Seasonal changes of root biomass, concentrations and masses of carbon and nitrogen in roots and root diameter classes (B2mm, 2_5mm and_5mm) were evaluated under four adjacent coniferous plantation sites: Oriental spruce (Picea orientalis L.), Austrian pine (Pinus nigra Arnold), Turkish fir (Abies bornmulleriana L.), Scots pine (Pinus sylvestris L.) and natural Sessile oak (Quercus petraea L.) forest. The root biomass, carbon and nitrogen concentrations, carbon and nitrogen masses, C/N ratio of roots were evaluated with regards to species, root diameter classes and seasons. The significantly (pB0.05) lowest annual mean root biomass was 258 g/m2 in Austrian pine compared with tree species. The fine root class has significantly the lowest carbon content, with the 42% value, among other root diameter classes. However, the temporal variations of root C (%) presented no statistical significance. The annual mean nitrogen concentrations ranged from 0.56% (Turkish fir) to 0.74% (Austrian pine) among species. We also found that root nitrogen concentrations in January were significantly different from those of other sampling times. C/N ratios showed statistically significant differences in root diameter classes that the ratios were found, 49 for fine roots, 111 for small roots and 143 for coarse roots, respectively.