Vertical distribution of microbes and nematodes in subsurface soils in an Australian woodland
Soil biota play an important role in ecosystem processes such as soil formation, biogeochemical cycling, contaminant degradation, and water infiltration and purification. Our understanding of belowground communities, and their functional roles, has increased significantly over the past couple of decades, particularly through the development of new sophisticated analytical techniques. However, most research is focused on the surface soils and our knowledge of belowground communities residing in the deeper subsurface soil horizons remains limited. Our aim with this study was to investigate microbial and nematode communities of subsurface soil horizons in a natural forest ecosystem. Our study site is the Eucalyptus FACE site (EucFACE) established in a remnant Cumberland Plain woodland in Western Sydney, New South Wales, Australia. Cumberland Plain woodland is an iconic Australian ecosystem type that is considered threatened due to land use changes. We collected soil samples along 12 soil profiles (0 to 5m depth) within the site, and used real-time PCR and metagenomics analysis to examine the size and structure of nematode, bacterial and fungal communities at different depths. We relate the observed changes to variability in edaphic variables along the profiles.
We observed a shift from loamy sand near the surface to clay at deeper depths with associated changes in soil moisture content, pH and nutrient concentration. Real-time PCR using 16S-rDNA, 18S-rDNA and the fungal ITS region suggested significant variation in both microbial and nematode communities throughout the soil profile but this variation was surprisingly consistent between profiles. Different taxa dominated at particular depths and nematodes were surprisingly abundant in some deeper soil layers. We discuss this in relation to changes in community composition and food availability. The influence of depth was soil nutrient dependent, with the most ancient soil horizons exerting a greater influence on nematodes and fungal microbial communities. Metagenomic analysis revealed three types of microbial community strategies in response to depth: stability, resilience and sensitivity. These ecological strategies lead respectively to the creation of nematodes, bacterial and fungal communities at stable state, alternate stable state and at regime shift. The communities at regime shift or highly sensitive together with the carbon made available by the deepest soil layers were triggering the regulation, starvation, death and growth in resilient and sensitive community in the soil surface layers.