Actinobacteria Emerge as Novel Dominant Soil Bacterial Taxa in Long-Term Post-Fire Recovery of Taiga Forests

Abstract

The long-term post-fire recovery phase is a critical stage for forest ecosystems to progress toward regeneration and mature succession. During this process, soil bacteria exhibit greater environmental adaptability, rapidly driving nutrient cycling and facilitating vegetation restoration. This study investigated the community structure and diversity of soil bacteria during long-term recovery after forest fires in the cold temperate zone, focusing on soils from the 2000 fires in Daxing'anling. Soil samples were classified into Low (L), Moderate (M), and High (H) fire damage intensity, with bacterial community composition and diversity analyzed using Illumina sequencing technology. After long-term fire recovery, the contents of soil organic carbon, black carbon, total nitrogen, alkaline nitrogen, available phosphorus, and available potassium were significantly higher elevated (p < 0.05), and water content was significantly lower, compared with that in the control check (CK) group. Soil urease, fluorescein diacetate, soil acid phosphatase, and soil dehydrogenase activities were significantly higher, and soil sucrase activity was significantly lower in H. There was a significant difference in the Alpha diversity index among the groups. Compared with CK, the Shannon index was significantly increased (p < 0.05) in L, while both Chao1 and Shannon indices were significantly decreased (p < 0.05) in M and significantly higher in H than CK. The results of the PCoA showed that there was a significant difference in the Beta diversity of the bacterial community among the groups (R² = 0.60 p = 0.001). The dominant bacteria groups were Proteobacteria and Acidobacteriota, while Actinobacteria became the new dominant group during the long-term post-fire recovery. AP, WC, DOC, MBC, S-DHA, and S-SC were significantly and positively correlated with soil bacterial diversity (p < 0.05). The results of the co-occurrence network analysis showed that all groups were dominated by symbiotic relationships, with M having the highest network complexity and strongest competitive effects. This study found that the physicochemical properties of soils recovered over a long period of time after fire returned to or exceeded the unfired forest condition. The Actinobacteria phylum became a new dominant bacterial group, with stronger network complexity and competition, in the process of forest recovery after moderate fire.

Keywords: diversity; dominant species; long-term post-fire recovery; soil bacteria; taiga forests.

Figure 1

The asterisk indicates the study site in Heilongjiang Province and China. The map was generated in QGIS 3.34, using WGS 1984 coordinates, with administrative boundaries and sampling points marked.

Figure 2

Alpha diversity of soil bacterial communities. Different letters within a row indicate significant differences (p < 0.05, ANOVA) among the different intensities of fire in this study.

Figure 3

Principal Coordinate Analysis (PCoA) of bacterial communities in soil of different intensity fire sites with control (CK, no fire), low fire (L), moderate fire (M), and high fire (H). For each fire site, seven independent samples were analyzed.

Figure 4

Soil bacterial community composition at the phylum level. Different letters above bars indicate significant differences (p < 0.05, ANOVA) among the different intensities of fire in this study.

Figure 5

Mantel test of soil bacterial community with soil physicochemical properties and enzyme activities. Left: Mantel test results showing correlations between soil physicochemical properties (TN, pH, etc.) and enzyme activities (FDA, S-DHA, etc.). Color gradient indicates Mantel’s r values, and significance is indicated. Right: α-diversity indices (Chao1, Shannon) and community composition correlations with environmental factors. Significance is indicated as * for 0.01 < p ≤ 0.05, ** for 0.001 < p ≤ 0.01, *** for p ≤ 0.001.

Figure 6

Heatmap of Spearman correlation between soil bacterial communities and physicochemical properties and enzyme activities. Red identifies positive and blue identifies negative correlations, with darker colors for stronger correlations. Significance is indicated as * for 0.01 < p ≤ 0.05, ** for 0.001 < p ≤ 0.01, *** for p ≤ 0.001.

Figure 7

Co-occurrence network analysis and topological characteristics of soil bacterial communities. (1) Significant correlations (Spearman’s r > 0.6, p < 0.05) between taxa, with node size proportional to connectivity degree and color indicating bacterial phyla; (2) relative abundance distribution of network-represented phyla; (3) Topological parameters including average path length, clustering coefficient, and modularity.