【Objective】Aliphatic acids are widely regarded as one of the most recalcitrant and stable components of soil organic matter, and their abundance and molecular composition provide molecular-level indicators for assessing the extent of soil organic matter decomposition and accumulation. Therefore, elucidating the dynamics and controlling factors of the relatively labile free aliphatic acid fraction in soils is therefore critical for advancing mechanistic understanding of soil organic matter degradation under long-term agricultural cultivation, particularly in the black soils. Thus, this study aimed to characterize changes in the content and composition of free aliphatic acids along a century-long cultivation chronosequence and to identify the key environmental drivers regulating their dynamics. 【Method】Soil samples were collected from a 5?100 year dryland cultivation chronosequence in the Sanjiang Plain, Northeast China. Changes in the concentrations and molecular characteristics of free aliphatic acids during soil organic matter depletion were quantified. Random forest models were applied to assess the relative importance of soil physicochemical properties in regulating short-chain and long-chain aliphatic acids. 【Result】The results show that long-term cultivation markedly reduced soil free aliphatic acid contents, with a 58.4% decline from 5 to 100 years after reclamation, substantially exceeding the decrease in SOC (47.6%). This indicates that free aliphatic acids are more sensitive to cultivation disturbance than bulk soil organic matter. Prolonged cultivation induced pronounced shifts in molecular composition, characterized by a 43.9% reduction in the relative contribution of short-chain aliphatic acids to SOC and a concurrent 30.2% increase in long-chain aliphatic acids, resulting in a significant increase in average chain length (ACL). After 100 years of cultivation, unsaturated aliphatic acids in black soils declined by 80.2%, accompanied by a significant decrease of 17.7% in the carbon preference index (CPI), indicating intensified decomposition losses of plant-derived aliphatic acids. Collectively, these molecular-level changes demonstrate a progressive enhancement of soil organic matter decomposition under conventional straw removal management in the Mollisols. Correlation analysis and random forest modeling further demonstrated that the rapid depletion of short-chain aliphatic acids was mainly driven by reduced availability of labile carbon substrates and enhanced microbial decomposition, whereas long-chain aliphatic acids were preferentially preserved due to their higher chemical recalcitrance and association with clay minerals. 【Conclusion】Enhancing external organic matter inputs, such as through straw return, may help replenish labile carbon pools and mitigate the progressive loss of stable soil organic matter fractions. Overall, this study provides new molecular-level insights into cultivation-induced soil organic matter degradation and offers a scientific basis for the sustainable management and conservation of black soils.