Objective Land-use changeis a key driver of the global carbon cycle. Understanding its effects on soil microbial carbon use efficiency (CUE) from a microbial metabolic perspective is therefore critical, as it can provide a theoretical foundation for assessing the impact of LUC on ecosystem carbon cycling.

Method This study analyzed soil samples collected from forest to cropland conversion sites in Maoershan, Heilongjiang Province. Primary forest soil served as the control group, while cropland soils with varying durations since conversion constituted the treatment groups. Key indicators including soil physicochemical properties and microbial traits were measured through laboratory experiments. Microbial CUE was calculated using stoichiometric methods to systematically analyze the key factors and mechanisms driving its variation.

Results (1) Following conversion from forest to cropland, microbial CUE decreased significantly by 12.61% relative to the original level. With increasing cultivation duration, microbial CUE exhibited a nonlinear trend characterized by an initial decline followed by a later increase, with the lowest point occurring at approximately 16.49 years after conversion. Prior to this threshold, microbial CUE declined overall by 34.78%, then increased by 53.33% by the 40th year. Specifically, microbial CUE in the plots cultivated for 13 and 24 years was significantly lower than in forest soil, showing reductions of 26.1% and 23.9%, respectively. (2) Soil carbon quality (ratio of aliphatic to aromatic carbon, rA2930:rA1635) was the most significant driver of microbial CUE changes post-conversion. As soil carbon quality increased, microbial CUE initially decreased and then increased at a threshold value of 0.06. (3) Soil carbon quality regulates microbial CUE by driving shifts in microbial β-diversity and r/K strategies. A significant shift in microbial β-diversity was observed at a carbon quality threshold of 0.06. Furthermore, r-strategists were dominant in post-conversion soils, but their dominance weakened as soil carbon quality fell below 0.06. Specifically, when soil carbon quality was below 0.06, he relative abundance of r-strategists was significantly lower than in high carbon quality soils; conversely, the relative abundance of K- strategists was significantly higher.

Conclusion Our research reveals that, following forest to cropland conversion, microbial CUE showed an initial decline followed by an increase, reaching its nadir at 16.49 years after conversion. This dynamic is primarily driven by soil carbon quality (the ratio of aliphatic to aromatic carbon) and exhibits a threshold effect at 0.06, below this threshold, microbial β-diversity undergoes a significant shift, with the balance of life history strategies tilting toward K-strategy. This, in turn, drives a gradual recovery of microbial CUE in the later stages of conversion by enhancing metabolic efficiency. This study demonstrates that soil carbon chemistry, by regulating microbial community structure and functional strategies, serves as a key mechanism determining soil carbon cycle efficiency under land-use change. Research demonstrates that soil carbon quality plays a fundamental role in regulating microbial community structure and function, thereby governing the efficiency of microbial CUE under land-use change. This mechanism enhances the mechanistic understanding of carbon cycling under tillage disturbance and offers a scientific basis for harmonizing agricultural production with ecological conservation.