Understanding how microbial biodiversity is generated and maintained is a central issue in microbial ecology. Microbes exist throughout the soil profile, and intensive studies have been focused on top-layer soils. In contrast, deeper microbes have received less attention, which have a crucial influence on longer-term soil carbon sequestration given that sub-surface horizons are huge organic carbon pools with long turnover times. Even though a few studies examined microbial distribution patterns through soil profiles, we still have only a limited understanding of how such pattern is generated, especially in marginal soils, which have been increasingly considered as a promising alternative for food and bioenergy production against limited land resources. To this end, we examined (1) how bacterial and fungal diversity (revealed by richness and compositional similarities) and complexity (revealed by association network features) change along soil depths (0-210 cm) in two marginal landscapes with different soil chemical conditions and histories; and (2) underlying mechanisms (revealed by stochastic/ deterministic relative importance) in determining the depth-related spatial distribution of both bacteria and fungi.
Results/Conclusions
Our result showed that microbial OTU richness (alpha diversity) and compositional similarities (beta diversity) of bacteria and fungi both decreased with soil depth, with association network complexity in both bacteria and fungi illustrating the same pattern, suggestive of depth-decay relationships between microbial diversity and complexity with increasing depths. Besides, the turnover rates (the slope of the linear regression between log-transformed compositional similarities and log-transformed geographic depths) varied across different microbial groups and landscapes, showing different responses of microbial groups to environmental conditions. The loss of bacterial and fungal diversity with soil depth was driven by deterministic process, suggesting the importance of environmental filtering in nutrient poor soils. However, dispersal limitation weighs more in fungi than bacteria, which may indicate different dispersal abilities of these two microbial groups across soil depths. Our study emphasized the importance of soil depth as an environmental gradient structuring soil microbial community.