The effects of carbohydrate supply, available nitrogen forms, and their interactions on the photosynthetic characteristics of Castanopsis hystrix seedlings were examined through manipulated pot experiments and phloem girdling. Three inorganic nitrogen sources (ammonia nitrate, AN; ammonium chloride, NH₄⁺-N; and potassium nitrate; NO₃^--N) and three organic nitrogen sources (Urea; arginine, Arg; and glycine, Gly), were applied at identical rates of 10g/m² for a 10d treatment. The carbohydrate supply, N forms, and their interactions had significant effects on the foliar net photosynthetic rate (Pn) of C. hystrix seedlings. Single girdling treatment significantly decreased leaf Pn, leaf stomatal conductance (Gs), leaf intercellular CO₂ concentration (Ci), leaf transpiration rate (Tr), relative chlorophyll content (RCC), and CO₂ use efficiency (CUE) of test seedlings, but it enhanced the leaf stomatal limitation values, as well as water use efficiency (WUE). The decreased Pn was mainly caused by leaf stomatal limitation, and reduction of foliar RCC. For non-girdling seedlings of C. hystrix, all treatments with N addition significantly decreased foliar Pn, Gs, Ci, and Tr, but increased foliar WUE and leaf stomatal limitation values. This is mainly due to stomatal limitation caused by high N loading. Moreover, all treatments with N addition significantly increased leaf CUE, except for the Arg addition treatment. In addition, the strongest stimulation effect on leaf CUE was caused by Gly addition. Correlation analysis indicated that Gs and Tr demonstrated significant correlations with leaf Pn, while Ci and RCC showed relatively weak correlations with leaf Pn. Additions of three organic N forms significantly reduced the inhibitory effect of the single girdling treatment on Pn of C. hystrix seedlings, with the most mitigated effect resulting from Arg addition. Due to the intense inhibitory effect of the single girdling treatment, the foliar Pn recovered to the control level upon Arg addition l, whereas all types of inorganic N additions did not have a significantly positive effect on the girdling-induced inhibition of Pn. Our results indicate that inhibition of carbohydrate supply could lead to reduction of the photosynthetic capacity of C. hystrix seedlings, while these inhibitory effects can be mitigated by organic N supply, such as Arg, Gly, and Urea, of which Arg had the greatest impact.