Changes in Lignin Chemistry during Photodegradation Revealed By Two-Dimensional NMR Spectroscopy.

Lignin comprises a large proportion of terrestrial biomass, yet it is also one of the most recalcitrant plant compounds during microbial decomposition. Lignin is hypothesized to be preferentially degraded during photodegradation, the process through which solar radiation breaks down organic matter, as lignin can strongly absorb both ultraviolet (UV) and shortwave visible radiation. However, contradicting results exist regarding changes in lignin concentration during photodegradation. Clearly, focusing on lignin concentration alone is not sufficient to generate a clear mechanistic understanding about the dynamics of lignin break down during photodegradation. Here we quantify the changes in chemical composition of lignin and other cell wall components during photodegradation using two-dimensional nuclear magnetic resonance (NMR) techniques. Litter of Bromus diandrus was exposed to two levels of UV radiation (with and without UV) and two durations of exposure (2.5 months during summer and one year) in the field. Afterwards, litter cell wall chemistry was examined using solution-state NMR spectrometry. Litter lignin, cellulose, and hemicellulose concentrations were measured using forage fiber techniques. With 2.5 months of exposure, neither litter cell wall chemistry nor litter fiber fractions was affected by UV treatments. One year of UV exposure decreased the abundance of the major lignin inter-unit linkage, beta-aryl ether, by 10%. Lignin concentration, however, was not affected by UV treatments. Interestingly, the abundance of acetylated xylan, a key group of hemicellulose, was reduced by UV exposure by 16%, which is consistent with the result that UV exposure decreased hemicellulose concentration. Our results show that photodegradation induces degradation of lignin in a manner not revealed by routine fiber analysis and that loss of hemicellulose is a key pathway through which photodegradation contributes to litter mass loss. Furthermore, our study demonstrates the feasibility of NMR spectrometry to rapidly quantify changes in cell wall chemistry due to photodegradation.