The formation of humic acid and micro-aggregates facilitated long-time soil organic carbon sequestration after Medicago sativa L. introduction on abandoned farmlands

The substantial carbon sequestration observed in abandoned farmland assumes a pivotal role in mitigating the impacts of global warming. However, it remains unclear how to effectively manage abandoned farmlands to achieve this goal, especially over the long term, and understand the underlying mechanisms. The introduction of legumes can augment vegetation coverage, mitigate soil erosion, and ameliorate soil quality. A long-term study has been conducted in the semiarid region of the Loess Plateau since 2003, with a focus on revegetation strategies involving the introduction of alfalfa (Medicago sativa L.) and sweet clover (Melilotus officinalis L.), while natural abandonment (fallow) served as a control. In this study, we utilized a physical–chemical combination approach to investigate the carbon sequestration process within the 0–20 cm soil layer. Our results demonstrated that, in comparison to the fallow, alfalfa introduction significantly increased soil organic carbon (SOC), light fraction organic carbon (LFOC), and heavy fraction organic carbon (HFOC) concentrations, whereas no significant differences were observed when comparing with sweet clover. The amount and proportion of humic acid (HA) within HFOC were notably higher in alfalfa fields compared to fallow fields. While soil wet-sieved macro-aggregates (>2 mm and 0.25–2 mm) showed no significant difference between alfalfa and fallow fields, soil wet-sieved micro-aggregates were significantly more abundant in alfalfa fields. These wet-sieved micro-aggregates displayed high levels of humification across all fields. As soil wet-sieved aggregate size increased, the proportions of humin (HM) and HA decreased, while the proportion of fulvic acid (FA) increased. We concluded that the formation of HM, HA, and soil wet-sieved micro-aggregates played key roles in promoting long-term soil carbon storage following alfalfa introduction. These findings enhance our understanding of soil ecosystem responses to future climate change and underscore the significance of species selection in restoration processes to effectively mitigate global warming.