The Influence Mechanism Of Soil Respiration In Alpine Grasslands In Southern Tibet

The Qinghai-Tibet Plateau,which has huge carbon stocks at the highest altitude,is one of the most sensitive regions to global warming,and its grassland carbon stocks account for about 4.0%of global grasslands.Soil respiration is an important pathway mediating the loss of soil organic carbon（SOC）in the context of climate warming,accounting for 60%of total ecosystem respiration.Previous studies on the response of soil respiration to SOC storage on the Tibetan Plateau have focused on the absolute content of soil organic carbon,while less attention has been paid to the quality of SOC.To address these issues,this paper analyzes the differences in soil physicochemical properties between the surface（0-10 cm）and bottom（20-30 cm）layers of different grassland types in Tibetan alpine grasslands through large-scale field soil respiration surveys and sampling,and uses lignin phenols and amino sugars to indicate soil carbon fractions derived from plants and microorganisms,respectively,to investigate the relationship between soil respiration and The distribution characteristics of soil respiration and biogenic carbon（plant-and microbial-derived carbon）in southern Tibetan grasslands were investigated by using lignin phenols and amino sugars as indicators of plant-and microbial-derived soil carbon components,respectively,to analyze the influences of soil respiration in alpine grasslands and to reveal the effects of different biogenic SOC on soil respiration.The results of this work are as follows:the loss and accumulation of SOC molecular fractions during incubation,the relationship between heterotrophic respiration rate and SOC molecular fractions,and the influence of different SOC molecular fractions on heterotrophic respiration.The main results of this thesis are as follows:（1）The indoor experiments and data analysis revealed that alpine meadow had the highest vegetation cover,net primary productivity,aboveground biomass and total biomass among the three grassland types in southern Tibet,while SOC,total N,C/N,total phosphorus,microbial biomass carbon and water extractable N were the highest,while desert steppe had the lowest.（2）Using lignin phenols and amino sugars to characterize plant-and microbial-derived SOC components（plant-and microbial-derived carbon）,respectively,and to analyze the relationship between them and soil respiration and the pathways by which this relationship is maintained through field investigations at large-scale field.The results show that soil respiration rates are large in alpine meadow with high plant-derived carbon content.Soil respiration would first consume plant-derived carbon,such as cinnamyl phenol,and this process would be influenced by vegetation characteristics and soil properties;however,if microbial-derived carbon accumulated to a certain level,soil respiration would choose to consume microbial-derived carbon,such as fungal residues,leading to the accumulation of plant-derived carbon,which would be achieved through vegetation characteristics.Based on the results,it is hypothesized that the short-term rapid accumulation of plant-derived carbon will significantly enhance soil respiration and also lead to a significant accumulation of microbial-derived carbon;however,when the accumulation of microbial-derived carbon exceeds a certain level,vegetation will choose to consume microbial-derived carbon,thus promoting the accumulation of plant-derived carbon,which may change the composition of SOC and make it unstable.These results provide a new research perspective on the relationship between soil respiration and SOC in grassland ecosystems on the Tibetan Plateau,and provide a scientific basis for predicting the feedback of SOC to climate change.（3）Based on indoor incubation experiments combined with Fourier transform infrared（FTIR）,we investigated the response of heterotrophic respiration to temperature change and the effect of SOC fractions on heterotrophic respiration in alpine grasslands in southern Tibet.The results showed that heterotrophic respiration varied most with temperature in alpine meadow with high SOC content.Increasing temperature narrowed the difference in temperature response between surface（0-10cm）and bottom（20-30 cm）soils,and the heterotrophic respiration rate and Q₁₀ values were significantly higher in the bottom layer than in the surface layer,and the heterotrophic respiration rate of alpine meadow soils incubated at 5°C was significantly higher than in other temperature zones.The aromatic C=C and aromatic C-H contents were the highest in the SOC fraction of the Tibetan grassland,and the aromatic C=C content was significantly higher in the bottom layer than in the surface layer.In the absence of exogenous carbon input,warming leads to an increase in SOC chemical recalcitrance across the soil profile（0-30 cm）through the accumulation of microbial-derived carbon.These findings emphasize that different carbon fractions have different effects on heterotrophic respiration,and in-depth analysis of the relationship between heterotrophic respiration and SOC fractions with temperature change is the key to understanding the mechanism of SOC accumulation in southern Tibetan grasslands and accurately predicting organic carbon dynamics in alpine grasslands.In summary,the Tibetan Plateau alpine meadow in southern Tibet have the highest SOC and plant-derived carbon content and the highest soil respiration work intensity.The accumulation of microbial-derived carbon content will lead to the accumulation of plant-derived carbon after a certain level.Temperature increase will lead to an increase in SOC chemical recalcitrance across the soil profile（0-30 cm）through the accumulation of microbial source carbon.The above findings are important for understanding the response mechanism of soil carbon cycle to climate change,and provide scientific basis for accurate assessment of SOC dynamics and full understanding of carbon sequestration function in alpine grasslands.