Background The occurrence of forest fires leads to increased soil erosion, and the effect of fire disturbance on the stability of soil aggregates is not yet clear. The Jinyun Mountains in Chongqing suffered a fire in August 2022, in which coniferous and broad-leaved mixed forests are widely distributed and highly flammable, and are the most vulnerable forest type to fire damage, so it is important to study the stability of soil aggregates under this forest.

Methods Three 20×20 m sample plots were set up in Jinyun Mountain coniferous and broad-leaved mixed forests with mild (S), moderate (M) and severe (L) fire disturbance and non-fire disturbed areas (CK). Soil samples of topsoil (0~10 cm) and subsoil (10~20 cm) were collected by S-type sampling method at the sample site. Used three different fragmentation mechanisms of fast wetting (FW), slow wetting (SW) and wet stirring (WS) in Le Bissonnais (LB) method to simulate the damage of soil aggregates. The stability of soil aggregates is evaluated by measuring the average mass diameter (MWD) of soil aggregates and the relative dispersion index (RSI), relative fragmentation index (RMI) and characteristic parameter (K_a) calculated using the MWD values. The average mass diameter values under different fire interference intensities are respectively expressed by MWD_S, MWD_M and MWD_L, and the average mass diameter values under different treatment methods are respectively expressed by MWD_FW, MWD_SW and MWD_WS.

Results 1) Under fire disturbance, soil aggregates in coniferous and broad-leaved mixed forests were damaged most under FW treatment, followed by WS, and the damage was least under SW treatment. Under FW treatment, the proportion of water stable aggregates >0.50 mm in coniferous and broad-leaved mixed forests showed S<M<L<CK. Compared with CK topsoil, S, M and L decreased by 2.94%, 1.51% and 1.59% respectively under SW treatment. Under WS treatment, S surface soil decreased the most by 18.87%. 2) Under the fire disturbance, the soil of the forest land showed the characteristics of MWD_FW < MWD_WS < MWD_SW as a whole. MWD values of soil agglomerates under different degrees of fire exhibit MWD_S < MWD_M < MWD_L except for the lower soil under FW treatment, which exhibits MWD_S < MWD_L < MWD_M. MWD values were significantly higher (P < 0.05) in all soil layers under light fire disturbance and in the surface layer under moderate fire disturbance under the SW treatment than under the other treatments. 3) The RSI value, RMI value and K_a value of the topsoil of the forest under fire disturbance were higher than those of the subsoil.

Conclusions The stability of soil aggregates in coniferous and broad-leaved mixed forests fire sites varied in response to different fire intensity disturbances, with light fire disturbances reducing the stability of soil aggregates and moderate and heavy fires increasing the stability of soil surface aggregates. The stability of soil aggregates is lowest under heavy or stormy rainfall, followed by mechanical disturbance and highest under light rainfall, and the stability of soil aggregates is higher in all the surface layers than in the lower layers.