The effect of fire scars on microbial diversity of fynbos soil

[摘要] ENGLISH ABSTRACT: Microbial communities (bacteria, archaea, fungi, protista and viruses) are essential for the maintenance of a healthy balance in soil ecosystems. There are many factors that influence and disrupt this balance, including invasive species and fire events which disturb the properties and microhabitats of soil. Riparian zones are not typically exposed to fire. However, when the riparian zones are exposed to fire, it may have significant consequences for the natural patterns and processes of a soil ecosystem and the soil microbial communities. Invasive alien woody species such as Acacia and Eucalyptus spp. have become ubiquitous across riparian environments, affecting water and nutrient cycling and reducing plant diversity. However, the approaches to clear invasive species may also have negative consequences for ecosystem functioning. The 'slash and burn' technique is a biomass management tool that uses the felling of invasive stands, which are then stacked to build a pile (from dead plant biomass) and burnt. This study determined the effect of burning (the 'slash and burn' technique) of invasive biomass (Acacia and Eucalyptus spp.) on soil bacterial and fungal diversity and community structure in fynbos riparian zones (Western Cape, South Africa).The sites chosen for this study were within fynbos regions invaded by Acacia mearnsii (also known as black wattle) or Eucalyptus camaldulensis (river red gum). Four study sites were chosen, each at different statuses of invasion. These sites were Bainskloof, Rawsonville, Robertson and Wellington. Before the mechanical removal of invasive species, the Bainskloof and Rawsonville sites consisted predominantly of A. mearnsii, with a small percentage cover of Eucalyptus spp. at the Rawsonville site. The Robertson and Wellington sites consisted predominantly of E. camaldulensis, with a small percentage cover of Acacia spp. at the Wellington site. Changes in the microbial diversity and community structure were assessed using automated ribosomal intergenic spacer analysis (ARISA) fingerprinting. Microbial diversity profiles of ARISA were determined by means of the Shannon (H') and Simpson's complement (1-D) indices. Microbial community structure profile of ARISA was evaluated by means of Analysis of Similarity (ANOSIM), cluster analysis and non-metrical multidimensional scaling (NMDS). The Pearson correlation coefficient (PCC) analysis was used for the correlation between the chemical properties and microbial diversity (H'). Whereas, the principle component analysis (PCA) was used to determine which chemical properties may explain the variation of microbial community structure post-fire.This study showed that the 'slash and burn' of Eucalyptus biomass had a greater impact on the soil microbial communities compared to the 'slash and burn' of Acacia biomass. The data indicated that the 'slash and burn' of Acacia biomass (Bainskloof) did not affect the bacterial diversity (H') post-fire. In contrast, the 'slash and burn' of Eucalyptus biomass (Robertson and Wellington; also Rawsonville, where some Eucalyptus biomass was present in the piles) led to a steep decrease in bacterial diversity (H') immediately post-fire which remained relatively low a year after the burn event. Furthermore, the 'slash and burn' of Acacia and Eucalyptus biomass had no effect on the fungal diversity (H'). This, in turn, resulted in no variation of fungal diversity (H') within and between invasion sites throughout the study.Post-fire, all sites demonstrated a shift in microbial community structure. In addition, all the sites showed three distinct bacterial community structures separated by different sample times. The unique microbial community structure in the Bainskloof site, a year after the burn event, could be due to the disturbance of a flood. The unique bacterial community structures in the Eucalyptus (Robertson and Wellington) and Rawsonville sites, a year after the burn event, are likely due to the successional changes of the bacterial communities after the 'slash and burn'. Furthermore, the fungal community structures post-fire and a year after the burn event in the Eucalyptus sites could not be delineated as separate clusters. This was in contrast to the results in the Rawsonville site where the post-fire fungal community structure was different from the community structure a year after the burn event. Moreover, the fungal community structures in the Eucalyptus and Rawsonville sites a year after the burn event were similar. This similarity could possibly be due to the post-fire dominant fungal species that are beneath the soil surface layer where fire occurred or from adjacent areas around the burnt piles. These post-fire dominant fungal species have the capacity to disperse into the burnt areas by means of mycelial expansion from deeper to surface soil profiles or from the margins of the burnt piles into the burnt areas.The sites exposed to the 'slash and burn' of Eucalyptus biomass showed that soil pH served as the strongest soil abiotic indicator for bacterial diversity (H'). This finding was not evident in the Bainskloof site, which was exposed to the 'slash and burn' of Acacia biomass. In this study, all the sites showed that the 'slash and burn' of Acacia and Eucalyptus biomass leads to an increase in soil pH. However, the bacterial diversity (H') showed different trends between invasion sites post-fire. The 'slash and burn' of Eucalyptus biomass resulted in a decrease in bacterial diversity (H'). Whereas, the 'slash and burn' of Acacia biomass did not affect the bacterial diversity (H') post-fire. As for the fungal communities, no soil abiotic properties served as a useful indicator for the fungal diversity (H').The soil pH, EC and PO4 concentration explained the most variation of microbial communities in the sites exposed to the 'slash and burn' of Eucalyptus biomass. These trends were not observed after the 'slash and burn' of Acacia biomass at the Bainskloof site. At this site, no variation in EC and PO4 concentration was recorded immediately post-fire. However, EC and PO4 concentration a year after the burn event was relatively higher compared to the conditions pre-fire. As for the sites exposed to the 'slash and burn' of Eucalyptus biomass, the soil pH, EC and PO4 concentration showed a steep increase immediately post-fire which remained relatively high a year after the burn event.'Slash and burn' of Eucalyptus biomass left a patch where the fynbos vegetation did not recover. It is possible that the 'slash and burn' of Eucalyptus biomass may have damaged the roots and mycorrhizal fungi in the soil that consequently decreased the rate and capacity of recolonization in burnt areas. For future research, it will be useful to investigate the effect of 'slash and burn' of invasive biomass on specific functional groups (i.e. mycorrhizal fungi, ammonifiers and N-fixers) in the riparian zones of fynbos. It will also be of value to evaluate the recovery of these functional groups (if possible) post-fire and to determine what it means for the restoration of fynbos vegetation.'Slash and burn' of Acacia biomass, however, is unclear due to the interference of a flood that occurred at the Bainskloof site during the trial period. The flood disturbed the burnt areas and led to the re-establishment of A. mearnsii. Therefore, for future research, an observational study may be considered to assess whether fynbos vegetation will recover after 'slash and burn' of Acacia biomass. Taken together, the results demonstrated a shift in microbial communities post-fire. However, the microbial diversity (H') remained the similar after the 'slash and burn' of Acacia biomass.

[发布机构] Stellenbosch University