Occupancy as a key attribute linking saprotrophic fungi to soil carbon decomposition.

Understanding how fungal biogeography shapes ecosystem processes is central to predicting biogeochemical responses to environmental change. However, the properties of saprotrophic fungi that link their distribution patterns to belowground carbon cycling remain unclear. Here, we provide a comprehensive assessment linking widespread (high-occurrence) and narrowly distributed (low-occurrence) saprotrophic fungi (saprotrophs) to carbon decomposition processes across a broad latitudinal gradient from tropical to boreal zones. Our results show that the diversity of saprotrophic fungi is shaped by distinct community assembly processes, which in turn generate contrasting geographic patterns across latitudes. Widespread saprotrophs are mainly structured by large-scale climatic and local soil variables (environmental filtering) and show increasing diversity toward higher latitudes, whereas narrow-ranged saprotrophs are primarily constrained by the regional species pool (dispersal filtering), leading to declining diversity with latitude. By integrating large-scale biogeographic data with DNA stable isotope probing experiment, we provide evidence that fungal taxa actively participating in straw decomposition are dominated by widespread saprotrophs, underscoring their essential role in carbon cycling. Our findings highlight occupancy as a key biogeographic attribute regulating fungal contributions to carbon cycling and provide a predictive framework for understanding how belowground biodiversity shapes soil carbon dynamics under global change.