Microbial distribution in an eroded landscape: Buried A horizons support abundant and unique communities

Hummocky landscapes are naturally susceptible to erosion by water and tillage. Downslope movement of material results in the accumulation of soil in depositional positions and exposed sub-soils at the surface of eroded positions, affecting landscape-scale C cycling and productivity. Prolonged or extreme erosion can create inverted soil profiles with deeply buried C-rich surface material. The susceptibility of this C to decomposition is largely controlled by the potential of microorganisms to degrade it with long-term implications for C dynamics in agroecosystems. Our objective in this study was to evaluate whether the redistribution of soil along eroded hill slopes creates differences in the structure of the soil microbial community, both laterally across the landscape and vertically through the soil profile. Using phospholipid fatty acid analysis (PLFA) and high-throughput DNA sequencing, we investigated the abundance, diversity and community structure of microbial communities along an eroded landscape. Microbial abundance and community structure were found to be strongly influenced by a depth gradient at the near-surface (0-20cm). In a depositional backslope position, viable microbial biomass was detected to the depth of the A horizon (85cm), with a C-rich buried layer that contained substantial viable biomass (10.1 -18.8μg PLFAg-1 soil). Soil organic carbon (SOC) concentration was significantly correlated (r=0.40; p<0.001) with PLFA concentration at all positions, indicating microbial abundance is determined by C availability. In contrast, community structure was related to the origin of the soil in the landscape, and may be regulated more strongly by SOC composition. Pyrosequencing of bacterial and fungal DNA showed that genetic diversity was largely maintained in former surface soils. Our work demonstrates that abundant microbial biomass is supported in C-rich buried soils while SOC is largely preserved for decades. The presence of an abundant and diverse community suggests that there is potential for enhanced C loss under changing conditions such as climate change or modified land-use. © 2014.