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Organic matter stabilization in soil microaggregates: implications from spatial heterogeneity of organic carbon contents and carbon forms


This study investigates the spatial distribution of organic carbon (C) in free stable microaggregates (20–250 μm; not encapsulated within macroaggregates) from one Inceptisol and two Oxisols in relation to current theories of the mechanisms of their formation. Two-dimensional micro- and nano-scale observations using synchrotron-based Fourier-transform infrared (FTIR) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded maps of the distribution of C amounts and chemical forms. Carbon deposits were unevenly distributed within microaggregates and did not show any discernable gradients between interior and exterior of aggregates. Rather, C deposits appeared to be patchy within the microaggregates. In contrast to the random location of C, there were micron-scale patterns in the spatial distribution of aliphatic C–H (2922 cm−1), aromatic C=C and N–H (1589 cm−1) and polysaccharide C–O (1035 cm−1). Aliphatic C forms and the ratio of aliphatic C/aromatic C were positively correlated (r 2 of 0.66–0.75 and 0.27–0.59, respectively) to the amount of O–H on kaolinite surfaces (3695 cm−1), pointing at a strong role for organo-mineral interactions in C stabilization within microaggregates and at a possible role for molecules containing aliphatic C-H groups in such interactions. This empirical relationship was supported by nanometer-scale observations using NEXAFS which showed that the organic matter in coatings on mineral surfaces had more aliphatic and carboxylic C with spectral characteristics resembling microbial metabolites than the organic matter of the entire microaggregate. Our observations thus support models of C stabilization in which the initially dominant process is adsorption of organics on mineral surfaces rather than occlusion of organic debris by adhering clay particles.

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This project was funded by grants from the National Science Foundation (BCS−0215890, DEB-0425995). NEXAFS spectra were obtained at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, at the X-1A1 beamline developed by Janos Kirz and Chris Jacobsen at SUNY Stony Brook (Department of Energy contract DE-FG02-89ER60858 and NSF grants DBI-9605045 and ECS-9510499). The FTIR data were collected at U10B of the NSLS, supported by the U.S. Department of Energy (contract DE-AC02-98CH10886). Many thanks to Lisa Miller and Randy Smith at NSLS for help with FTIR measurements, to Sue Wirick, Chris Jasobsen, and Mirna Lerotic for assistance with the NEXAFS measurements and data analysis, and to Yuanming Zhang and Julia Dathe for invaluable help with sectioning. We are indebted to two anonymous referees for constructive comments on earlier versions of the manuscript.

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Correspondence to Johannes Lehmann.

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Lehmann, J., Kinyangi, J. & Solomon, D. Organic matter stabilization in soil microaggregates: implications from spatial heterogeneity of organic carbon contents and carbon forms. Biogeochemistry 85, 45–57 (2007). https://doi.org/10.1007/s10533-007-9105-3

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  • Aliphatic C, Aromatic C
  • FTIR
  • Minerals
  • Free microaggregates