Retention of Fine Woody Debris Reduces Stability of Soil Organic Carbon Pool by Changing Soil Organic Carbon Fractions and Enzyme Activities in Urban Picea koraiensis Plantations

The importance of urban forest management and carbon cycle research has increased amidst ongoing urbanization. Understanding the potential impact of fine woody debris (FWD) retention as a management strategy on the soil organic carbon (SOC) levels and stability in urban forests is crucial. In this study, four FWD retention treatments (no retention, CK; low retention, LR; medium retention, MR; and high retention, HR) were implemented in Harbin urban Picea koraiensis Nakai plantations to investigate the stability of the SOC pool in response to these treatments. The FWD retention treatment had no significant effect on the soil’s physical and chemical properties and SOC concentration, but significantly reduced the total potassium and NO3− concentrations. The FWD retention treatment increased active SOC fractions and carbon-degrading enzyme activities, while reducing leucine aminopeptidase, polyphenol oxidase enzyme activities, and the stability of the SOC pool. The random forest model showed that FWD retention, particulate organic carbon, cellobiohydrolases, and β-xylosidase enzyme activities were factors that significantly affected the stability of the SOC pool. These findings suggest that retaining a large amount of FWD in northeast China can benefit the soil carbon cycle in urban plantations by accelerating the turnover of active SOC fractions.