| The Yellow River Delta(YRD)is an alluvial plain formed by the deposition of a large amount of sediment carried by the Yellow River at the entrance of the Bohai Sea.The groundwater level in this area is relatively high,and groundwater is directly attacked by tide and recharged laterally.In addition,evaporative climate and unreasonable irrigation and other agricultural production activities have formed a large area of typical coastal saline-alkali soil,which is an extremely important reserve land resource in China today when land resources are scarce.During the development of the YRD,a large amount of lands have been transformed from natural wetland to farmland or woodland,and the change of land use types also has a certain impacTt on the structure,diversity and function of bacterial community in soil.In addition,the newly reclaimed saline-alkali farmland in this area in the past 50 years has low background value and capacity of soil nitrogen,and the soil ecosystem is very fragile.In the process of farmland development and utilization.excessive nitrogen fertilizer input makes excess nitrogen fertilizer form non-point source pollutants,enter the saline-alkali farmland ecosystem.reduce the utilization rate of nitrogen fertilizer and pollute groundwater and surface water,at the same time,it also poses a potential threat to human health.Therefore,understanding the structure and diversity of microbial communities involved in soil nitrogen cycling in this area,and mastering the bacteria that play a major role in nitrogen cycling in different agro-ecosystems have theoretical guiding significance for rational application of nitrogen fertilizer in this area.In this paper,bacterial 16S rRNA Illumina Miseq sequencing and PICRUSt function prediction analysis were carried out on 38 soil samples from three land use types:natural wetland,forest land and agricultural land.The differences of bacterial community structure and diversity in different land use types and their driving factors were compared.At the same time,the function of soil bacterial community was studied by land use change.Secondly,we used Illumina Miseq to sequence the functional genes(cnnoA.niirS.nirK and nifH)revolved in nitrogen cycling in 22 samples collected from five typical agroecosystems in the YRD(corn-wheat rotation.cotton,soybean,fruit-vegetable and rice).to study the structure and diversity of nitrogen-cycling microbial communities in different agroecosystems and their influencing factors,and tried to find out the key microbes in nitrogen cycling in different agroecosystems.(1)Study on the structure and function of soil bacterial community in natural wetland,woodland and agricultural land.Alpha diversity analysis showed that there were significant differences in soil bacterial diversity under different land use types.Agricultural land was significantly larger than natural wetland and woodla:nd,and natural wetland had the lowest bacterial diversity.Beta diversity analysis showed that the composition of soil bacterial community in natural wetland was significantly different from that in forest land and agricultural land.Soil physical and chemical factors such as total nitrogen,organic matter,electrical conductivity,soil water content and cation exchange capacity could explain the difference by 23.6%,20.6%,18.7%,17.4%and 8.2%,respectively.Heavy metals content(copper,chromium and zinc)in soils also contributed to the difference.Chromium,zinc and chromium could explain 11.7%,11.0%and 7.3%of the differences,respectively.The relative abundance of genes related to ion-coupled transporters and sporogenesis in natural wetland was significantly higher than that in agricultural land and woodland,while the abundance of oxidative phosphorylation genes was significantly lower than that in other two areas.(2)Study on functional microorganisms participating in nitrogen cycle in soils of five different agricultural ecosystems.The structure of functional microbial communities involved in nitrogen-cycling in paddy soils was significantly different from that of the other fours.Soil water content and electrical conductivity are the main factors contributing to the difference.Soil water content explains 45.4%,21.6%and 54.2%of the differences in ammonia oxidation,denitrification and nitrogen-:fixing functional microbial communities,respectively.Soil electrical conductivity explains 15.3%and 13.7%of the differences in the structure of ammonia oxidation and denitrification f'unctional microbial communities,respectively.Nitrosopumilus of AOA plays a leading role in the ammonia oxidation process in fruit-vegetable soils.while Nitrosospira and Nitrosovihrio of AOB in corn-wheat Rotation system and soybean planted soils.In paddy soils.Roseateles and Nitrosomonas of AOB.Candidantus Nitrosocosmicus and Candidatus_Nitrolenuis of AOA play a key role in the ammonia oxidation process.NirS-type denitrifying bacteria A.zoarcus and Seudomonas,nirK-type denitrifying bacteria Rhizobium and Alcaligenes are the dominant microorganisms in rice soil.Rhocdanobacter,Rubrivivax,Cupriavidus,Azospirillum.Magnetospirim of nirS and Ensifer,Bradyrhizobium.Chelatiivorans,Rhodopseudomonas.Sinorhizobium,and Chelatococcus of nirK were the key players of denitrification in soybean planted soils.Paraburkholderia,Azotobacter,Ideonella,Agrobacterium,Rhizobium.Burkholaderia,Skermanella.Geobacter,Azospirillum,Pseudacidovorax,Dechloromonas and Azoarcus were the dominant microbes in nitrogen-fixing process in soybean soils,while they were Desulfuromonas,Azospira,Pseudomonas,Brcrdyrhizobium,Leptolyngbya,Sideroxydans,Desulfovibrio,Desuulfobulbus and Pelobacter.RDA results showed that the first two axes of RDA explained 59.85%,43.88%and 48.36%of the differences of dominant microbes in ammonia oxidation,denitrification and nitrogen-fixing process,respectively.The results of this study enable us to effectively predict the impact of land use on ecosystem services and stability in the YRD.We provide the microbial mechanism and data for the rational application of nitrogen fertilizer,which is helpful for the sustainable development of this area. |
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