Epilithic mosses are early colonizers of the terrestrial biosphere, which constitute a special ecosystem regulating rock-atmosphere interactions. Terrestrial mosses can take up nitrate (NO3-), a major form of bioavailable N, from soil substrates. However, the importance of substrate NO(3)(-)relative to atmospheric NO(3)(-)remains unclear in moss NO(3)(-)utilization. This has prevented the understanding of moss NO(3)(-)dynamics and their responses to environmental N loadings. This study investigated monthly concentrations, delta N-15, and delta O-18 of NO(3)(-)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 to evaluate fractional contributions of atmospheric NO3-(CYRILLIC CAPITAL LETTER EFatm) and soil NO3-(CYRILLIC CAPITAL LETTER EFsoil), moss NO(3)(-)uptake flux (F-influx), moss NO(3)(-)reduction flux (F-reduction), and the percentage of NO(3)(-)reduction in moss NO(3)(-)uptake (f(reduced)). The monthlyCYRILLIC CAPITAL LETTER EF(soil)values averaged 53 +/- 13% and the monthlyf(reduced)values averaged 50 +/- 35%. Both the monthlyF(reduction)andf(reduced)increased as the monthlyF(influx)increased, particularly when theCYRILLIC CAPITAL LETTER EF(soil)values were higher thanCYRILLIC CAPITAL LETTER EF(atm)values. However, the amount of annual NO(3)(-)reduction (219.7 +/- 30.5 mu g-N/g, dw) accounted for only 1.0 +/- 0.2% of the bulk N of the mosses. We conclude that half of the NO(3)(-)in epilithic mosses is derived from the soil NO(3)(-)and that NO(3)(-)uptake from the soil induces moss NO(3)(-)reduction, but the total NO(3)(-)assimilation contributed a low fraction of the total N in the studied mosses. These findings are important for understanding N sources and N dynamics in terrestrial mosses.