Epilithic mosses are early colonizers of the terrestrial biosphere, constitute a special ecosystem regulating rock-atmosphere interactions, and may be more restricted in their nitrogen (N) supply than other mosses. Terrestrial mosses can take up nitrate (NO₃^-), a major form of bioavailable N, from soil substrates. However, the importance of substrate NO₃^- relative to atmospheric NO₃^- remains unclear in moss NO₃^- utilization. This has prevented the understanding of moss NO₃^- dynamics and its responses to environmental N loadings. Here we investigated the monthly concentrations, δ¹⁵N, and δ¹⁸O of NO₃^- in four epilithic moss species from August, 2006 to August, 2007 in Guiyang, southwestern China. We developed a non-steady state isotope mass-balance model based on dual N and O isotopes to evaluate fractional contributions of atmospheric NO₃^- (Ф_atm) and soil NO₃^- (Ф_soil), moss NO₃^- uptake flux (F_influx), moss NO₃^- reduction flux (F_reduction), and the percentage of NO₃^- reduction in total NO₃^- uptake of mosses (expressed as f_reduced). Monthly Ф_soil values averaged 53 ± 13% and monthly f_reduced values averaged 50 ± 35%. Both monthly F_reduction and f_reduced values increased with monthly F_influx values, particularly when Ф_soil values were higher than Ф_atm values. However, the amount of annual NO₃^- reduction (219.7 ± 30.5 μg-N/g, dw) accounted for only 1.0 ± 0.2% in bulk N of mosses. We conclude that half of NO₃^- in epilithic mosses is derived from soil NO₃^- and that NO₃^- uptake from soil induces moss NO₃^- reduction, but the total NO₃^- assimilation contributed a low fraction to total N of study mosses. These findings are important for understanding N sources and dynamics in terrestrial mosses.