All-season warming is assumed to advance snowmelt and delay snow accumulation; additionally, coupled with warming-induced drought stress, all-season warming could extend both the beginning and the end of the fire season. Using fire data updated for 1968–2018, we found an asymmetrical expansion of the lightning fire season in the boreal forest of Northeast China. Lightning fires have not advanced into the early fire season (May–June) but have largely extended into the snowless late fire season (July–September) since the late 1990s (mean end Julian date delayed by 51.1 days for 1998–2018 compared with 1968–1997, p < 0.001). Despite significant warming, the Julian days of snowmelt have changed only slightly, which has prohibited the fire season from advancing into early spring. The expansion of lightning fires into July–September was associated with a warming-induced significant increase in evapotranspiration and a decrease in soil/fuel moisture.

The Nanwenghe Nature Reserve is located in the Daxing’anling forest region, which is abundant meadow resource which is prone to fire in fire season every year. In this paper, the change in the soil chemical properties of burned stands was emphasized. The results showed that: (1) Soil organic matter (SOM) concentration in burned stands showed an extremely significant difference from that in the control stands (p < 0.01). Alkali-hydrolysable N, available K and total N in the burned stands were significantly lower than those in the control stands (p < 0.05). (2) Total P, total K, pH and available P in the burned stands were not significantly different from the concentrations in the control stands (p > 0.05). (3) Available K, total N, total P and total K concentration showed no significant difference from June to September. The pH values increased with a variance of 8%, and pH values in July, August and September were significantly different from those in June. SOM concentrations obtained in July, August and September increased significantly, compared to the concentrations in June (p < 0.05). Alkali-hydrolysable N showed a decreasing trend for all conditions.