Effects of Phenolics on seedling growth and ¹⁵N nitrate absorption of Cunninghamia lanceolata

Chinese fir (Cunninghamia lanceolata) has replant problem under field conditions and has been attributed to phenolic allelochemicals, which cause its autotoxicity. To determine the mechanism of inhibitory effects of phenolics on Chinese fir, we studied their effects on growth and ¹⁵N nitrate absorption by each organ of its seedlings, cultured in pots using ¹⁵N-labelled isotope trace technique. The 1×10⁻³ mol·L⁻¹ vanillic acid and phenolics mixture (0.5×10⁻³ mol·L⁻¹ vanillic acid and 0.5×10⁻³ mol·L⁻¹ ρ-hydroxybenzoic acid) significantly inhibited the seedling growth of Chinese fir. Phenolics solution of 1×10⁻² mol·L⁻¹ concentration decreased total N content of roots, stems and leaves. ¹⁵N derived from the fertilizer (NDFF) in roots, stems and leaves treated with 1 × 10⁻² mol·L⁻¹ vanillic acid was 36.1, 42.7 and 38.5% less, respectively, and corresponding values for phenolics mixture at the same concentration were 14.1, 17.2, and 17.0% lower, respectively, compared to control. In contrast, vanillic acid and ρ-hydroxybenzoic acid at 1×10⁻⁵ mol·L⁻¹ stimulated the seedling growth, enhanced total ¹⁵N content and increased ¹⁵N nitrate absorption. These treatments also increased the ¹⁵N nitrate distribution in roots and reduced its distribution in stems and leaves. We suggest that accumulated phenolics in soil could inhibit seedling growth through reducing nutrient absorption by Chinese fir and consequently leading to low productivity of replanted Chinese fir.