Background and aims <p>Fine woody debris (FWD) can provide faster nutrient release and carbon (C) turnover in the soil and improve soil quality due to its faster decomposition rate. However, the potential of retaining fragmented FWD to improve the soil quality in urban forests remains uncertain.</p> Methods <p>This study investigated the effects of retaining fragmented FWD (control (CK), 0 gC·m⁻²; low C retention (LC), 250 gC·m⁻²; medium C retention (MC), 500 gC·m⁻²; high C retention (HC), 1000 gC·m⁻²) on soil and enzyme properties, soil quality index (SQI) and multifunctional index of soil enzymes (MIE) in six urban plantations in Harbin, Heilongjiang Province, China.</p> Results <p>Compared to CK, FWD retention improved soil properties, nutrient availability, soil organic C (SOC) content, and enzyme activities, while alleviating phosphorus limitation but increasing nitrogen (N) limitation. In <i>Larix gmelinii</i> and <i>Betula platyphylla</i> plantations, FWD reduced microbial C limitation, while it increased C limitation in <i>Picea koraiensis</i>, <i>Pinus sylvestris</i> var. <i>mongolica</i>, and <i>Juglans mandshurica</i> plantations. Retaining FWD significantly improved SQI and MIE, with the strongest effects observed in MC and HC treatments. SQI showed a positive correlation with MIE, emphasizing their interdependence. Using random forest models and partial least squares path modeling, it was found that enzyme properties have the strongest direct impact on MIE, followed by microbial metabolic limitations and soil properties.</p> Conclusion <p>Therefore, the application 500–1000 gC·m⁻² (approximately 1.5–3 kg·m⁻² of FWD) can improve soil quality and ecosystem functions in urban forests in Heilongjiang province. The results provide reference for improving ecosystem services and sustainable management strategies.</p>