Objective The aims were to explore the effects of straw returning to the soil on soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), and to clarify the key mechanisms and regulatory thresholds of these effects based on the chemical properties of the straw itself.

Method This study combined meta-analysis with Random Forest models to conduct a comprehensive analysis of articles published from January 2000 to June 2025, synthesizing 480 studies and extracting 2,555 observations to evaluate the effects of straw return on farmland soil MBC, MBN and their ratio (MBC∶MBN).

Result Results indicated that straw incorporation increased MBC, MBN and the MBC∶MBN ratio by 29.1%, 26.5% and 0.6%, respectively. Random Forest model analysis further revealed that straw chemical properties served as the dominant regulatory factors: straw C∶N ratios exerted significantly positive effects on MBC, MBN and MBC∶MBN, with respective increases of 27.5%, 24.4% and 6.1% when the C∶N ratio was below 20. MBC increased with higher straw incorporation rates, whereas MBN was significantly enhanced when straw water-soluble carbon content reached a threshold of 4.7%. Rice and wheat straw preferentially promoted MBN accumulation, while maize straw significantly elevated MBC. Regarding environmental conditions and duration, straw incorporation demonstrated more pronounced effects in regions with mean annual temperatures of 5 - 20℃ and annual precipitation of 250 - 1000 mm. Long-term straw incorporation (≥ 10 years) exhibited superior enhancement of MBC, MBN and MBC∶MBN compared to short-term incorporation (< 10 years).

Conclusion Straw return could significantly enhance the contents of MBC and MBN in farmland soil, with its effectiveness being comprehensively regulated by straw intrinsic characteristics (including C∶N ratio, water-soluble carbon content, and straw type) as well as temperature-moisture environmental conditions and return duration. Therefore, matching appropriate straw types, return durations, and planting regions based on straw chemical characteristics represents a key pathway to improving the soil fertility enhancement effects of straw return.