Thermodynamics of Microbial Decomposition of Persistent Carbon in Erosion-Buried Topsoils

Abstract

Hillslope erosion in hummocky landscapes can lead to the accumulation of C-rich topsoil in depositional positions that eventually becomes buried if erosion persists. Our objective in this study was to evaluate the persistence of SOC and the thermodynamic efficiency of the microbial community in C-rich buried surface horizons from five sites with varied texture and organic matter contents. Surface Ah (0-10 cm) and buried surface (Ahb) horizons were isolated from intact cores, sieved (<2 mm) and incubated under ideal conditions of temperature and moisture. Ahb soils had an average organic C content (25.6 mg OC g^-1 soil) similar to the corresponding Ah soil (30.9 mg OC g^-1 soil). Using isothermal calorimetry, we determined that Ah horizons produced significantly more heat and CO₂ but had smaller calorespirometric ratios than Ahb soils, under both basal (841 vs 3106 kJ mol^-1 CO₂-C) and glucose metabolism (627 vs. 697 kJ mol^-1 CO₂-C).100-day basal respiration was nearly four times greater in Ah vs. Ahb horizons. While MAOM correlated with basal heat production in both horizons, it only correlated with C persistence in the Ah horizons (Rho = 0.67, p < 0.01), suggesting variability in C persistence was not primarily driven by organo-mineral bonds in Ahb horizons, although energy use efficiency is. Microbial community structure in Ahb horizons was distinct from the surface soils, and changed minimally during incubation, suggesting co-development of the community as decomposition proceeded over the decades of burial, leading to persistent C. These relatively large volume buried surface soils may provide unique opportunities to understand microbial hotspot C processes that are typically difficult to isolate at a spatially explicit scale (e.g., an aggregate interior). We propose that the co-development of distinct microbial communities in C-rich buried horizons leads to more thermally stable SOC, but further research is required to test this hypothesis.