The potential drivers of decomposition depend on the ecosystem and stage of decay. In arid or semi-arid grassland systems, for example, where water availability limits both microbial activity and the transport of compounds, mass loss rates are comparable to more humid environments due to photodegradation. Photodegradation is the process by which light breaks photoreactive carbon compounds into either volatilized carbon compounds, such as CO2, or smaller organic compounds that are consumable by microorganisms. Lignin has been shown to preferentially decay through the photodegradation pathway, which is the opposite trend found under biological decomposition dominant in mesic systems. With maximum solar radiation reaching the forest floor over the winter, we examined the potential for photodegradation to prime spring/summer biological litter decomposition in a temperate forest. We measured mass loss and lignin compounds of Quercus and Acer litterbags on both north- and south-facing slopes in the mountains of southwestern North Carolina under three UVB levels (100%, 80%, and 20%). We hypothesized that litter on south-facing slopes and 100% UVB would lose more mass and lignin over the winter months due to photodegradation (five months), which would then prime higher mass loss of those same litters at ten months, following biological decomposition.
Results/Conclusions
We effectively reduced the amount of UVB reaching forest floor litter (100%, 75%, and 20%). However, patterns in mass loss across litter types did not align with UVB treatments. Mass loss in both reduced UVB treatments was significantly lower than the control, but indistinguishable from each other signaling a potential effect of the UVB-reducing plastics. However, soil moisture and soil temperature did not vary by UVB treatment, only by aspect. Consequently, we examined changes in lignin compounds for each litter type × collection date × aspect pooled across UVB treatments. The proportion of total lignin in the litter, decreased at 5 months before increasing again at 10 months. This was the same pattern for two monomers, Syringyl S6 and Guaiacyl G6, known derivatives of lignin. In other words, the proportion of lignin in the litter relative to other compounds decreased during the winter when decomposer activity was low and no tree canopy was present to block radiation. The proportion of lignin then increased at 10 months following the growing season under a full tree canopy and high expected decomposer activity. Our results suggest the possibility of photodegradation within a temperate forest and unexpected lignin loss during early stages of decomposition.