Postfire soil convergence toward an unburned reference varies with fire severity and soil depth

Background Wildfire effects on soil can differ among soil properties, soil depths, and fire-severity classes, yet their longer-term trajectories remain poorly resolved in subtropical pine plantations. We evaluated temporal convergence of burned soils toward an unburned reference following an April 2022 wildfire in a Pinus massoniana plantation in central China. Twenty permanent plots representing unburned, low-, moderate-, and high-severity conditions were monitored across eight surveys over 43 months and three soil depths. Results Severity-associated differences were strongest and most consistent in the 0–10 cm soil layer during the early monitoring period. Under high severity, surface-soil organic carbon (SOC) at the first survey was 19.08 ± 1.49 g kg −1 compared with 38.61 ± 0.59 g kg −1 in the unburned reference, corresponding to a 50.6% deficit. By 43 months, SOC under high severity had increased to 35.10 ± 0.27 g kg −1 , while the unburned reference was 38.96 ± 0.93 g kg −1 , leaving a residual difference of 3.86 g kg −1 , or approximately 9.9%. pH and available phosphorus approached reference values more rapidly, whereas SOC showed the clearest persistent difference. Complete eight-survey trajectories showed an overall narrowing of burned–unburned differences, although short-term and seasonal fluctuations occurred. Integrated multivariate analyses similarly indicated decreasing departure from the unburned reference, with the largest initial displacement and greatest persistence under high severity. Conclusions Over the 43-month monitoring period, burned soils showed substantial but incomplete convergence toward the selected unburned reference, and the magnitude and persistence of this convergence varied with fire severity, soil depth, and soil property. Because no prefire measurements were available, this convergence should not be interpreted as demonstrated restoration of the original prefire condition. Faster convergence of pH or available phosphorus therefore did not indicate complete soil convergence while SOC remained depleted. These results support multi-indicator, depth-explicit monitoring of postfire soil condition, particularly following high-severity wildfire.