The Effect of Microbial Diversity on Organic Matter Transformations in Soils

Microorganisms play a critical role in soil organic matter (SOM) formation, however fundamental relationships between the ecology of microbial communities and SOM transformations are largely unknown. In macroecology, increased biodiversity results in increased ecosystem functionality (e.g., productivity, SOM accumulation), which has been explained by greater community niche breadth. Translating these concepts to microbial communities, we predicted that greater species richness would result in greater rates of C cycling and SOM formation, a trend underpinned by a relationship between richness, access to substrates, and microbial carbon use efficiency (CUE).

Soils were collected from a remnant forest and perennial pasture on the Fleurieu Peninsula, South Australia, a viticulture and pasture region with a Mediterranean climate. To explore the relationship between microbial diversity and SOM transformation, we manipulated biodiversity in microcosms by sterilizing soils then inoculating them with a dilution-series of native microbial communities. During five harvests over a year-long incubation, we observed the relationships between biodiversity and microbial transformations of plant detritus and SOM using isotopically-labelled plant litter (mix of grasses and eucalypts; 20% ¹³C), a novel nuclear magnetic resonance (NMR) C7 pulse sequence that detects only ¹³C-¹³Cbonds, and sequencing of the 16S rRNA and ITS genes.

The effect of microbial richness on C-cycling exhibited contrasting trends based on land cover and varied for litter- and soil-C cycling. In forest soils, higher levels of biodiversity resulted in more CO₂ production from both litter and SOM (p<0.001), and results from the NMR C7 pulse experiment suggest that while more litter was microbially transformed, more litter derived-C was also retained in soils (p<0.05; r²= 0.3). In the pasture soil, higher biodiversity resulted in lower SOM cycling (p<0.01; r²= 0.7), but there was no effect of diversity on the quantity of litter-C microbially transformed and retained. Importantly, CUE (as assessed with NMR) was not consistently related to microbial species richness and C cycling, suggesting that SOM retention in soils is mediated by other factors, such as microbial competition for soil nutrients and OM sorption and desorption on soil minerals.