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In grassland ecosystems, many plants gradually senesce in autumn, forming standing dead material (standing litter) that remains through the winter rather than immediately falling to the ground as litter. However, limited research has focused on the decomposition of standing litter during winter and its subsequent effects on decomposition after being shed in the following spring. We conducted a one-year experiment in the typical steppe of Inner Mongolia, China, to investigate the decomposition processes of litter from the dominant plant Leymus chinensis and Stipa grandis litter in autumn and spring. During the experiment, autumn-shed litter was placed directly on the soil surface, while spring-shed litter remained suspended above the soil surface for the first seven months of winter until it detached and fell to the ground. The results showed that throughout the study, the decomposition rate of spring-shed litter was consistently faster than that of autumn-shed litter. Notably, the lignin content in spring litter decreased significantly during the winter standing period, while no such change was observed in autumn litter. This suggests that photodegradation played a crucial role in lignin decomposition during the winter, facilitating the subsequent breakdown of plant litter. These findings highlight the significant differences in decomposition rates between litter shed in autumn and spring.