Evaluation of nitrogen use efficiency at seedling stage of excellent clones of Chinese fir trees

To screen nitrogen-efficient fir species, field experiments were conducted to study the growth traits and nitrogen utilization of four excellent fir asexual lines under different nitrogen treatments at different growth stages of the seedling stage. The results showed that the growth traits of different asexual fir lines differed to different degrees under nitrogen treatment conditions. In general, the growth indexes of the four asexual lines in the fast-growing period under different N treatments did not differ significantly. However, in the late stage of growth, the diameter, height, and biomass of Yang 049 increased with the increase of nitrogen application, with maximum increases of 2.17%, 14.93%, and 48.15%, respectively; the height of Yang 061 decreased slightly with the increase of nitrogen application, with the maximum decrease of 7.54%, and there was no significant difference in its diameter and biomass under different nitrogen treatments. The diameter, height, and biomass of Yang 047 increased slightly with the increase of nitrogen application, with a maximum decrease of 7.54%, and there was no significant difference in its diameter and biomass under different nitrogen treatments. The diameter, plant height, and biomass of Ocean 047 increased slightly with the increase of nitrogen application, but its average value was lower than that of the other three asexual lines; among the four asexual lines, there was no obvious change rule for the diameter, plant height, and biomass of Ocean 6421 under different nitrogen treatments. Further results of water content and NPK content showed that the coefficients of variation of water content of Ocean 061 and Ocean 047 were 0.74% and 5.71%, respectively, under nitrogen treatments, and that the water content of Ocean 049 was higher than that of other three asexuals and tended to increase with the increase of nitrogen application, but the differences were not significant. Aboveground and belowground N contents of the four asexual lines showed different changes under N treatments, but the magnitude of the changes was small. The phosphorus and potassium contents of Yang 049, especially in the above-ground part, varied greatly under the influence of nitrogen application, with the ranges of 4.24%～47.26% and 2.74%～25.16%, respectively. Based on the above growth traits and physiological indexes, it was preliminarily determined that Ocean 049 was highly responsive to nitrogen application and could be used as a high-nitrogen and high-efficiency candidate species, suitable for afforestation under conditions of high nitrogen fertilizer accessibility; when afforesting in difficult habitats, nitrogen fertilizer inputs should be increased to ensure that its productivity is maximized. In the case of difficult sites, N input should be increased to ensure its productivity is maximized. Yang 061 and 6421 are not sensitive to the amount of N applied so they can be used as low-N high-efficiency candidates, and there is no need to pay too much attention to the background level of N in silvicultural sites during the cultivation process.