LAUSR

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. Chinese fir seedlings were respectively subjected to the addition of ammonium (NH4+), nitrate (NO3−), p-hydroxybenzoic acid (pHBA) and their combined additions. The pHBA addition alone promoted Chinese fir seedlings growth, but this effect was differentially affected by the interaction with NH4+ or NO3−. Both NH4+ and NO3− application improved plant growth, but the stimulatory effect of NO3− was weakened with addition of pHBA. Although NO3− addition combined with pHBA 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 pHBA and NO3−, thereby aggravating C − N imbalance. The Chinese fir seedlings shifted their N uptake preference from NO3− to NH4+ when pHBA were added in soil. The dominant N source and allelochemicals interaction should be considered for Chinese fir growth.