<b>Stable carbon degradation strategies drive microbial carbon use efficiency after afforestation</b>

Microbial carbon use efficiency (CUE) is a critical metric to assess soil microbial participation in the carbon cycle. However, the mechanistic links between microbial functional traits and CUE under land-use change remain poorly understood. By selecting soil samples from a 45-year <i>Robinia pseudoacacia</i> (RP) afforestation chronosequence and combining an ¹⁸O-H₂O labeling method with metagenomic techniques to explore how afforestation alters soil substrate conditions and microbial metabolism. We found that CUE increased due to a relatively high increase in microbial biomass and a relatively low increase in microbial respiration after afforestation. This outcome is jointly determined by soil stoichiometry and carbon substrate characteristics. Nutrient imbalances, especially phosphorus limitation, stimulated microbial communities to mine stable organic matter for essential elements. Since the breakdown of stable substrates requires higher activation energy, microbes must enhance their metabolic efficiency, thereby increasing CUE. Functional gene analysis further revealed that forest soils with higher abundances of stable carbon-degrading genes exhibited enhanced CUE, while farmland soils with fewer such genes showed low CUE. These findings provide empirical evidence for microbial mediation of soil substrate changes and emphasize that the influence of the stable carbon pool on microbial metabolism is not negligible in soil carbon cycle.微生物碳利用效率（microbial carbon use efficiency, CUE）是评估土壤微生物参与碳循环的核心指标。然而，土地利用变化背景下，微生物功能性状与CUE之间的内在机制关联仍未得到充分阐明。本研究选取为期45年的刺槐（*Robinia pseudoacacia*, RP）造林时间序列土壤样品，结合¹⁸O标记水（¹⁸O-H₂O）标记法与宏基因组学技术，探究造林过程如何改变土壤底物条件与微生物代谢模式。研究结果显示，造林后土壤CUE得以提升，其原因在于微生物生物量的相对显著增长，以及微生物呼吸的相对小幅增加。该效应由土壤化学计量特征与碳底物特性共同调控。养分失衡尤其是磷限制，会驱动微生物群落分解稳定有机质以获取必需营养元素。由于稳定底物的降解需要更高的活化能，微生物必须提升自身代谢效率，进而提高CUE。功能基因分析进一步表明，携带丰富稳定碳降解基因的森林土壤表现出更高的CUE，而此类基因丰度较低的农田土壤则呈现较低的CUE。本研究为微生物介导土壤底物变化提供了实证依据，并强调在土壤碳循环过程中，稳定碳库对微生物代谢的影响不容忽视。