Abstract
Microbial communities are the main catalysts of ecosystem carbon dynamics. The contribution of sub-soil microorganisms to relatively stable fine-fraction carbon content may be critical for soil carbon sequestration. Here we assessed bacterial, fungal and archaeal communities at three different soil depths (over 90 cm) in a 25-year old crop-tillage trial in southeastern Australia. We explored the effect of soil depth on microbial community composition, network complexity and interkingdom associations to test whether microbial community characteristics could be linked to relatively stable fine-fraction carbon content. Our results showed that soil depth had a stronger impact than cropping practice on microbial abundance, diversity, taxonomic composition and community structure. Both bacterial and fungal abundance and diversity declined significantly along the depth continuum, whereas soil archaeal diversity increased below the top 10 cm. Network complexity declined with increasing soil depth, with network parameters such as the number of nodes, edges, and maximum degree decreasing with depth. In particular, mean degree, closeness centrality, and mean clustering coefficient were the highest at 10 cm and declined thereafter while betweenness centrality increased. A core group of OTUs also comprising keystone taxa was found across soil depths and cropping practices. Interestingly, the relative abundance of keystone taxa increased with soil depth, suggesting potentially important roles in maintaining network complexity in sub-soils. Fine-fraction soil carbon was strongly associated with microbial biomass, network connectivity, and the keystone taxa. Overall, our results suggest that despite the decrease in microbial abundance and diversity, the sub-soil microbial networks are highly complex with many associations. Our study also reveals a link between fine-fraction carbon and keystone taxa, suggesting the importance of such taxa for soil carbon dynamics.
Banerjee, Samiran