Wildfire-Induced alterations in soil physical, chemical, and micromorphological properties in forest ecosystems of Yunnan, Southwestern China

Wildfire is a major ecological disturbance with profound implications for forest ecosystems, particularly through its effects on soil quality and structure. This study examined the impacts of a severe wildfire (2019) on soil physical, chemical, and micromorphological properties in a mountainous forest region of Yunnan Province, southwestern China. Soil samples were collected from post-fire and adjacent control sites at three depths (0-5, 5-15, and 15-30 cm) and analyzed using a factorial design. Results indicated that the most pronounced fire-induced alterations occurred in the surface layer (0-5 cm). Post-fire soils exhibited increased sand content but reduced clay content, porosity, and field capacity. Chemically, wildfire increased soil pH while reducing electrical conductivity (EC), soil organic carbon (SOC), available phosphorus (AP), and available potassium (AK) in the surface horizon, with limited changes in deeper layers. Micromorphological observations revealed modified aggregate structures, carbonized root fragments, dispersed fungal hyphae, and an abundance of charcoal particles, reflecting both physical and biological disturbances. The soil mass fractal dimension (D) also increased in post-fire soils, indicating greater structural fragmentation and aggregate breakdown. This increase in D is consistent with the observed decreases in porosity and aggregate stability, reflecting a more heterogeneous and degraded pore network. These findings demonstrate that wildfire predominantly affects surface soil horizons, altering infiltration capacity, nutrient dynamics, and microbial activity. Integrating physical, chemical, and micromorphological assessments offers a more comprehensive perspective on fire-induced soil changes, and may provide a useful foundation for post-fire management strategies aimed at supporting the resilience of forest ecosystems under increasing wildfire frequency and intensity.