Lignin degradation during plant litter photodegradation

Abstract:

Lignin is the second most abundant compound, after cellulose, synthesized by plants. Numerous studies have demonstrated that initial lignin concentration is negatively correlated with litter decomposition rate under both laboratory and field conditions. Thus lignin is commonly considered to be a “recalcitrant” compound during litter decomposition. However, lignin can also serve as a radiation-absorbing compound during photodegradation, the process through which solar radiation breaks down organic matter. Here, we synthesize recent studies concerning lignin degradation during litter photodegradation and report results from our study on how photodegradation changes lignin chemistry at a molecular scale. Recent field studies have found that litter with high initial lignin concentration does not necessarily exhibit high mass loss during photodegradation. A meta-analysis (King et al. 2012) even found a weak negative correlation between initial lignin concentration and photodegradation rate. Contradicting results have been reported with regard to the change in lignin concentration during photodegradation. Some studies have found significant loss of lignin during photodegradation, while others have not. In most studies, loss of lignin only accounts for a small proportion of the overall mass loss. Using NMR spectroscopy, we found significant loss of lignin structural units containing beta-aryl ether linkages during photodegradation of a common grass litter, Bromus diandrus, even though conventional forage fiber analysis did not reveal changes in lignin concentration. Both our NMR and fiber analyses supported the idea that photodegradation induced loss of hemicellulose, which was mainly responsible for the litter mass loss during photodegradation. Our results suggest that photodegradation induces degradation, but not necessarily complete breakdown, of lignin structures and consequently exposes hemicellulose and cellulose to microbial decomposition. We conclude that lignin concentration is a poor predictor and poor indicator of photodegradation rate, whereas lignin degradation during photodegradation likely plays a key role in "priming" litter for microbial decomposition.