Soil Organic C Speciation in Surface and Subsurface Soils of North-South and West-East Transects of Continental United States: Carbon K-Edge Nexafs Spectroscopic Investigation.

The mass of organic matter in subsurface soils (below 30 cm) is estimated to account for approximately 50% of the global soil organic C. However, because there have been relatively few studies of subsurface C pools, the factors controlling the stability and turnover of subsurface C pools are largely unknown and thus cannot be accurately integrated into existing ecosystem biogeochemical models. In particular it is expected that temperature and precipitation, two critical factors impacted by global climate change, are related to the subsurface differently than surface C pools. The objective of this study was to investigate how these variables are related to the total C pools and their molecular speciation within each pool in surface and subsurface soils across different climate zones of continental United States. Different from destructive wet chemical analysis, synchrotron-based carbon K-edge near edge x-ray absorption fine structure (NEXAFS) spectroscopy has been shown to be a selective, sensitive, and nondestructive method capable of providing molecular-scale information on C speciation. Organic C speciation of a subset (160 out of 4871 sites) of surface (0-5cm) and C-horizon soil samples collected from 2007 to 2010 for the USGS Geochemical Landscapes Project were investigated using the carbon K-edge NEXAFS spectroscopy. The carbon K-edge NEXAFS spectra were collected on the Hermon beamline at the Synchrotron Radiation Center of University of Wisconsin-Madison. Aromatic-C, phenolic-C, aliphatic-C, carboxylic-C, and O-alkyl-C were detected in the samples. The impact of temperature and precipitation on the C speciation in surface and subsurface soils will be discussed.