Addition of N to an initially N-limited forest increases foliage biomass, demand for water and the probability of water stress. Effects of water and N on tree growth are thus compounded. The 13C abundance of plant tissues is directly correlated with water use efficiency (WUE), and could be used to disentangle the effect of water alone on carbon fixation. However, the 13C abundance may also be directly influenced by changes in rates of photosynthesis related to variations in N status, and by variations in N metabolism via non-RuBisCo carboxylations, and indirectly by effects of N source on WUE. We studied the 13C abundance of current needles from top whorls in two long-term fertilization experiments, one in Norway spruce (Picea abies Karst.) and one in Scots pine (Pinus sylvestris L.). As predicted, N fertilization increased foliage biomass and δ (‰). In the experiment with spruce this effect on 13C abundance was correlated with volume production and foliage biomass in a dry year, but was not seen in a wet year after 19 years of continuous annual N fertilization, which rules out the possible influences of N metabolism and changes in rates of photosynthesis. In the experiment with pine, which was at a drier site, needles from N-fertilized plots had a higher 13C abundance in three dry years, but not significantly so in a wet year. We suggest that effects of N source (NH4+ or NO3−) on 13C abundance are unlikely to be important under these experimental conditions. The balance between demand and supply of water should thus be the major determinant of the 13C abundance of current needles on top whorls. This opens possibilities to conduct retrospective studies of the role of water supply in fertilization experiments.