Response of Non-Structural Carbohydrates and Carbon, Nitrogen and Phosphorus Stoichiometric Characteristics of Ochroma lagopus Leaves to Nitrogen Addition

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.