Visualizing the Stability of Char: Molecular- to Micron-scale Observations of Char Incubated in a Tropical Soil

Monday, 15 December 2014
Katherine A Heckman1, Christina Ramon2, Peter K Weber3, Margaret S Torn4, Jennifer Pett-Ridge2 and Peter S Nico5, (1)USDA Forest Service, Vallejo, CA, United States, (2)Lawrence Livermore National Laboratory, Chemical Sciences Division, Livermore, CA, United States, (3)Lawrence Livermore National Laboratory, Livermore, CA, United States, (4)Berkeley Lab/UC Berkeley, Berkeley, CA, United States, (5)Lawrence Berkeley Laboratory, Berkeley, CA, United States
The persistence of pyrogenic materials (hereafter referred to as char) in terrestrial ecosystems is of interest both from a carbon cycle modelling perspective and a climate change mitigation standpoint. However, the fate of newly introduced char in soils remains unclear. Recent reviews attempting to summarize trends in char decomposition have come to differing conclusions, further stressing the complexity of factors dictating char stability in soils. The current dataset specifically addresses the stability of char additions to a tropical clay-rich soil, possible priming effects, and interactions among char, microbial communities and the mineral matrix. 13C- and 15N-labeled Acer rubrum(red maple) wood was combusted at 400°C and added to surface (0-10 cm) and subsurface (20-30 cm) soils from the Luquillo Experimental Forest, Puerto Rico. Soils were incubated for 13 and 345 days at 26°C. Following incubation, intact microaggregates were frozen and cryosectioned into thin sections of approximately 5 µm thickness and mounted on gold-coated quartz slides. Thin sections were examined by synchrotron-based Fourier transform infrared spectroscopy (SR-FTIR), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), and high resolution secondary ion mass spectrometry (nanoSIMS). The combination of these µm to nm scale techniques allowed us to create corresponding spatial maps of native organic matter, char, and mineral phase distribution, track spatial variability in organic matter molecular structure, and dispersion of 13C and 15N isotopic labels. We present preliminary results indicating a high degree of stability of char in these wet tropical soils throughout the incubation period, suggesting that applied char may persist for long periods of time in similar soils.