Tropical legume trees and their soil-mineral microbiome: biogeochemistry and routes to enhanced mineral access

To summarise the contents of this thesis briefly: the introduction discusses the evolution of the first legume-rich tropical forests in the early Cenozoic (58-42 Mya) and the effect they had on biogeochemical cycles in general and silicate rock weathering and climate in particular. This work puts forward a hypothesis that early Cenozoic forests rich in N2-fixing legumes caused enhanced weathering regimes globally, stimulating the drawdown of atmospheric CO2.
Chapter 1 utilises a large-scale replicated weathering field study in Neotropical secondary forests in Panama rich in fixing legume trees demonstrating that N2-fixers exerted 2-fold greater silicate rock weathering than non-fixers linked to significant differences in soil acidity and belowground microbial community structure and function.
Chapter 2 summarises our findings from another tropical system along a secondary forest chronosequence in the Australian Wet Tropics, showing that N2-fixing and ectomycorrhizal monodominant Acacia celsa drives greater P and K-specific weathering dissolution from basalt silicate rocks throughout the chronosequence. Highly nodulated Acacia trees were also linked to greater total basalt and dunite weathering rates than non-fixers. Analyses from chapters 1 and 2 employ Next-Generation sequencing technology and omics-driven approach in demonstrating a consistent effect of N2-fixing legume trees on their belowground microbiome in tropical forests that is that high inputs of fixed N are a strong determinant in functionally entraining the microbial community to high levels of mineral weathering. Consequently, this work also highlighted target candidate microbial genes and metabolic pathways linking those differences to enhanced weathering.
In order to test some of the highlighted candidate weathering genes in vitro, the next study, described in detail in Chapter 3 deployed the large transposon-mutant collection of the tropical soil β-proteobacterium Burkholderia thailandensis E264, selecting 11 mutant lines for further analysis. Data from replicated in vitro weathering experiments implicated several genes in the process of bacteria-mediated weathering. Further phylogenetic and taxonomic analyses of the soil pools of these genes indicated many elusive and uncultured bacterial lineages as carriers of those weathering genes thus highlighting their potential role in soil mineral weathering. This included the proposition and description of a new class within the Acidobacteria phylum, Ca. Acidipotentia, cl. nov.
The final Discussion and Conclusion chapter covers meta-analyses of data from my original datasets as well as already published literature to establish the role of N2-fixers in ecosystem succession beyond their ecosystem N enrichment effects. The results from this chapter provide evidence for enhancements in acidification, soil lithotrophy, microbial respiration and anaerobic metabolism in soil beneath N2-fixers that may all converge in a second previously unrecognised ecosystem service carried by N2-fixers: that of enhanced weathering and subsequent increase in available nutrient stocks in such early successional systems.