Soil quality assessment and constraint diagnosis of Schima superba firebreaks.

Biological firebreaks are essential for forest fire prevention, with Schima superba serving as a dominant species in southern China. However, the dynamics of soil quality across stand ages in these firebreaks and their capacity to sustain long-term fertility and ecological functions that support forest stability remain poorly understood. In this study, a space-for-time substitution approach was employed to investigate three Schima superba firebreaks of different ages located on ridge sites, integrating soil quality assessment and constraint diagnosis indices. The results revealed that most soil indicators increased significantly with stand age. The minimum dataset (MDS) included available nitrogen (AN), soil organic carbon (SOC), sand, silt, pH, electrical conductivity (EC), bulk density (BD), microbial biomass phosphorus (MBP), and soil water content (SWC). The retained indicators accounted for 47.06% of the original total set. The soil quality index (SQI) values ranged from 0.29 to 0.64 and increased significantly with stand age. Despite these improvements, the overall SQI predominantly fell within Levels III and IV, reflecting generally poor soil quality. The primary contributors to SQI were AN, SOC, SWC, MBP, BD, and pH. Constraint diagnosis across stand ages identified SOC, SWC, silt, and MBP as major limiting factors. Nutrient-related constraints (SOC, AN, and MBP) accounted for 35.62%-40.59% of the total limitations, highlighting nutrient deficiency as the principal restrictive factor. However, physical constraints (sand, silt, and BD) and EC may pose increasing risks to the long-term sustainability of Schima superba firebreaks. These findings provide a robust scientific foundation for assessing the ecological function of Schima superba firebreaks and for guiding soil management and improvement strategies.