Buried Soil Carbon Vulnerability to Decomposition with Landscape Disturbance

Buried layers of ancient soil organic carbon (SOC) can store significant amounts carbon (C). Persistence of this C is favored by burial, which disconnects the soil from atmospheric conditions and limits plant derived C inputs, thus reducing microbial activity. However, erosion exposes buried paleosols to modern surface conditions and results in influx of root-derived C through the processes of root exudation and root turnover. These C inputs stimulate microbial activity and leave paleosol C vulnerable to decomposition. Understanding turnover of ancient soil C is critical for predicting the response of this large C reservoir to environmental change and feedbacks to climate. Yet, the effects of root-derived C inputs on decomposition of buried C is not well established. With this study we aim to quantify how root derived C inputs affect decomposition of paleosol C located along varying degrees of isolation from modern surface conditions, Our field site is located in Wauneta, NE where erosion has brought a Pleistocene era soil –the Brady soil- closer to the surface. We collected Brady soil from 0.2m, 0.4m, and 1.2m below the modern surface, and conducted two controlled laboratory incubations, Soils were amended with (1) a lab synthesized 13C labeled (12 atom% 13C) solution to mimic root exudates (0.3 mg C g-1 soil), and (2) root litter enriched with 92% atom% 13C (0.3mg C g-1 soil), in 30 day, and 240 day laboratory incubation experiments, respectively. We measured 13C-CO2 respiration from airtight microcosms throughout the incubations and used the isotopic label to partition between root derived C and Brady soil C respiration. Our data show that Brady soil C is highly vulnerable to decomposition via soil C priming upon addition of root-derived C regardless of burial depth, indicating that exposure of paleosols to modern surface conditions may result in a positive C cycle feedback to climate