Aims Nitrogen (N) application is often known to influence various properties in Chinese fir growth, but the regeneration failure and productivity decline in Chinese fir plantations are often affected by allelochemical compounds. Little is known about the interactive effect of allelochemicals and N sources on the plant growth in Chinese fir seedlings. Methods Chinese fir seedlings were respectively subjected to the addition of ammonium (NH 4 + ), nitrate (NO 3 − ), p -hydroxybenzoic acid ( p HBA) and their combined additions. Results The p HBA addition alone promoted Chinese fir seedlings growth, but this effect was differentially affected by the interaction with NH 4 + or NO 3 − . Both NH 4 + and NO 3 − application improved plant growth, but the stimulatory effect of NO 3 − was weakened with addition of p HBA. Although NO 3 − addition combined with p HBA increased N accumulation in roots, the enzymes activities related to N assimilation were significantly decreased. These negative influences were also manifested through the decreased photosynthetic capacity, the reduction in photosynthates production and accumulation, and the destruction of chloroplast structural integrity. Moreover, the C/N ratio in root was obviously decreased and N assimilation was inhibited under the combined treatment of p HBA and NO 3 − , thereby aggravating C − N imbalance. Conclusions The Chinese fir seedlings shifted their N uptake preference from NO 3 − to NH 4 + when p HBA were added in soil. The dominant N source and allelochemicals interaction should be considered for Chinese fir growth.