Abstract:One-year old seedlings of Machilus pauhoi were used to determine the effect of adding a combination of N and P on the morphology of different orders of fine roots and soil microbes. For this, the phospholipid fatty acid (PLFA) analysis and root system scanning methods were used. The results were as follows:(1) Combined addition of N and P significantly improved (P < 0.05) specific root length and specific root area of the first-order and second-order fine roots, while it reduced specific root area of the third-order and fourth-order roots. (2) Root tissue density for the first-order and second-order fine roots tended to decline, contrary to that for the third-order and fourth-order. (3) Root average diameter of the first-order to the fourth-order roots was significantly reduced (P < 0.05). (4) Total microbial, fungal, and bacterial biomass all showed a tendency to reduce after first increasing, all reaching the maximum when treated with N:P in the ratio of 10:1. (5) Although the bacterial and fungal and actinomycetes biomass and total microbial were significantly and positively correlated with specific root length and specific root area for the first-and second-order roots, they were negatively correlated for the fourth-order roots. Although Gram-positive bacteria and fungal biomass were positive correlated with root tissue density for the third-order and the fourth-order, no significant correlations with the first-and second-order roots were observed. Root average diameter for all orders had no significant correlation with soil microbes. Our results suggest that the effect of short-term addition of a combination of N and P was the best when treated with N:P in the ratio of 10:1, which can improve the efficiency of nutrient uptake of Machilus pauhoi seedlings' fine roots. Plants may adapt to N deposition by adjusting the fine root morphological characteristics. Meanwhile, the response of underground biological communities such as soil microbes, and the relationship of microbes with fine roots, to N deposition also changes, which affects C and N cycles and the nutrient flow in underground ecosystem.