It has been established that plant diversity affects ecosystem functioning by controlling nutrient and C cycling. However, little is known about the effect of plant diversity on belowground processes such as root turnover and root decomposition, and the effects that diversity loss could have on C cycling. Thus, this PhD thesis focused on disentangling how plant diversity and plant identity affect two important mechanisms: root turnover and root litter decomposition. The overall hypotheses were: i) fast-growing species characterized by high quality roots have faster turnover and decomposition, compared with slow-growing species, ii) Plant communities with higher diversity have higher root litter decomposition and root turnover because of complementary effects and iii) high root turnover rates are linked to higher ecosystem C uptake rates. A set of two glasshouse-based experiments focusing on root turnover and root litter decomposition were performed using two fast-growing species and two slow-growing species were grown as monocultures and mixtures. Finally, using a 20-year long-term field experiment the effect of plant diversity on root turnover was assessed. The findings show that root decomposition and turnover follow the âroot economic spectrumâ but neither root turnover nor root litter decomposition was correlated with plant growth strategy. Diverse communities had higher root turnover than monocultures while root litter decomposition was slower suggesting that root quality varied between living and dried dead roots. Surprisingly, high root turnover rates were not linked to high ecosystem C uptake rates while root traits reflecting higher C investment and high nitrogen were strongly correlated with higher net ecosystem exchange. Overall, the results presented in this thesis show the complexity of belowground interactions and the need for further studies with a higher species spectrum that helps to understand the underlying mechanism of these interactions.