Abstract: Nitrogen (N) and phosphorus (P) are important nutrients affecting plant growth. Fine root is an important organ to absorb nutrients and water. Here, we analyzed the effects of different N and P application ratios (15∶1, 12∶1, 10∶1, 8∶1) on the growth and fine root traits of 1-year-old Machilus pauhoi seedlings in a pot experiment. The results showed that: (1) All the N and P addition treatments significantly increased leaf biomass, total biomass, and leaf mass fraction, but decreased root-shoot ratio, especially at the treatments with N/P ratios of 10∶1 and 8∶1. (2) Combined applications of N and P significantly increased relative growth rates of both total biomass and biomass of each organ, with a strongest effect under N/P ratio of 10∶1. (3) Combined applications of N and P significantly increased N and P concentrations in fine roots (P<0.05), with no difference among treatments. Fine root N concentration was the highest in the treatments with N/P ratios of 15∶1 and 10∶1. Fine root P concentration was the highest in the treatment with N/P ratio of 10∶1. Combined applications of N and P reduced the average diameter of each fine root order and specific surface area (SRA) of the 1st-2nd order roots, but increased specific root length (SRL) of the 2nd-4th order roots and SRA of the 3rd-4th order roots. The changes of root tissue density (RTD) of each order was opposite to that of SRA. (4) The relative growth rates of total biomass and biomass of each organ were significantly positively correlated with RTD but negatively correlated with SRL and SRA after combined N and P application. The results indicated that application of different N/P ratios affected biomass allocation pattern, relative growth rate, and the relationship between relative growth rate and some fine root traits of M. pauhoi seedlings. The most suitable ratio of N/P for the growth of M. pauhoi seedlings was 10∶1. Our results provide an important reference for the development of precise nutrient management and efficient cultivation of M. pauhoi seedlings.

Key words: nitrogen/phosphorus ratio, relative growth rate, biomass allocation, fine root trait, Machilus pauhoi.