Photodegradation has emerged as a key driver of litter decomposition and aboveground carbon (C) fluxes in terrestrial ecosystems, yet its latitudinal variation and underlying mechanisms remain poorly understood amid climatic heterogeneity across gradients. We conducted an in situ spectral-attenuation experiment across three sites along a latitudinal gradient in eastern China: south-subtropical (Low), north-subtropical (Middle), and temperate (High). Leaf litter from two widespread tree species (Larix olgensis and Populus davidiana) with contrasting traits were used. After 319 days, litter mass loss was highest at the Low site (60%) and lowest at the Middle site (52%). Ambient solar radiation accelerated decomposition relative to attenuating ultraviolet (UV)-B radiation or combined UV and blue light, contributing 49% to total mass loss on average in a non-monotonic latitudinal pattern, with stronger effects at Low and High sites. UV-A and blue light primarily drove mass, C, nitrogen (N), and lignin losses (21–32%), exceeding UV-B radiation (10–18%). Partial least squares path modelling, validated through sensitivity analyses, identified solar radiation as the dominant driver, modulated by microbial communities and latitude: enhancing activity and decomposition at the Low site, but counteracted by fungal suppression at the High site. Overall, our study reveals a latitudinal transition from microbially mediated photofacilitation in humid low latitudes to direct photomineralization in arid high latitudes, providing mechanistic insights to refine ecosystem C models under changing radiation and climate regimes.