Objectives Phyllostachys heteroclada f. solida is an excellent bamboo species used for both shoot production and timber, yet there is a lack of effective management and operation practices for it. We investigated the impact of different nitrogen application levels on the growth and nutrient characteristics of Ph. heteroclada f. solida's branches and leaves.

Methods The experiment was conducted at Xikou Forest Farm in Longyou County, Zhejiang Province. The bamboo forest under test was one-year-old mother bamboos with a ground diameter of approximately 12 mm and rhizomes (bamboo whips) about 20 cm long. Four urea application levels were set up: 0 (CK), 20 (N₁), 40 (N₂), and 60 g/plant (N₃). After new bamboo shoots sprouted branches and leaves, one branch cluster was sampled from the upper, middle, and lower sections of each bamboo plant's canopy to measure the growth indicators of the bamboo plants. Simultaneously, intact mature leaves were collected to measure their saturated fresh weight, as well as C, N, and P contents. The software, WINRhizo 2009, was used to analyze the morphological quality of branches and leaves, the efficiency of branch sprouting and leaf unfolding, and the allometric growth relationships of biomass in the main and lateral branches of bamboo.

Results N application significantly increased the N and P contents and decreased the C/N and C/P ratios of the leaves of main and lateral branches, did not change the C content and N/P significantly. With the increase of N application levels, the leaf starch and cellulose contents remained stable, while the soluble sugar content showed a trend of first increasing and then decreasing. The lignin content in the main branches increased with the increase of nitrogen application rate, with N₃ increasing by 57.51% compared to CK, while the lateral branches gradually decreased, with N₃ decreasing by 27.46% compared to CK. Leaf length, leaf width, leaf area, leaf perimeter, and single-leaf dry weight of main and lateral branches exhibited an N-shaped response to increased N application levels, reaching their maximum values under the N₃ treatment. Compared to CK, these parameters increased by 21.41%−22.76%, 8.99%−10.03%, 27.07%−33.50%, 17.60%−19.51%, and 4.82%−5.62%, respectively. The leaf shape index of main branches increased from 6.17 to 6.87, while that of lateral branches increased from 6.02 to 6.76, indicating a trend toward more elongated leaves. In contrast, the effect of N application rates on morphological traits of bamboo branches was not significant. N application significantly increased the leaf emergence intensity and leaf area ratio of bamboo, with the highest values increased by 0.71 times and 1.09 times respectively compared with CK, thus enhancing the efficiency of branching and leaf spreading. N application changed the growth relationship between leaf and branch biomass of bamboo from isotropic growth to anisotropic growth, and promoted the accumulation of branch biomass.

Conclusions Nitrogen application can significantly boosts the nutrient content in the leaves of Ph. heteroclada f. solida, without significant impact on the morphological traits of the main and lateral branches, yet it exerts a marked effect on leaf morphology. When nitrogen is applied at high rate, the bamboo leaves become more elongated and thinner, leading to a significant expansion in leaf area and perimeter. Moreover, as the nitrogen application rate increased, there is a remarkable enhancement in leaf growth and development, leaf unfolding efficiency, leaf emergence intensity, and leaf area ratio. N application also can enhance the transport capacity of branches, promote biomass accumulation, and optimize the resource allocation of branch and leaf biomass. Therefore, applying nitrogen at a relatively high rate is an effective measure to promote the growth and development of the branches and leaves of Ph. heteroclada f. solida.