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Hidden Lignin in Soils

Hidden Lignin in Soils. Peter J. Hernes 1 Klaus Kaiser 2 Rachael Y. Dyda 1 Chiara Cerli 3 1 University of California, Davis 2 Martin Luther University Halle-Wittenberg 3 University of Amsterdam. Lignin Biomarkers Unique tracers for vascular plants & terrigenous organic matter. Vanillyl.

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Hidden Lignin in Soils

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  1. Hidden Lignin in Soils Peter J. Hernes1 Klaus Kaiser2 Rachael Y. Dyda1 Chiara Cerli3 1University of California, Davis 2Martin Luther University Halle-Wittenberg 3University of Amsterdam

  2. Lignin BiomarkersUnique tracers for vascular plants & terrigenous organic matter Vanillyl • Diagnostic ratios: • Syringyl:Vanillyl (S:V), Unique to Angiosperms • Cinnamyl:Vanillyl (C:V), Unique to soft tissues • Acid:Aldehyde (Ad:Al)v and (Ad:Al)s

  3. Lignin source and diagenesistraditional interpretation (Hedges and Mann, 1979) (numerous studies)

  4. Lignin in soil • Old Dogma: lignin is inherently degradation-resistant, therefore important in SOM stabilization • New Dogma: lignin turns over as fast or faster than bulk SOM (based on compound-specific 13C measurements of lignin in successional systems)

  5. Lignin in soil – Dogma problems • New Dogma is largely based on surface soils, which constitutes 10-30% of SOM and a considerable portion of organic matter is discrete, not sorbed • How good are our methods at measuring sorbed lignin (and other surface active compounds) in subsurface soils where most SOM is sorbed? • Lignin that is irreversibly sorbed in hot alkaline conditions (CuO oxidation) is almost certainly going to be stable in soils.

  6. Lignin fractionation, part 1 • Leached several plant litters, sorbed the leachate to bleach-cleaned soils, observed fractionation Hernes et al., 2007

  7. Lignin fractionation, part 1 • Important conclusions: fractionation significantly alters the molecular signature, producing highly degraded-appearing SOM from fresh material • Two conundrums:1) In many soil/leachate combinations, acid:aldehyde ratios increased in BOTH the supernatant and sorbed soil2) Measurement mass balance didn’t compute. The total lignin recovered never matched the total input.

  8. Irreversibly sorbed lignin • How do you measure what you can’t “see”? DOM inputs Sorption Measurements

  9. Indirect: Measurementby difference • Prior experiments were not optimal1) Designed primarily to look at DOM, not solids2) Subtracting large numbers to get a small one3) No replication

  10. Better experiments • Everything in duplicate • Total lignin chosen more carefully to avoid subtracting large numbers • Minimized sample handlingLeachate and mineral added directly to CuOrxn vessels, agitated for three hours, vacuum centrifugation, CuO oxidation • SMD as percent of duplicate averages ranged from 0.04% to 7.5%, average 2.5%

  11. Mineral differences

  12. Irreversible lignin • Clearly could be an important component of stable SOM • What is its age/turnover time? • How do you measure it in actual soil samples?

  13. Compositional Fractionation

  14. Bulk carbon & Aromatics • Irreversible lignin did not correspond to total carbon sorptive capacity, indicating compositional complexity of sorptive processes • HOWEVER, total carbon sorption appears to be linked to SUVA254, which is a proxy for the percentage of aromatic compounds (like lignin) that make up DOM.

  15. Conclusions • Lignin—and aromatics in general—may be more important toward SOM stabilization than we can easily measure, especially in deep soils where sorptive processes dominate • Fractionation due to partitioning leads to significant shifts in composition, in particular Ad:Al interpretations in soil must be revisited.

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