI)in litter added groups were both greater than 0 and higher than that of the litter unadded groups,showing a deterministic aggregation effect.For both bacteria and fungi,the difference of beta diversity was mainly manifested in the three groups of RP17 and RP27,CK17 and CK27,RP42 and CK42,and the differences in fungi showed more obvious.Mantel test and redundancy analysis showed that the driving factors to soil microbial community diversity were different in litter added and unadded groups.The response of soil factors to bacteria and fungi also different which C: P and C: N,STP,AN mainly affect bacteria and the AP,AN,STP,ln BG: ln AP,BG,NN mainly affect fungi.(4)The addition of litters changed the composition and structure of soil microbial community,and its composition and structure were affected by a variety of soil factorsLitter addition significantly improved the the relative abundance of Proteobacteria,Actinobacteria which of the rich nutritional bacteria,and reduced Acidobacteria,Chloroflexi,significantly improved the relative abundance of Ascomycota and Basidiomycota and reduced Mortierellomycota.Lefse analysis found that Proteobacteria,Actinobacteria were the indicator species of bacteria and Ascomycota,Basidiomycota were the indicator species of fungi in the litter added group.For the litter unadded group,the Acidobacteria,Chloroflexi were the indicator species of bacteria,Mortierellomycota was the indicator species of fungi.Redundancy analysis found that it had difference in different groups of the main impact factors combination,and the key factors influence to the bacterial and fungal community structure were similar.The molecular ecological network further found that the the number of nodes,links,modules,key indicator species were significantly higher in bacteria than that of fungi.Litter addition enchancd the network properties.The positive interaction relation between species increased first and then reduced in bacterial molecular ecology network,and fungi showed strong positive interaction relations.Proteobacteria,Actinobacteria were the key network nodes in all groups,and Acidobacteria,Chloroflexi were the key network nodes in litter unadded group.There were no key network nodes in fungal networks,but Ascomycota as the main dominant species in the whole groups which played important roles in the networks,furthermore,Mortierellomycota occupied an important proportion in the network of litter unadded groups.This study showed that the litter properties have important influence on microbial community diversity and composition,at the same time,the litter addtion could contributed to the accumulation of soil nutrients,improved the soil extracellular enzyme activity,changed the diversity of microbial community,especially fungal phylogenetic diversity,changed the community structure of species at the same time,raised the relative abundance of the eutrophic species.Microbial community diversity and species composition are affected by multiple soil factors in the whole process of change and at different temperature stages.The combination of factors affecting bacteria and fungi is common,but the weight of specific factors is different. |
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