Nitrogen Addition Modulates Adaptive Responses of Chinese Fir Roots to Phosphorus Deficiency and Promotes Nutrient Absorption Efficiency

Intensified atmospheric nitrogen (N) deposition, coupled with increased application of N fertilizers in forest plantations, has resulted in N and phosphorus (P) imbalance in the forest soil system. However, it is seldom investigated whether an addition of N counteracts the effects of low P stress on N and P nutrient absorption and utilization efficiencies. Thus, we conducted a greenhouse experiment on three clones of Chinese fir (clones 061, 020 and 2 C) with four levels of N addition (2, 5, 10, and 15 mmol·L− 1 sodium nitrate) under normal P supply (1.0 mmol·L− 1 KH2PO4), moderately P deficiency (0.5 mmol·L− 1 KH2PO4) and severely P deficiency (0.1 mmol·L− 1 KH2PO4) conditions. The results showed that addition of moderate concentration of N (10 mmol·L− 1 N) significantly promoted root elongation, root dry mass accumulation, root surface area and root APase activity of Chinese fir seedlings under P-deficient conditions, with distinct clonal variation. Seedlings of clone 061 showed the most root APase activity in response to addition of moderate concentration of N under P deficient condition while clones 061 and 020 had the longest roots and the largest root dry mass accumulation. Under severe P deficiency, the N and P absorption efficiencies (NAE and PAE) of Chinese fir seedlings increased with increasing N concentration, while the N and P utilization efficiencies (NUE and PUE) decreased. The findings demonstrate that addition of moderate level of N modulates the adaptive responses of Chinese fir to P deficiency and enhances the absorption efficiency of P. The variation among clones suggests that the responses are genetically controlled and there exists great potential for breeding highly phosphorus efficient genotypes under coupled conditions of P deficiency and increased atmospheric nitrogen deposition.