2D spectroscopy to assess effects of UV radiation on cell wall chemistry during litter decomposition

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 two-dimensional nuclear magnetic resonance (2D NMR) spectroscopic methods, we quantified the molecular-level 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 (2.5 months during summer, and 1 year). Through fiber analysis by sequential digestion, we found that the litter hemicellulose fraction decreased significantly from 31.6 to 24.9 % 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 (23.8 vs. 25.9 %). 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 β-aryl ether inter-unit linkages by 9 % and decreased the relative abundance of lignin p-hydroxyphenyl 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 processes. Abstract Litter chemistry is one of the most studied controls on decomposition in terrestrial 14 ecosystems. Solar radiation has been shown to increase litter decomposition rates in arid 15 ecosystems through the process of photodegradation. However, it remains unclear how 16 photodegradation affects litter chemistry, especially the abundance and composition of 17 lignin, which is thought to play a key role in photodegradation. Using two-dimensional 18 nuclear magnetic resonance (2D NMR) spectroscopic methods, we quantified the 19 molecular-level changes in litter chemistry associated with photodegradation. Litter of 20 Bromus diandrus was exposed in the field to two levels of radiation (with and without 21 ultraviolet (UV) wavelengths) and two durations of exposure (2.5 months during summer, 22 and one year). Through fiber analysis by sequential digestion, we found that the litter 23 hemicellulose fraction decreased significantly from 31.6% to 24.9% after one year of 24 decomposition. In litter exposed for one year, the hemicellulose fraction was significantly 25 lower in litter with UV exposure compared to litter without UV exposure (23.8% vs. 26 25.9%). These results indicate that UV photodegradation has a small but significant effect 27 on litter chemistry compared to other decomposition processes. Even though fiber 28 analysis showed no loss of total lignin, 2D NMR analysis demonstrated that UV exposure reduced the major lignin structural units containing β -aryl ether inter-unit linkages by 9% 30 and decreased the relative abundance of lignin p -hydroxyphenyl units by 20%. The 2D NMR analysis also revealed that lignin guaiacyl units were preferentially lost after one 32 year of decomposition relative to the reference material, but no effects of UV exposure on 33 guaiacyl 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 processes.