Signatures of an abiotic decomposition pathway in temperate forest leaf litter

Photodegradation helps explain why leaf litter mass loss rates are comparable between mesic environments and drylands, despite water availability limiting decomposer activity in drylands. With photodegradation, light energy directly mineralizes photoreactive organic molecules such as lignin, which contrasts with biotically-driven decomposition where lignin persists in decomposing leaf litter relative to other compounds. Photodegradation of leaf litter has largely been studied in dryland systems, but whether it occurs in mesic, deciduous forest systems is uncertain. As such, we examined potential photodegradation in maple and oak litter in the Southern Appalachian Mountains (USA) during the non-growing season, when newly fallen leaves are exposed to maximum solar radiation without deciduous canopies. The proportion of oxidized lignin in the leaf litter relative to other carbon compounds decreased with maximum solar radiation during the non-growing season. With a closed canopy, however, the proportion of total lignin relative to other compounds in the leaves increased. This pattern was particularly strong for the syringyl- S6 and guaiacyl-G6 lignin monomers. The relative proportion of S6 and G6 in the litters was reduced by 10–65% in the first 5 months. The largest reductions in lignin occurred on south-facing mountain slopes where solar radiation was highest (relative to north-facing slopes) in the Northern Hemisphere. Overall, however, the changes in lignin did not alter cumulative litter mass lost. Our results suggest that photodegradation promotes the transformation and relative loss of lignin in the early stages of litter decomposition in mesic deciduous forests.