UV-B exposure alters the lignin chemistry of plant litter: a potential mechanism for UV-B effects on decomposition rates in arid ecosystems

Background/Question/Methods

Decomposition rates occur in three stages.  During the initial stage microbial enzymes convert cellulose into labile sugars. However cellulose is often physically inaccessible to enzymes because it is encased in lignin. In natural ecosystems oxidative enzymes usually degrade lignin, but recent studies suggest that UV-B exposure can also degrade lignin. Mechanisms controlling potential losses due to UV-B remain unclear with some studies showing exposure to UV-B resulting in significant mass loss while others show no effect. In this study we examined the effects of UV-B on surface chemistry of litter as a possible mechanism for increased litter loss. We exposed homogenized switchgrass (Panicum virgatum), to three levels of UV-B for three months in a controlled greenhouse environment: no UV, ambient UV, and elevated UV. UV exposures simulated outdoor intensities found at Short Grass Steppe in Colorado using Caldwell weighted measurements.  Elevated levels were no greater than the highest day of UV recorded during the summer.  After UV-B doses equivalent to 6 months of exposure, samples were harvested for spectroscopic analysis.  We used Fourier-Transformed Infrared Spectrometry (FT-IR) to detect changes in the frequency of specific bonds as an index of surface chemistry of exposed biomass. 

Results/Conclusions

Results from the FT-IR indicate that surface chemistry was significantly changed (p<0.05) in the 1138-1190 cm^-1 and 1703-2181 cm^-1 regions for the elevated UV-B treatment.  The 1148-1190 region indicates increases in cellulose and lignin ether bonds, and increases in the 1703-2180 indicate changes in lignin. Principal component analysis (PCA), a method for determining correlation, of all spectra suggested moderate changes in overall chemistry due to UV-B exposure.  However, for the carbonyl region, PCA indicates strong treatment effects.  Increases in carbonyl bonds indicate that oxidation of C bonds occurred during UV-B exposure. Increase in carbonyl groups may alter lignin structure thereby increasing enzymatic access to cellulose and subsequent microbial decomposition.  Linking cell wall changes to decomposition could provide a mechanism for understanding UV-B effects and a possible opportunity for inclusion of UV-B affects in arid ecosystem C models.