Significance Understanding controls on carbon storage in terrestrial ecosystems is essential to predict and mitigate human impacts on the global carbon cycle. This study identifies a general unifying mechanism to explain how photochemical mineralization of plant litter, photodegradation, affects subsequent biotic decomposition. Previous work has shown that photodegradation can be a major direct control of carbon loss in arid lands. We now demonstrate that, across a wide range of species, exposure to sunlight has large “photopriming” effects, enhancing microbial decomposition, especially due to blue–green light. Photodegradation increased availability of carbohydrates to hydrolytic enzymes, likely through reducing the bottleneck imposed by lignin in secondary cell walls. This mechanistic insight suggests that photodegradation is globally important for carbon turnover in terrestrial ecosystems.