Using 2D NMR spectroscopy to assess effects of UV

Authors: Yang Lin Jennifer Y King Steven D Karlen John Ralph

Publish Date: 2015/09/08

Volume: 125, Issue: 3, Pages: 427-436

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Abstract

Litter chemistry is one of the most studied controls on decomposition in terrestrial ecosystems Solar radiation has been shown to increase litter decomposition rates in arid ecosystems through the process of photodegradation However it remains unclear how photodegradation affects litter chemistry especially the abundance and composition of lignin which is thought to play a key role in photodegradation Using twodimensional nuclear magnetic resonance 2D NMR spectroscopic methods we quantified the molecularlevel changes in litter chemistry associated with photodegradation Litter of Bromus diandrus was exposed in the field to two levels of radiation with and without ultraviolet UV wavelengths and two durations of exposure 25 months during summer and 1 year Through fiber analysis by sequential digestion we found that the litter hemicellulose fraction decreased significantly from 316 to 249  after 1 year of decomposition In litter exposed for 1 year the hemicellulose fraction was significantly lower in litter with UV exposure compared to litter without UV exposure 238 vs 259  These results indicate that UV photodegradation has a small but significant effect on litter chemistry compared to other decomposition processes Even though fiber analysis showed no loss of total lignin 2D NMR analysis demonstrated that UV exposure reduced the major lignin structural units containing βaryl ether interunit linkages by 9  and decreased the relative abundance of lignin phydroxyphenyl units by 20  The 2D NMR analysis also revealed that lignin guaiacyl units were preferentially lost after 1 year of decomposition relative to the reference material but no effects of UV exposure on guaiacyl units were observed These results suggest that photodegradation causes partial degradation not necessarily complete breakdown of lignin structures Our data also demonstrate that applications of 2D NMR methods are valuable for acquiring detailed information on lignin and polysaccharide chemistry during both biotic and abiotic decomposition processesWe thank Dad RouxMichollet Keri Opalk Sarah Liu and Jerry Hu for their assistance in the field and laboratory We thank Oliver Chadwick and Carla D’Antonio for their valuable comments on the experimental design and on this manuscript We thank Kate McCurdy Eric Massey and the University of California’s Sedgwick Reserve for providing the study site Comments from Associate Editor Marc Kramer and four anonymous reviewers greatly improved this manuscript YL and JK were supported by the National Science Foundation under DEB0935984 and DEB1406501 JR and SK were supported by the DOE Great Lakes Bioenergy Research Center DOE Office of Science BER DEFC0207ER64494

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