Effects Of Nitrogen Addition On Carbon, Nitrogen, Phosphorus And Microbial Characteristics Of Soil Aggregates In Stipa Baicalensis Grassland

As an important phenomenon of global climate change,atmospheric nitrogen deposition has had a serious impact on the global nitrogen cycle,and may even play a decisive role in the future climate change.Its impact on grassland ecosystem has become one of the hotspots of ecological research.To grasp the influence of nitrogen deposition on the ecological stoichiometric characteristics of carbon?C?,nitrogen?N?and phosphorus?P?of grassland soil aggregates can provide basic data for the comprehensive analysis and assessment of the impact of nitrogen deposition on grassland ecosystem.The uneven distribution of nutrients in soil in different aggregate components provides a spatial heterogeneous microhabitat for microorganisms.In order to reveal the ecological stoichiometric characteristics and microbial community distribution of carbon,nitrogen and phosphorus in different particle size aggregates of grassland soil and its response to nitrogen addition.This study was conducted on stipa baicalensis grassland of Inner Mongolia.Since2010,Six nitrogen addition treatments,including N₀(0 kg N·hm^-2·yr^-1)?N₁₅(15 kg N·hm^-2·yr^-1)?N₃₀(30 kg N·hm^-2·yr^-1)?N₅₀(50 kg N·hm^-2·yr^-1)?N₁₀₀(100 kg N·hm^-2·yr^-1)and N₁₅₀(150 kg N·hm^-2·yr^-1),were conducted in the field control test of simulated nitrogen deposition for 8 years.The effects of nitrogen deposition on particle size distribution,ecological stoichiometric characteristics and microbial community structure of the soil aggregates in baikal naxia chinensis were studied.The results show:?1?The addition of nitrogen significantly improved the stability of soil aggregates and the proportion of>2 mm aggregates?P<0.01?.Microaggregates occupied the majority of the soil,and microaggregates decreased significantly with the increase of nitrogen content?P<0.05?.Soil aggregates of 0.25² mm increased with the addition of nitrogen.Compared with N100 treatment,the proportion of>2 mm soil aggregates in N150 treatment showed a decreasing trend,while the proportion of<0.25 mm micro-aggregates showed an increasing trend,and the addition of high concentration of nitrogen may damage the stability of the aggregates.?2?The organic carbon and total nitrogen contents of 0.25² mm aggregates were significantly higher than other particle sizes?P<0.05?,while the total phosphorus contents were not significantly different in the aggregates.Compared with the control group,the addition of nitrogen significantly increased the organic carbon and total nitrogen content of>0.25mm soil aggregates?P<0.05?,but had no significant effect on total phosphorus.The addition of nitrogen resulted in the decrease of C/N of0.25 mm>soil aggregates,and the increase of C/P and N/P of 0.25²mm soil aggregates?P<0.05?.?3?Nitrogen addition significantly increased the carbon and nitrogen content of 0.25² mm soil aggregates,as well as the total phospholipid fatty acid content of microbial community,the phospholipid fatty acid content of fungi,and the ratio of fungi/bacteria,gram-positive bacteria/gram-negative bacteria?P<0.05?,and decreased the microbial Margalef richness index of soil aggregates?P<0.05?.Correlation analysis showed that the contents of total PLFAs,PLFAs,G⁺/G^-,F/B of soil aggregates were positively correlated with soil organic carbon and total nitrogen;but negatively correlated with C/N.Two-way ANOVA of variance showed that both nitrogen addition and particle size of aggregates had significant effects on the microbial community structure of soil aggregates?P<0.05?,but there was no interaction between the two factors.Through comprehensive analysis,the addition of nitrogen for 8consecutive years significantly affected the physical and chemical properties,microbial community structure and diversity of soil aggregates,and the fixation of soil organic carbon and total nitrogen was closely related to the microbial community.To some extent,nitrogen addition promoted the carbon fixation potential of soil and increased the mineralization rate of organic matter in soil aggregates.With the increase of nitrogen addition level,P element in soil aggregates became the main limiting factor for the growth of grassland plants.