Impact of fire on soil microbial biomass of Pinus tabuliformis forest in Pingquan County, Hebei of northern China

In the burned area of Pinus tabuliformis forest in Pingquan County, Hebei of northern China, we measured soil microbial biomass carbon (C_mic), nitrogen (N_mic) of different fire intensity regions with a fumigation-extraction method after burned half a year. The study area was classified into different levels according to the smoked height and tree mortality: high-severity sites, middle-severity sites, low-severity sites and nearby unaffected sites. The purpose of our research was to explore the effects of different severity fire on C_mic and N_mic. The results showed that both C_mic and N_mic decreased after different intensity burning and gradually declined with the soil depth. C_mic ranged from 34-205 mg/kg; N_mic ranged from 7-40 mg/kg; C_mic/N_mic was wholly at 5-6. Two-way ANOVA showed that fire severity, soil layer and the interaction of the two factors had significant effects on soil organic carbon (SOC), soil total nitrogen (TN), NH₄⁺-N, NO₃^--N (P < 0.05). However, soil pH was only positively correlated with fire severity (P < 0.05). Meanwhile, fire severity, soil layer and the interaction of the two factors had significant effects on C_mic and N_mic (P < 0.01). Soil nutrients were distributed mainly in the 0-10 cm layer and dropped sharply after the high-severity fire. Besides, soil pH increased significantly after the high-severity fire. The correlation analysis result showed that SOC and TN influenced the C_mic and N_mic extremely significantly (P < 0.01). The soil microbial biomass was positively correlated with SOC and TN. Principal component analysis further prove that SOC and TN are the main factors affecting C_mic and N_mic.