Root-soil interactions are crucial parts of the terrestrial carbon (C) cycle. To get a better understanding of their role in forest ecosystem feedbacks to climate change, this dissertation explored (1) how tree species variation in root phenolic profiles influence soil C cycling and (2) to what extent seasonal drought affects fine root growth dynamics in a mixed-species forests of European beech (Fagus sylvatica [L.]) and Norway spruce (Picea abies [L.] Karst). For the first question, I modified a root exudate collection system in order to measure root phenolic composition of two coniferous and four deciduous forest tree species using high-performance liquid chromatography (HPLC). I tested the effects of nine of these phenolic compounds on soil microbial respiration. These findings helped inform the next incubation experiment, which made use of 13C-labeling techniques and different microbial assays to analyze the mechanistic effects of phenolic-glucose interactions on soil organic matter (SOM) decomposition and associated soil microbial dynamics. Overall, my experimental results showed that root phenolic composition is highly species-specific and can have both stimulatory and inhibitory effects on soil microbial respiration. Moreover, I found that root phenolic effects on soil biogeochemistry are more persistent than glucose, influencing SOM decomposition rates and associated enzyme activities as well as inducing shifts in bacterial community composition. To answer the second question, I studied root growth dynamics in a mature mixed beech-spruce forest in southern Germany under throughfall-exclusion over a four-year period by means of minirhizotron imaging. While fine root production of both F. sylvatica and P. abies decreased under throughfall-exclusion, I found an increase in fine root lifespan in response to seasonal water limitation. Yet, in the interspecific stands of F. sylvatica and P. abies, a decline in root production was not observed, suggesting that the belowground productivity of mixed-species forests is less affected by seasonal drought than monospecific forest stands. In conclusion, these findings show the varying magnitude and direction of root phenolic effects on SOM decomposition and the responsiveness of fine roots to drought, emphasizing the importance of studying root-facilitated C cycling in a changing climate.