Sources,Turnover And Driving Factors Of Soil Organic Carbon In Alpine Grasslands On The Tibetan Plateau

The alpine grassland ecosystem of the Tibetan Plateau is a major portion of the terrestrial ecosystem,contributing 2.5%to global soil organic carbon（SOC）stocks but covering only 0.3%of the Earth’s total terrestrial area.It plays an important role in the global carbon cycle and in achieving carbon peaking and carbon neutrality goals.In addition,the Tibetan Plateau is a sensitive and critical area for global climate change,and mineral soils with low degrees of weathering have a high potential response to climate warming.Plant and microbial necromass carbon are the main sources of SOC,and SOC turnover times effectively characterize SOC stability,which is a key parameter in predicting soil carbon pool changes.However,the contribution and preservation mechanism of carbon components from different biological sources to SOC in alpine grasslands and the driving mechanism of soil geochemical and climatic factors on SOC turnover remain unresolved,particularly for the biogeochemical processes involved in SOC dynamics and weathering,which hinders the accurate assessment of SOC dynamics in alpine grasslands under climate change.Here,a large-scale field survey was conducted along a 3500 km transect and the samples were analyzed by combining them with biological markers,Fourier transform infrared spectroscopy,carbon stable isotopes,elemental analysis,and other technical means.Plant-versus microbe-derived biomarkers were employed（including lignin phenols and amino sugars）to illustrate its controlling factors and the relative contribution of plant versus microbial necromass to SOC in alpine grasslands.This study investigates the distribution patterns and driving factors of SOC turnover time in alpine grasslands.Furthermore,weathering-mediated topsoil（0–10 cm）and subsoil（20–30 cm）organic carbon driving mechanisms were revealed.The main results are summarized as follows.（1）This study used lignin phenols and amino sugars to trace plant-and microbial-derived SOC components,respectively,and their absolute concentrations and preservation characteristics in the topsoil and subsoil of alpine grasslands and alpine meadows were investigated.The results showed that the absolute concentrations of lignin phenols and amino sugars in the alpine meadow soil were significantly higher than those in the alpine steppe,and the concentrations in the topsoil were higher than those in the subsoil.Aridity index is an important climatic factor that controls the distribution of absolute concentrations of lignin phenols and amino sugars.Increasing plant species richness is favorable for the accumulation of carbon in plants and microbial necromass,however,its promoting effect on subsoils is weakened.The soil cation exchange capacity was the main factor affecting the distribution of SOC molecular composition.Higher nutrients and lower p H are beneficial for the accumulation of lignin phenols and amino sugars in the soil.Soil minerals have protective effects on both plant-and microbial-derived carbon components;however,clay and iron-aluminum oxides are more closely related to amino sugars than lignin phenols.Drought and low temperatures weakened the decomposition of lignin phenol and the accumulation of amino sugars.The above findings indicate that the positive effect of species abundance on amino sugars and lignin phenols weakened with the deepening of the soil layer,and the protective effect of minerals on microbial residue carbon was stronger.（2）The contribution of microbial residue carbon to the SOC of alpine grassland was quantified,and its spatial distribution pattern was clarified.The contribution of microbial residual carbon to SOC in alpine grasslands（35%）was lower than that in global grasslands（47%）,and the relative concentration of lignin phenol was higher than that in global grasslands.As the drought index increased,the relative concentration of lignophenols decreased,while the contribution of microbial residue carbon to SOC increased.The contribution of fungal necromass carbon to SOC in alpine grassland（27%）was greater than bacterial necromass carbon（8%）.The relative lignin phenol concentration was significantly negatively correlated with SOC content,whereas the values of amino sugar/lignin phenols and fungal residues/bacterial residues were significantly positively correlated with SOC.Overall,the accumulation of SOC in alpine grasslands was not attributable to lignin phenol,and fungal residue carbon played an important role in the accumulation of SOC.（3）The SOC turnover times（τ）in the 0–30 cm soil layer of alpine grasslands on the Tibetan Plateau were estimated,and the key drivers ofÏ„were explored in combination with climatic and geochemical variables.This thesis found thatÏ„ranged from 4 to 289 years on the Tibetan Plateau and showed a decreasing trend from the northwest to southeast and an increasing trend with altitude.The estimatedÏ„was 70₆₂⁷⁷years（mean with 95%confidence interval）in alpine meadows,which did not significantly differ from alpine steppes(75₆₄⁸⁶ yr).Overall,using boosted regression tree analysis,geochemistry was the most important controlling factor forτ（54%of the relative effect onτ）,followed by climate（36%）,and altitude（10%）.When examining the relative contribution of individual variables,we found that mean annual precipitation was the primary predictor ofÏ„,followed by soil Si content,and altitude.Notably,variation inÏ„was explained by precipitation rather than temperature.Altitude indirectly affectedÏ„by regulating climatic and soil geochemical conditions.The direct negative effect of climate onÏ„was opposite the positive indirect effect of climate onÏ„via soil geochemistry.These results highlight the importance of considering the interactions of climatic and geochemical factors as well as hydrological conditions when predicting how carbon turnover in the soils of semi-arid alpine grasslands will respond to future climate change.（4）The relationships between weathering intensity and soil properties,vegetation characteristics,molecular composition of SOC biological sources,and turnover time were systematically investigated.This study revealed that plant species richness,aboveground biomass,soil iron and aluminum oxides,relative concentrations of amino sugars,and values of glucosamine/muramic acid were significantly positively correlated with chemical weathering intensity,whereas soil p H and lignin phenol were significantly negatively correlated with chemical weathering intensity.The increase in soil nutrients released by minerals during the weathering process leads to an increase in grassland productivity and an increase in soil Fe and Al oxides and soil clay,which promotes the enhancement of the mineral stability of SOC and the increase in the contribution of fungal residue carbon to SOC,which affects SOC turnover and sequestration.Structural equation modeling showed that aridity index was the dominant factor for SOC dynamics in the alpine grasslands on the Tibetan Plateau.Weathering plays a key role in regulating SOC dynamics,however,the driving paths of topsoil and subsoil organic carbon are different.Aridity index and vegetation characteristics had a greater impact on SOC content in topsoil than in subsoil.The influence of climatic factors weakened with increasing soil depth.The molecular composition of SOC and soil mineralogy characteristics became more important for SOC sequestration in the bottom layer relative to the topsoil.This study reveals the regulatory role of weathering in SOC and provides novel insights into the mechanisms of SOC source,turnover,and sequestration.In summary,the contribution of microbial necromass carbon to SOC in alpine grasslands on the Tibetan Plateau is smaller than that in global grasslands,and fungal necromass carbon plays a dominant role in the accumulation of SOC.SOC turnover times increased with altitude and were controlled by the interaction between climate and soil geochemistry.Weathering is a key process regulating SOC cycling,which controls carbon input,loss,and SOC stability.The above findings have important implications for understanding the response mechanism of the soil carbon cycle to climate change during the weathering process and provide a scientific basis for accurately assessing the dynamic changes in SOC and enhancing carbon sequestration in alpine grasslands.