| Soil carbon fixation and emissions play a crucial role in the global carbon cycle and climate change.Meanwhile,the soil organic carbon(Soil organic carbon,SOC)cycle is tightly interlinked with the nitrogen(Nitrogen,N)and phosphorus(Phosphorus,P)cycles.With increasing fertilizer application and atmospheric N deposition rates,additional N and P inputs to soils are significantly higher.It was found that N and P addition promoted the decomposition of recalcitrant SOC and increased the content of labile SOC,but it was also suggested that P input inhibited the decomposition of SOC.Consequently,the response of SOC to N and P addition is uncertain.The increased N and P inputs will alter the community composition and function of plants and microorganisms to affect their contributions to SOC.Because of the differences in molecular stability and stabilization mechanisms of organic carbon components from different sources in soils,biomarker techniques were used to extract labile SOC components(neutral sugar),plant-derived carbon(lignin phenol)and microbial-derived carbon(amino sugar)with the aid of a long-term experimental platform for nutrient addition in an alpine grassland on the Tibetan Plateau.We investigated the responses of different SOC components in an alpine grassland on the Tibetan Plateau to 10-year nutrient addition and their response mechanisms.The key findings are as follows:(1)The N and P addition stimulate the depletion of neutral sugar in soilsThe N and P addition significantly decreases the concentration of neutral sugar in soils,which did not significantly change with the NP addition.Nutrient addition increases the ratio of hexose to pentose(GM/AX).SOC is negatively correlated with soil neutral sugar concentration but positively correlated with the GM/AX ratio.These results suggest that the concentration of SOC is higher at lower neutral sugar concentration and higher GM/AX ratio.It is reasonable that microorganisms produce relatively small amounts of microbial-derived hexose by consuming more labile plantderived pentose induced by imbalanced nutrients with N and P addition.This would lead to lower neutral sugar concentration and regulate the transformation of labile SOC pool to stable SOC pool.(2)The P and N combined P(NP)addition suppress the accumulation of microbial necromass but stimulate the accumulation of lignin phenol in soilsThe P and NP addition significantly decreases the concentration of amino sugar in soils while increasing the concentration of lignin phenol in the topsoil(0–10 cm).The N addition significantly decreases the concentration of lignin phenol in the subsoil(20–30 cm).Redundancy analysis and random forest model indicate that total soil phosphorus(Total phosphorus,TP)is the most important factor affecting the concentration of amino sugar and lignin phenol.(3)The P and NP addition weaken the relative stability of SOCThe N addition in the subsoil significantly increases the ratio of amino sugar to lignin phenol(AS/VSC).The P and NP addition significantly decreases the ratio of AS/VSC(except P addition in the subsoil)and the ratio of fungal necromass carbon to bacterial necromass carbon(FNC/BNC)in the subsoil.According to amino sugar being more stable in soils compared to lignin phenol and fungal necromass carbon having higher molecular stability compared to bacterial necromass carbon,these results may indicate that 10-year P and NP addition has increased SOC instability and may improve the potential for SOC loss in the future,but further verification is needed.In conclusion,the effects of nutrient addition on plant-and microbial-derived SOC components and the influencing mechanisms were examined with the help of biomarker techniques.The aforementioned findings contribute to a better knowledge of SOC change and its mechanism of accumulation in the context of increasing soil N and P inputs,and they may enhance the precision of future soil carbon storage predictions to successfully combat climate change. |
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