Grass-clover mixtures are essential in many low-N-input cropping systems, but the importance of various root fractions for the below-ground N dynamics are not well understood. This may be due to the difficulties of studying root longevity and turnover in situ in mixtures. The present field study, investigated (1) the development in root biomass over two growing seasons and (2) the turnover of dual 15N- and 14C-labelled ryegrass and white clover root material. Litter bags containing various dual-labelled plant materials were incubated in cylinders inserted in the topsoil of a young ryegrass-clover ley. Disappearance of 14C and 15N from the litter bag material were studied for 1 year following incubation. Four times during two growing seasons, roots were divided into two classes: large roots, retained on a 1-cm sieve, and small roots, passing a 1-cm sieve but retained on a 100-µm sieve. Large root biomass increased during the two growing seasons, and small root biomass increased during the growing seasons but decreased during autumn and winter. White clover roots lost 14C and 15N almost twice as fast as ryegrass roots. The disappearance pattern of 14C and 15N from dual-labelled ryegrass and white clover roots and the C and N contents of the recovered root material indicate that large roots are determining soil C pool build-up, whereas small roots determine soil N pool build-up.