Fresh tree root decomposition induced by tillage is an important source of soil nutrients in agroforestry systems. Here we examined the effects of tree species, root size and soil N enrichment on fresh root decomposition under laboratory conditions. Fresh roots with two diameters (<2 and 2-5 mm) of Populus euramericana cv. 'N3016' (poplar) and Pinus tabulaeformis (pine) collected from agroforestry systems in Northeast China were used in the experiment. For each root treatment, four N levels (0, 50, 100 and 150 mu g N g(-1) soil) were added. We recognized N concentration and C/N ratio as the root quality variables, and determined decomposition rates as cumulative CO(2) production and mass loss. Poplar roots had higher N concentration and lower C/N ratio and decomposed faster than pine roots, and smaller roots decomposed faster than the corresponding larger roots. The effect of N addition on root decomposition varied from positive to negligible to negative, and depended on root quality and N addition rates. Increased N availability did not accelerate and even suppressed poplar root decomposition, whereas generally stimulated pine root decomposition. Our results suggest that root quality should be incorporated into the design of agroforestry systems. Moreover, the differential responses of N addition on decomposition of fresh roots with different quality provide insights into soil nutrient management in agroforestry practices.