The Bio-accessibility of Synthetic Fe-Organo Complexes in Subsurface Soil with Elevated Temperature: a Proxy for the Vulnerability of Mineral Associated Carbon to Warming Rachel C. Porras, Peter S. Nico, and Margaret Torn Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA

Abstract:

Globally, subsurface soils (>30 cm) represent an important reservoir of soil organic carbon (SOC). However, the vulnerability of this deep SOC and, in particular mineral-associated SOC, to warming, and its potential to amplify the effects of climate change is highly uncertain. To gain insight into the bio-accessibility and temperature sensitivity of mineral-associated organic C, we conducted a series of incubations using soils collected from three depths (0-10, 50-60, and 80-90 cm) under coniferous forest. The soils are moderately acidic (mean pH=6.5) sandy, mixed, mesic Ultic Haploxeralfs.

To understand how mechanisms controlling SOC bio-accessibilty or temperature sensitivity differ with depth and with the properties of Fe-organo complexes (i.e.,degree of crystallinity, amount of reactive surface area, or surface saturation), we used a ¹³C labeled glucose substrate to prepare synthetic Fe-organo complexes spanning a range of crystallinity and mineral surface saturation. The synthetic Fe-organo complexes were then added to soil from three depths. The soils containing the ¹³C labeled Fe-organo adduct were incubated at two temperatures (ambient and +4°C) and respired ¹³CO₂ was measured and used to estimate flux rates. Differences in measured ¹³CO₂ fluxes as a function of depth, surface loading, and mineral properties are discussed in terms of their implications for the temperature sensitivity of mineral protected organic carbon in subsurface soils.