Effect Mechanism Of Nitrogen Addition On Soil Organic Carbon Priming Effect In An Alpine Meadow

The increase of atmospheric nitrogen（N）deposition is one of the most important characteristics of global change,which has an important impact on food production,carbon and nitrogen cycle and environmental quality at the global scale.The Qinghai-Tibet Plateau is the"third pole of the world",and alpine meadows play an important regulatory role in the carbon sink process of the Qinghai-Tibet Plateau ecosystem.However,due to the harsh climatic conditions and simple ecosystem structure in this region,the composition and diversity of vegetation and microbial communities are extremely sensitive to the increasing nitrogen deposition.Exogenous organic matter input can cause a priming effect on soil organic carbon（SOC）and thus alter soil carbon mineralization.Increased atmospheric nitrogen deposition will not only affect exogenous organic carbon input by altering plant communities and changing litter properties,but also regulate the priming effect of soil organic carbon by affecting the structure of biological communities and their interactions,which will have important effects on soil organic carbon stability and ecosystem carbon cycling in alpine meadows.However,the current research on the intensity and direction of the priming effect of soil organic carbon in alpine meadow is still in the exploratory stage,especially with the participation of plant-microbial community,there are few studies on the controlling mechanism of soil priming effect.In this study,based on a long-term simulated N deposition experimental platform（established in 2014）,an artificially simulated multi-gradient N deposition experiment(0,7,20,40 and 80 kg N ha^-1 yr^-1,denoted as N0,N7,N20,N40 and N80,respectively)was conducted in alpine meadows on the Tibetan Plateau,mainly monitoring the alpine meadow community characteristics and litter decomposition characteristics,and collected topsoil（0-15 cm）for indoor incubation experiments of soil organic carbon excitation effect with ¹³C-labeled glucose as exogenous carbon addition.Soil nutrients and microbial community characteristics were measured in the original soil,at the first（day 3）and second（day 42）stages of incubation,as well as the direction and intensity of the organic carbon priming effect during incubation.Through the combination of field monitoring and indoor culture,¹³C isotope tracer technology and microbial high-throughput sequencing and other methods,the dynamic changes of soil organic carbon priming effect in an alpine meadow in response to nitrogen deposition and the potential regulation mechanism of plant-microorganisms on it were further studied.The main results are as follows:1.Nitrogen addition significantly increased above-ground biomass（AGB）,Shannon-Wiener index and Pielou evenness index in alpine meadow.The response of different functional groups of plants in alpine meadows to nitrogen addition varied,with N7 and N20 increasing legume biomass and importance value,N7 decreasing sedge importance value,and N40 increasing grass biomass and importance value.The results indicated that light nitrogen addition helped to promote the growth of legumes and forbs,and moderate nitrogen addition promoted the growth of legumes and grasses,thus ensuring a steady increase in community biomass and diversity under nitrogen addition.The effect of N addition on the decomposition rate of litter in the alpine meadow gradually decreased with increasing decomposition time and was higher than that of the control（N0）by 40.2%,23.0%and 64.2%in N7-N40,respectively（P<0.05）;the effect of N addition on the mass fraction of litter showed that N7-N40 significantly promoted the turnover of cellulose fraction and N80 promoted the turnover of C fraction;N addition also changed the stoichiometric ratio of litter,showing that N20-N80 significantly increased the C:N and N:P of litter（P<0.05）.2.Dissolved organic carbon（DOC）,total nitrogen（TN）and ammoniacal nitrogen（NH₄⁺-N）contents of alpine meadow soils under the influence of N addition were significantly lower in the early and late stages of soil incubation compared to the original soil（P<0.05）,and differed among N addition treatments.Compared to N0,TN was significantly lower in N80 in the early stage and increased significantly in N7-N40 in the late stage;NH₄⁺-N was significantly higher in N7-N40 in the early stage and decreased significantly in N40-N80 in the late stage（P<0.05）.Soil C:N and C:N:P increased significantly（P<0.05）in both the pre and post incubation periods,and increased significantly in N80 in the pre