The response of non-structural carbohydrates and stoichiometric characteristics of Ochroma lagopus to nitrogen addition is currently unclear. In this study, a 2-year-old O. lagopus was selected, and seven nitrogen addition treatments were set up to investigate the effects of nitrogen addition on the non-structural carbohydrates and stoichiometric characteristics. O. lagopus tree height and diameter increased by 2.00%–14.00% and 3.74%–16.93%, respectively. Nitrogen addition significantly increased leaf soluble sugar, starch, and NSC (non-structural carbohydrates) contents of O. lagopus forests. Their changing trends showed a first increasing and then decreasing trend with increasing fertiliser application in both urea and slow-release fertilisation treatments. Nitrogen addition significantly increased soil N content, leaf N content, and leaf N/P in O. lagopus forests, all of which increased with the increase in urea and slow-release fertiliser application; while leaf P content in O. lagopus forests was significantly lower in nitrogen addition treatments compared with CK (no fertiliser treatment), and decreased with the increase in urea and slow-release fertiliser application. O. lagopus leaf NSC had highly significant positive correlations with soil N, leaf N content, and N/P, and significant negative correlations with leaf P content and C/N. As a result, the application of both urea and slow-release fertiliser increased the soil N content, improved N uptake and utilisation, and promoted the growth of O. lagopus, with the application of slow-release fertiliser at 450 g/plant being the best treatment, and the slow-release fertiliser being superior to urea.

As a fast-growing tree species, Ochroma lagopus Swartz has a greater demand for nutrients, and urea and slow-release fertilizer addition can promote the growth and development of O. lagopus. This experiment explored the effects of urea and slow-release fertilizer on the growth and nutrient uptake of O. lagopus plantation forests. The results of the study can provide a theoretical basis for the cultivation and management of O. lagopus plantation forests. At the end of June 2023, a two-year-old O. lagopus was used as the study’s object at Mengwing Farm, Mengla County, Xishuangbanna Dai Autonomous Prefecture, Yunnan Province, China. Fertilizer is applied through root outer ring application. Seven treatments of no N control (CK) and urea and slow-release fertilizers 300 g/plant (N1, H1), 450 g/plant (N2, H2), and 600 g/plant (N3, H3) were set up to determine the breast height, diameter at breast height, as well as the nutrient contents of leaves, branches, roots, and soils of the trees. The DBH and tree height were higher under N addition than in the CK, and both were highest in H3 at 90 d after fertilization, with 16.92% and 14.64% higher than CK, respectively. The overall change in C content of each organ was not obvious, N content increased with the increase in fertilizer application, and the change pattern of P content was not consistent. Soil N content showed a significant increase with the increase in N application, while P and K content showed a trend of increasing and then decreasing, and soil N content of slow-release fertilizer treatments was higher than that of urea treatments under the same amount of N application. The soil N content was higher in the slow-release fertilizer treatment than in the urea treatment regardless of the amount of N applied. The soil N content was highly significantly positively correlated with the N content of leaves, branches, and roots. There was a highly significant allometric growth relationship among the C, N, and P content of each organ. The N addition had a significant effect on the growth and the nutrient content of each organ and soil. O. lagopus mainly adapted to changes in the soil N content by adjusting the N and P content of each organ, and the optimal effect was observed in H3. In the future, an appropriate amount of P fertilizer should be applied to complement N fertilizer, urea and slow-release fertilizer can also be applied.