The Great Xingan Mountain is one of the biggest forest regions in China and serves as an important wood resource base. However, in recent years, frequently experienced severe forest fires have caused great losses in this area. Forest restoration is a very important problem that draws attention from many scientific disciplines. Restoration of soil quality in burned areas could affect vegetation restoration directly. It is thus important to monitor soil quality. Soil microorganisms are one of the most useful indicators of soil quality. Here we focus on the response of microbial populations to changes in the soil environment of recently burned areas in the Great Xingan Mountain, in which the restoration was at the early regeneration stage. The purpose of this study is to examine the effects of a range of fire severities on soil microbial biomass distribution, soil microbial community structure, and soil microbial functional diversity in these areas.
Soil samples were collected from high and moderate severity burned forest sites (2003 fires), and an unburned site. Standard experimental methods were used to measure typical physical and chemical indicators, such as total nitrogen (TN), total carbon (TC), soil organic carbon (SOC), soil moisture (SM), soil available nitrogen (AN), and electrical conductance (EC). Fumigation, extraction, and Biolog methods were used to detect soil microbial biomass, soil microbial community structure, and soil microbial functional diversity, respectively.
Our results showed that there were significant differences in soil nutrients (total nitrogen, total carbon, soil organic carbon, available nitrogen) and soil water content between burned and unburned sites. Soil microorganism biomass carbon on a high severity burned site and moderate severity burned site had values of (3143.93±381.020) mg/kg and (3204.19±54.798) mg/kg,respectively, which are significantly higher than those on the unburned site ((1639.60±468.818) mg/kg). Soil microorganism biomass nitrogen on burned sites yielded (313.56±103.599) mg/kg on the high severity burned site and (383.12±43.509) mg/kg on the moderate severity burned site; these values were also higher than the value on the unburned site ((165.22±29.752) mg/kg). However, only the difference between moderate severity burned site and unburned area reached a statistically significant level.Soil microorganism metabolic activities and carbon use capacities of microorganism community, which are assessed by average well-color development (AWCD), also showed the same pattern, that is, the values of AWCD were 1.45 on high severity burned site, 1.44 on moderate severity burned site, and 0.77 on the unburned site, respectively. However, the diversity of soil microorganism community among the three types of sites was not significantly affected. The results of statistical analysis indicate that the changes in soil microorganisms of burned sites compared with the unburned site were closely related to the soil nutrients (e.g., soil carbon, soil nitrogen, available nitrogen and soil organic carbon) and soil water contents. The Principle Component Analysis showed that fire had a major effect on carbon use by soil microorganisms. The higher fungi ratio among study sites may be due to soil acidity, with soil pH values at the Great Xingan Mountain ranging from 4.12 to 4.68. Soil nutrients, soil water contents, and soil microbial community characteristics, such as the soil microorganism biomass (carbon and nitrogen), metabolic activities, and diversity of microorganism community, were not statistically significantly influenced by the extent of fire (high or moderate severity fire) after 6 years. We only found the different carbon use by soil microorganism between high severity burned and moderate severity burned sites.