Abstract Molinia caerulea (L.) Moench is a well-known resource competitor to the detriment of tree regeneration in many boreal or temperate forests of the Northern hemisphere. This study aimed to investigate to what extent soil nitrogen uptake in interaction with light availability drives the early (i.e. in two months) establishment of competition between oak (Quercus petraea (Matt.) Liebl.) and M. caerulea saplings. Two-year-old oak was grown in a greenhouse in 20 L pots, either alone or in combination with M. caerulea, at two irradiance levels (11% and 55% of incident photosynthetically active radiation, iPAR). Leaf photosynthesis measurements and soil 15N-labeling were used to monitor changes in carbon assimilation and soil nitrogen uptake between and within species under well-watered conditions. Presence of M. caerulea had no significant effect on short-term oak sapling growth regardless of irradiance availability level. However, under 55% iPAR, M. caerulea had higher nitrogen uptake and photosynthesis capacity, whereas sessile oak grown with M. caerulea under 55% iPAR showed the same N uptake and a strong decrease in photosynthesis capacity compared to 11% iPAR. The findings reported here point to a very short-term response of photosynthesis and N uptake in a mixture of M. caerulea – oak saplings under 55% iPAR), which might partly result from greater nitrogen uptake by M. caerulea.