Objective Land-use change is 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 land use change on ecosystem carbon cycling.

Method This study analyzed soil samples collected from forest and cropland site 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, and the key factors and regulatory mechanisms driving its changes were systematically analyzed.

Result (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 16.5 years after conversion. Prior to this threshold, microbial CUE declined overall by 34.78%. 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. Using 0.06 as the threshold for the soil carbon quality, there were significant differences in the β-diversity of microbial communities between high and low carbon quality cropland soils. Overall, r-strategists dominated in post-conversion soils; however, when soil carbon quality fell below 0.06, the dominance of r-strategists diminished, while the relative abundance of K-strategists microorganisms increased significantly.

Conclusion After forest conversion to cropland, soil microbial CUE first decreased and then recovered, reaching its minimum at 16.5 years. 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 finding enhances the mechanistic understanding of carbon cycling under tillage disturbance and offers a scientific basis for harmonizing agricultural production with ecological conservation.