Labile substrates of root exudates affect soil organic matter (SOM) decomposition by regulating microbial activity. However, whether this effect is negative or positive remains controversial. Labile substrate may help microbes produce SOM-degrading enzymes. Moreover, it may also shift microbial C-use preference, leading to a biomass growth but not decomposition. N is released during SOM decomposition, thus the allocation pattern of root exudates between microbial growth and enzyme production is critical for N cycling. We combined a field study at Yamashiro Experiment Site and lab incubations together to elucidate microbial growth, enzyme production and N decomposition under the addition of root exudates. Root-exudate C enhanced microbial biomass both in the lab and in situ. However, enzyme activities were decoupled with N decomposition rate in the incubation. The activities of C-degrading enzyme were related with qco₂, which suggested a preferential C-use between SOM and root exudates. The activities of N-degrading enzyme were high when microbial NC ratios were high. Although there was uncertainty in the allocation of root exudates to enzymes, N decomposition was well explained by microbial biomass N both in the lab and in situ. Microbial growth demand may be the driving force of N decomposition.