period and decreased significantly in N40 in the post period compared to the control.Correlation analysis of soil nutrient contents with plant community and litter decomposition characteristics showed that soil NH₄⁺-N and NO₃^--N contents were significantly positively correlated with Shannon-Wiener index,litter decomposition rate,and cellulose content,while significantly negatively correlated with species richness,forbs biomass,and litter dry weight loss rate（P<0.01）.3.The effect of nitrogen addition on the microbial community diversity of alpine meadow soil differed during incubation,showing that the fungal Chao index was significantly lower than N0 at N80 in the early stage（P<0.05）,while there was no significant effect on bacterial diversity（P>0.05）.After exogenous carbon addition,the composition of bacterial community changed significantly compared with the original soil（P<0.01）,and bacterialα-diversity was significantly increased by more than 30%（P<0.001）in both pre-and post-culture;fungalα-diversity（Chao index）was significantly increased by more than 10%（P<0.05）in both pre-and post-culture;microbial biomass carbon and nitrogen was significantly increased in both pre-and post-culture,and N7 and N40 were significantly higher than N0（P<0.05）.Nitrogen addition significantly increasedβ-N-acetylaminoglucosidase,leucine aminopeptidase,acid phosphatase,β-1,4-glucosidase,and phenol oxidase activities in an alpine meadow（P<0.05）.Bacterial diversity（Chao index）was significantly and positively correlated with soil DOC and MBC content in the pre-culture period;fungal diversity（Shannon index）was significantly and positively correlated with soil DOC and NO₃^--N content（P<0.05）;and bacterial richness（Chao index）was significantly and positively correlated with gram importance value and litter cellulose content in the late culture period（P<0.05）.The results indicated that soil fungal and bacterial community composition responded significantly to nitrogen addition after exogenous carbon input,and fungal diversity was significantly reduced at high nitrogen addition,suggesting that soil microorganisms may change community composition structure to adapt to soil nitrogen changes in organic carbon priming effect.4.The soil organic carbon mineralization showed a positive priming effect（PE）(4-9 mg C kg^-1 d^-1)after exogenous carbon input and showed a gradual decrease with increasing incubation time,which was significantly higher in the early incubation period（3 d）than in the late incubation period（42 d）（P<0.05）in an alpine meadow.Nitrogen addition significantly inhibited PE（P<0.05）in the early incubation period,reducing it by about 29%to 46%,while there was no significant effect in the late incubation period.Regression analysis showed that soil PE gradually increased with increasing soil NH₄⁺-N content,sedge family importance value,grass family importance value,litter decomposition rate,litter cellulose and N content,and with increasing soil DOC and MBC content,β-1,4-glucosidase,plant community diversity（Simpson index and Richness）,forbs and legume importance value,aboveground biomass,and bacterial diversity decreased gradually（P<0.05）.Structural equation model（SEM）results show that soil microbial mass carbon and nitrogen,fungal and bacterial diversity,and soil enzyme activity were the key factors regulating the response of the organic carbon priming effect to nitrogen addition.The microbial regulation of organic carbon priming effect in the early stage of culture is mainly"microbial activity mechanism"and"co-metabolism mechanism",while in the late stage of culture it is mainly"nitrogen mining mechanism".Vegetation characteristics had a small role in regulating the priming effect,mainly the decomposition of litter,the decomposition and release of cellulose and nitrogen components,and the increase of the dominance of sedges and gramineae all promoted the organic carbon priming effect,while forbs and legume populations suppressed the organic carbon priming effect.Based on the coupling relationship between soil microorganisms and plant community ecological strategies,this study highlights that the soil bacterial community-mediated organic carbon priming effect in alpine meadow ecosystems with long-term nitrogen limitation may weaken under the future trend of increasing nitrogen deposition,which is important for understanding the feedback effect of alpine meadow soil organic carbon stability mechanism on nitrogen deposition,and then provides an important scientific basis for accurately predicting the dynamic changes of carbon cycle in alpine meadow ecosystems on the Tibetan Plateau.