CZO National Meeting, Arizona, May 2011 Distribution and Speciation of Soil Organic Carbon along Hillslope l Transects: Importance of Fe-Redox Cycling Chunmei Chen, Peter Leinweber, Anthony K. Aufdenkampe, Donald L. Sparks
Introduction Organic matter-mineral linteraction: ti One of major mechanisms for C stabilization Impacted by coupled geomorphological, hydrological & microbial processes Precipitation olite Sapro bedroc ck Soi il Well-drained Poorly-drained Mineralogical transformation (Fe oxides) Slow C decomposition, eroded C accumulation Enhancing DOC release Lignin Preservation C fractionation
Objectives Investigate soil organic matter molecular composition at varying landscape topographic positions Assess interactions of C with specific soil minerals at the molecular scale Determine Fe speciation of soils along redox gradients
Field Site 3 4 2 1 6 5 Base Top
Advanced Characterization Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS) Soft ionization enables detection of non-fragmented molecules which can be assigned to compound classes and reflect composition of original sample. Temperature-resolved pyrolysis indicates thermal stability of compounds, reflects biological stability. Near-edge X-ray Absorption Fine Structure (NEXAFS) Characterize C functional groups of soil organic matter without any pretreatment. Scanning Transmission X-ray Microscopy (STXM) -NEXAFS Map distribution ib ti of C and C forms and the major elements (K, Ca, Fe, Al, Si) in soils at nanometer scale. Assess the interactive mechanisms of C with specific soil minerals.
Soil Properties Depth (cm) Amorphous Fe Crystalline Fe oxides (g kg -1 ) oxides (g kg -1 Clay content (%) ) Top Base Top Base Top Base 0-15 3.91 3.00 5.47 3.84 16% 22% 15-25 4.14 3.92 7.20 3.43 22% 25% Reduction of Fe oxides occurred at the base of the hillslope.
Carbon and Soil Mineral Surface Area Depth (cm) 0-15cm 15-25 cm Fraction % C Mineral surface area (m 2 g -1 ) C loading (C/mineral surface area) (mg m -2 ) Top Base Top Base Top Base Bulk soil 3.06 2.76 14.52 11.52 2.11 2.46 clay 7.26 6.87 70.79 53.22 1.02 1.29 Bulk soil 175 1.75 052 0.52 19.41 10.44 090 0.90 050 0.50 clay 4.91 1.33 72.23 50.40 0.68 0.26 Lower C concentration at the base of the hillslope is likely due to enhanced DOC leaching as a result of reduction of Fe oxides. levation Soil Increased Fe reduction Soil Reduced mineral surface area and %C E Downhill distance
OM compound classes from Py-FIMS 12 10 8 6 4 2 Top Base Soil ild depth: 0-15cm Bulk soil Clay Relative abundance of organic compouns as % 0 25 20 15 10 Bulk soil Soil depth: 15-25cm Clay Carbohydrates N compounds Lignin Dimers Lipids Phenols+Lignin nin Monomers AlkylAromatics Carbohydrates N compounds Lignin Dimers Lipids Phenols+Lignin in M Monomers AlkylA lkylaromatics 5 Relative abundance of organic compouns as % 0
Volatilization Temperatures by Py-FIMS Soil depth: 0-15cm Soil depth: 15-25cm Bulk soil clay Bulk soil clay 0.30 Intensity (%TI II) 025 0.25 Top Base 0.20 0.15 0.10 0.05 Peptides 000 0.00 0 200 400 600 800 0 200 400 600 800 0 200 400 600 800 0 200 400 600 800 Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Intensity (%TII) 0.6 0.4 0.2 Carbohydrates 0.0 0 200 400 600 8000 200 400 600 8000 200 400 600 8000 200 400 600 800 0.6 Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Inte ensity (%TII) 0.5 0.4 0.3 02 0.2 0.1 N compounds 0.0 0 200 400 600 8000 200 400 600 8000 200 400 600 8000 200 400 600 800 Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Temperature ( ο C)
Volatilization Temperatures by Py-FIMS Soil depth: 0-15cm Soil depth: 15-25cm Bulk soil clay Bulk soil clay Intens sity (%TII) 0.6 0.5 0.4 0.3 0.2 0.1 Top Base Lipids 0.0 0 200 400 600 800 0 200 400 600 800 0 200 400 600 800 0 200 400 600 800 Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Intensity (% %TII) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Top Base 0 200 400 600 800 0 200 400 600 800 0 200 400 600 800 0 200 400 600 800 Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Temperature ( ο C) Phenols and Lignin Monomers
C NEXAFS spectra of clay fractions Soil ildepth: 0-15 cm Aromatic-C Aliphatic-C Hill base soil: enriched in aromatic C depleted in aliphatic C
Conclusions C concentration and speciation along the pasture hillslope transect are linked to redox cycling of Fe oxides. The stability of soil organic carbon is related to carbon content and/or the normalized carbon content by mineral surface area.
Acknowledgements Kyungsoo Yoo at the University of Minnesota Beamline scientists at the Canadian Light Sources: Jian Wang, J.J.Dynes, and Tom Regier Environmental Soil Chemistry Group at the University of Delaware National Science Foundation (NSF)