Litter decomposition is critical for carbon (C) cycling in terrestrial ecosystems. Sunlight, as the essential energy for the biogeochemical cycling, can directly break down natural organic matter and accelerate decomposition through photodegradation. However, the role of photodegradation in litter decomposition has been neglected especially in productive mesic ecosystems, where litter is exposed to a heterogeneous radiation environment. Obtaining a more-complete picture of the drivers of litter decay is key to predicting how terrestrial C and nutrient cycles respond to climate changes.