| At present,global warming has become a common concern of scientists all over the world,and rising temperature has an important impact on nitrogen circulation and distribution in terrestrial ecosystems.The Qinghai-Tibet alpine meadow soil contains huge nitrogen,which plays an important role in the global nitrogen cycle.Therefore,it is necessary to study the spatial distribution of soil nitrogen in the Qinghai-Tibet alpine meadow and its response to simulated warming.This study was divided into three parts: In the first part,a grassland transect was set up on the Qinghai-Tibet Plateau,and nine sampling points were set up within the grassland transect,including alpine meadow soil,typical grassland soil and desert steppe soil.All sampling points in the transect were natural grassland without any agricultural and animal husbandry production.On this basis,the content distribution characteristics of all nitrogen forms(available nitrogen,total nitrogen,acidolytic total nitrogen,amino nitrogen,ammonia nitrogen,amino sugar nitrogen and acidolytic unknown nitrogen)were determined,and the single factor difference,Pearson correlation analysis,redundancy analysis and structural equation model analysis were carried out in combination with the basic properties of soil in sampling points and environmental impact factors.In the second part,a transect was set up to determine the distribution characteristics and influencing factors of soil nitrogen in the steppe at different elevations on the Qinghai-Tibet Plateau.In the third part,a 5-year field simulation experiment was carried out in an alpine meadow in Xizang Province.The effects of winter warming and annual warming on soil total nitrogen and nitrogen components were studied in an open top box.The effect of simulated warming on soil nitrogen in the alpine meadow of Qinghai-Tibet Plateau was studied.The results showed that(1)the organic nitrogen(SON)in the grassland transplantations of the Qinghai-Tibet Plateau were mainly amino acid nitrogen(AAN),ammonia nitrogen(AN)and acid-hydrolyzed unknown nitrogen(AUN),and the proportion of amino sugar nitrogen(ASN)was the smallest.The total nitrogen(STN),available nitrogen(ALHN)and soil organic nitrogen(SON)of alpine meadow soil were all higher than those of typical and desert steppe soil.Therefore,the nitrogen(N)availability of alpine meadow soil was higher than that of typical and desert steppe soil.Average annual rainfall(MAP),average annual temperature(MAT)and Clay(Clay)were the main influencing factors of soil total nitrogen(STN)and available nitrogen(ALHN),while average annual temperature(MAT),soil organic carbon(SOC)and Sand(Sand)were the main influencing factors of soil organic nitrogen(SON).Grassland type had direct effects on soil organic nitrogen and soil available nitrogen,while annual mean air temperature,plant aboveground biomass(AGB)and altitude had indirect effects on soil total nitrogen,available nitrogen and organic nitrogen through grassland type.(2)Bacteria,annual mean air temperature(MAT),annual mean rainfall(MAP),Sand particles(Sand)are the main factors affecting the composition of total nitrogen(STN),available nitrogen(ALHN)and organic nitrogen(SON)in the alpine meadow soil.Among them,Sand content was the main factor that caused the significant decrease of all nitrogen components at the altitude of5200 m.The rise of altitude contributed to the ecosystem function of alpine meadow in different altitudinal gradients.The rise of altitude resulted in the weakening of single function and the decrease of multifunctional index of ecosystem.Make the temperature,moisture and soil physical and chemical factors change regularly.It also affects soil microorganism and soil enzyme activity at the same time,and finally affects the transformation and distribution of soil nitrogen.(3)Compared with winter warming and annual warming,the three different warming treatments had no significant effects on the contents of total nitrogen(STN),ammonium nitrogen(NH4+-N),nitrate nitrogen(NO3--N)and total organic nitrogen(TON).The contents of available nitrogen(ALHN)and total nitrogen(ammoniacal nitrogen,amino acid nitrogen,amino sugar nitrogen and unknown nitrogen)in winter and year warming were significantly lower than those in no warming treatment,while the contents of acid insoluble nitrogen(AIN)were significantly higher than those in no warming treatment.Redundancy analysis showed that aboveground biomass was the most important factor affecting soil total nitrogen(STN)and nitrogen composition.The structural equation model showed that fungi had a direct negative effect on acidolysis of unknown nitrogen,while actinomyces had a direct positive effect on aboveground biomass.Our results suggest that climate warming promotes the accumulation of highly stable organic nitrogen components,which may lead to a decrease in soil nitrogen supply capacity.This study provides a new understanding of the response of alpine grassland to future climate change from the perspective of nitrogen cycle.In future warming scenarios,the optimal nitrogen(N)management strategies by providing additional nitrogen(N)sources or promoting native soil organic nitrogen(SON)mineralization should be explored to maintain grassland productivity in the alpine meadow ecosystem on the Tibetan Plateau(TP)in response to future climate change. |
