Effects of Fire Disturbance on the Structural Stability of Plantation Soil Aggregates

Three different fragmentations of Le Bissonnais(fast-wetting sieving, slow-wetting sieving and wet-stirring sieving were used to simulate the damage of soil aggregates under 3 different rainfall conditions: heavy rain, light rain and disturbance. The effects of burning on the stability of soil aggregate structure of Pinus massoniana plantation were characterized by the characteristics of 3–5 mm dry sieving aggregates after experimental treatment. The results showed that FW caused the largest degree of breakage, WS took the second place, while SW processed the smallest degree of breakage. After the burning, the number of > 0.5 mm large aggregates in soil increased, while the number of small aggregates decreased. Under FW and WS treatment, the content of > 0.5 mm water-stable aggregate in the 0–5 cm soil layer of low and medium intensity burning was higher than that of control sample sites, the FW and WS treatments were respectively 3.31% and 1.97% higher than those in the control area, and 2.85% and 1.32% higher than those in the control experimental sites. The proportion of > 0.5 mm water-stable aggregate in each layer of different intensity burning decreased under SW treatment, the 0–5 cm soil layer of site 1 with moderate intensity burning decreased by the most, which was 11.36%, while the 10–20 cm soil layer of site 2 with medium burning intensity decreased by the least, which was 2.17%. MWD analysis showed that under the fragmentations of FW and WS, the soil stability of each layer of P. massoniana plantation disturbed by different intensity burning was higher than that of the field without fire. Under the WS fragmentation, the surface soil stability of P. massoniana plantation disturbed by low and medium intensity burning was greater than that of high intensity burning. Therefore, the response of soil aggregate stability to different fire intensity were different in P. massoniana plantation of north subtropical zone. The water stability of soil aggregates in the low- and medium-intensity burning sites was higher than that of the unburned control site under heavy rain and light rain conditions, but it was lower than that of the unburned control site under disturbance conditions. The stability of forest surface soil under different fragmentation mechanisms was higher than that of subsoil, it indicated that the surface soil had relatively strong anti-erosion ability, which is related to its high organic matter content.