Effect of prescribed burning on soil chemical properties and enzyme activity
Soil properties of samples collected before fire (BF), after fire (AF), and 2-3 months after fire (AF2) are shown in Table 2. Soil pH decreased significantly from BF to AF (p < 0.001), but remained unchanged afterwards. Soil total C decreased at AF2 by 30% compared to BF (p = 0.03), but AF values were intermediate between sampling events. There was a significant increase in soil total N from BF (0.1%) to AF (0.14%), but it decreased at AF2 (0.1%) (p = 0.02). C to N ratio decreased by 43% (p <0.001) AF, but returned to BF values at AF2. NH4+ concentration was reduced from 11.2 mg kg -1 BF to 5.5 mg kg -1 AF2 (p <0.001). Extractable P decreased significantly by 57% (p < 0.001) from BF to AF, but no changes were observed in total P, extractable Ca, K, Mg, Mn, Fe, and Al concentrations.
In AF samples, soil enzyme activity of BGA declined by 75% (p < 0.001; Table 2) relative to BF measures, but returned to those levels at AF2. There was no significant change in NAG activity from BF to AF, but it doubled from AF to AF2 sampling (p = 0.006; Table 2). The BGA: NAG ratio decreased from BF to AF (p =0.03), but no differences were observed between AF and AF2. A PCA analysis showed that soil properties and enzyme activities differed significantly between sampling events (R2= 0.24, p =0.001). Ordination of the sampling events was related to the first canonical axis (PC1=29%) while the second axis explained 18% of the variation (Figure 1). Significant positive correlations were detected between BGA activity and soil pH (r= 0.6, p= 0.004), C: N (r= 0.7, p = 0.002), extractable P (r= 0.6, p= 0.01), and extractable Ca (r= 0.6, p = 0.02) (Supplementary Fig. S2). NAG activity was positively correlated with NH4+ (r= 0.7, p= 0.002). β-glucosidase: NAG ratio showed positive correlation with soil pH (r= 0.6, p =0.02) (Supplementary Fig. S2). During the study period, the soil temperature after fire (AF and AF2) ranged between 25-30° C whereas the temperature before fire had a range from 21-23° C.
Effect of prescribed burning on soil microbial community composition
Microbial diversity and relative abundance of soil bacterial communities
No significant differences in any of the alpha diversity indices of the bacterial community were observed from BF to AF (Fig. 2A). The average observed richness was 392 and 328 ASVs at BF and AF, respectively (p = 0.16). Values of the Shannon index varied from 5.58 (BF) to 5.46 (AF) (p = 0.45). The average Simpson diversity index remained unchanged from BF (0.99) to AF (0.99) (p = 0.97). A NMDS analysis on Bray-Curtis distances (Fig. 3A) showed that bacterial beta diversity was not affected by the fire (PERMANOVA, R2= 0.06, p = 0.21).
Across all samples, Acidobacteria, Actinobacteria, Planctomycetes, and Proteobacteria were the predominant phyla BF with mean relative abundances of 19%, 18%, 12% and 30%, respectively (Supplementary Fig. S3A). There were no significant changes in relative abundance of any phyla from BF to AF (p > 0.05) (Supplementary Fig. S3A). Fire significantly increased the relative abundance of the bacterial order Desulfurellales by 26% AF (p = 0.009), and impacted the relative abundance of Variibacter genera, which significantly increased from 2.1% (BF) to 2.6% (AF) (p = 0.01) (Supplementary Fig. S4A).
Microbial diversity and relative abundance of soil fungal communities
There were no significant differences in alpha (Fig. 2B) diversity of the fungal community from BF to AF. The average observed richness was 485 ASVs BF to 547 ASVs AF (p = 0.29). The average Shannon diversity index was 4.81 and 4.78 at BF and AF, respectively (p = 0.93). Values of the Simpson index varied from 0.97 to 0.96 from BF to AF, respectively (p = 0.53). Soil fungal beta diversity did not change with fire as shown by a NMDS together with a PERMANOVA analysis (R2= 0.07, p = 0.07) (Fig. 3B).
Across all samples, Ascomycota, Basidiomycota, Mortierellomycota, and Rozellomycota were the dominant phyla BF with mean relative abundances of 81%, 15%, 2%, and 1% of the total relative abundance, respectively (Supplementary Fig. S3B). However, significant changes in the relative abundance of fungal phyla were not detected after the fire event. At the order taxonomic level, fire significantly reduced the relative abundance of Pleosporales by 65% (p = 0.007) and Eurotiales by 28% (p = 0.002). The relative abundance of the Taloromyces genera decreased from 10.7 % to 6.1 % (p = 0.005; Fig. S4B).
Relation between soil microbial communities and soil properties
Redundancy analysis showed that BF and AF samples clustered together suggesting that changes in soil variables due to fire had no effect on the composition of soil bacterial (Fig. 4A) and fungal (Fig. 4B) communities (p > 0.05). However, the RDA analyses showed there were specific bacterial (Fig. 4A) and fungal (Fig. 4B) genera significantly correlated with soil properties (Supplementary Tables S1 and S2). Only significant correlations are described below. Soil pH was positively related to Methanobacterium and statistically negatively related to Acidothermus and Sorangium. Total C was positively linked to various genera including Bacillus, Candidatus Xiphinematobacter, Haliangium, Isosphaera, Jatrophihabitans. A set of positive correlation was found between total N and Bradyrhizobium, Variibacter, Candidatus Koribacter, Candidatus Solibacter, Methanobacterium, Methylocystis and Syntrophorhabdus, but a negative correlation with Sorangium. Total P was positively correlated with Anaeromyxobacter. Bacterial genera that were positively linked to NH4+ were Acidibacter, Acidicaldus, Acidothermus, Bryobacter, Burkholderia-Paraburkholderia, Candidatus Methylomirabilis, Opitutus, Planctomyces, Rhodanobacter, Singulisphaera. Apart from these soil properties, soil enzyme activity BGA negatively correlated with Acidothermus, Sorangium and positively correlated with Tumebacillus. NAG activity was negatively linked to Candidatus Koribacter, Candidatus Solibacter, Methanobacterium, Methylocystis, Syntrophorhabdus, and positively linked with Sorangium. The BGA: NAG ratio was negatively linked to Acidothermus and Sorangium and positively linked with Methanobacterium.
For the fungal community, a negative correlation was found between soil pH and Cladophialophora and a negative correlation between total C, total N, Lophiostoma, and Myrothecium. The C: N ratio positively correlated with Myrothecium and negatively correlated with Archaeorhizomyces, Gymnopus, Pseudeurotium, and Tolypocladium. The fungal genera Archaeorhizomyces, Clavulina, Gymnopus, Inocybe, Laccaria, Pachyphloeus, Pseudeurotium, Tolypocladium, and Tomentella had a negative correlation with total P and a positive correlation to Curvularia and Paraphaeosphaeria. NH4+ was positively linked to Apiotrichum, Chaetomium, and Fusarium whereas, Fusarium was negatively correlated with soil moisture. Soil enzyme activity BGA was positively linked to Curvularia and Paraphaeosphaeria and negatively linked to Archaeorhizomyces, Clavulina, Gymnopus, Inocybe, Laccaria, Pachyphloeus, Pseudeurotium, Tolypocladium, and Tomentella. The presence of Archaeorhizomyces, Gymnopus, Pseudeurotium, Tolypocladium was positively associated with NAG activity and negatively with Lophiostoma and Myrothecium. BGA: NAG ratio was positively linked to Myrothecium and Penicillium and negatively linked to Archaeorhizomyces, Gymnopus, Pseudeurotium, and Tolypocladium.