Effects Of Nitrogen Deposition On Soil Carbon And Nitrogen Transformation And Microbial Characteristics In Stipa Baicalensis Steppe

Grassland ecosystem is the most important component of the terrestial ecosystem.The increase of nitrogen?N?deposition threatens the biodiversity and ecosystem function of grassland ecosystem.The field control experiment of simulated N deposition by N addition was conducted of Stipa baicalensis steppe in Inner Mongolia.In order to reveal the characteristics of soil carbon?C?and N transformation,the succession of soil microbial communities and their coupling relationships under the background of increasing N deposition in Stipa baicalensis steppe.The main results are listed as follows:1.High N addition?N100-N300?increased soil organic C,NO₃^--N,NH₄⁺-N and available phosphorus contents in 0-10 cm soil layer and 10-20 cm soil layer,and decreased soil pH in both soil layer.2.N addition promoted soil nitrification and inhibited soil ammonification.N15-N30 increased the conversion rate of organic C,N50-N300 decreased the conversion rate of organic C.The transformation rates of soil organic C had significant effects on the soil MBN conversion rate,and it can be well simulated by model of linear regression equation.3.High-N addition?N100-N300?decreasd soil urease,catalase,acid phosphtase,peroxidase and invertase activity in 0-10 cm soil layer,and decreased soil peroxidase and invertase activity in 10-20 cm soil layer.Soil enzyme activities were mainly affected by soil pH,NO₃^--N,NH₄⁺-N,microbial biomass C and microbial biomass N.4.The B PLFAs/F PLFAs ratio of N addition treatments in the two soil layers was lower or significantly lower than that of the control N0,and the difference among different N addition treatments was obvious.There were very significant differences between high-N treatments and low-N treatments and the control in the C utilization ability of soil microbial community.Moreover,100 kg N·hm^-2·a^-1was the threshold with which the effect on microbial activity shift from stimulation to inhibition in the study area.5.High-N addition?N150-N300?changed significantly the structure of soil bacterial community.N addition?N30-N300?decreased the relative abundance of Acidobacteria,Verrucomicrobia and Chlorofleri,and increased the relative abundance of Proteobacteria,Actinobacteria and Gemmatimonadetes.Bacterial communitystructure was mainly influenced by the pH,NO₃^--N,NH₄⁺-N and total P.6.N addition altered soil fungal community structure,and decreased the Shannon indexes.0-10 cm soil layer of N addition treatment,10-20 cm soil layer of N addition treatment of the relative abundance of fungi in the phyla,class and genus level were significantly different.The response of soil fungal community to N addition at the 0-10 cm soil layer and 10-20 cm soil layer was significantly different,and there was obvious soil gradient effect.N addition treatments significantly decreased the relative abundances of Basidiomycota,and increased the relative abundances of Glomeromycota in 10-20 cm soil layer.High-N addition?N100-N300?significantly decreased the relative abundances of Ascomycota in 0-10 cm soil layer.Soil pH,soil total organic C,NO₃^--N and available P were key environmental factors leading to the shift in the soil fungal community composition.7.Ammonium nitrate N addition(0-200 Kg N·hm^-2·a^-1)was in favor of the abundance of nitrogen-fixation organism?nifH?gene.High-N addition?N100-N300?significantly increased the abundance of ammonia-oxidizing bacteria?AOB-amoA?gene,and decreased the abundance of ammonia-oxidizing archaea?AOA-amoA?gene.high-N addition?N150-N300?decreased the abundance of denitrifiers?nirK?gene.With the increase of N addition level,mowing promoted AOB-driven ammonia oxidation,while lower abundance of denitrifiers mitigated the accumulation of nitrate originated from ammonia oxidation,resulting in high soil nitrate content.The above conculusions indicate that continuous nitrogen deposition increase could promote soil organic carbon sequestration,and increase soil NO₃^--N,while total N increase relatively small.Soil MBC and MBN significantly affected the conversion rate of soil C and N.The increase of nitrogen deposition is not conducive to the maintenance of the microbial community structure,the balance of flora and the continuous stability of the original soil microbial community of the Stipa baicalensis steppe,and has a negative effect on the soil carbon-nitrogen cycle.